- 


SYSTEMATIC    HANDBOOK 


OF 


VOLUMETRIC  ANALYSIS; 

OR, 

THE  QUANTITATIVE   DETERMINATION 

OF  CHEMICAL  SUBSTANCES   BY  MEASURE,  APPLIED 

TO   LIQUIDS,  SOLIDS,   AND   GASES, 


ADAPTED    TO    THE   REQUIREMENTS    OF    PURE    CHEMICAL   RESEARCH, 
PATHOLOGICAL    CHEMISTRY,     PHARMACY,     METALLURGY,     MANUFACTURING 

CHEMISTRY,    PHOTOGRAPHY,    ETC.,    AND    FOR   THE    VALUATION 
OF   SUBSTANCES    USED    IN    COMMERCE,    AGRICULTURE,    AND   THE    ARTS. 


FRANCA'  gut  TON,'"  Fri:c:,  F.C.S., 

PUBLIC  ANALYST  FOR  THE  COUNTY  OP  NORFOLK  ;  ETC. 


TENTH   EDITION 

REVISED    THROUGHOUT,    WITH   NUMEROUS    ADDITIONS,    BY 

W.   LINCOLNE   SUTTON,   F.I.C., 

PUBLIC  ANALYST  FOR  THE  COUNTY  OF  SUFFOLK,   NORWICH,  IPSWICH,  F.TC. 
AND 

ALFRED   E.   JOHNSON,   B.Sc.  LOND.,  F.I.C.,  A.R.C.Sc.I. 


PHILADELPHIA 
P.   BLAKISTON'S   SON   &   CO. 

1911. 

[All  rights  reserved.'] 


First  Edition 1863 

1871 
1876 

1882 
1886 
1890 
1896 

1900 

1904 
French  Edition,  translated  by  Dr.  C.  Mehu  (Masson,  Paris),  1885. 


Third       , 

Fourth      , 

Fifth        „      

Sixth        „ 

Seventh  "&  tr'.V  .  . 
Eighth 

<  c  \    <  T    c 
...<.f.{.<^^.<...<(  

'''To-tal  ;  .''. 

f5,750  copies. 

PRINTED   IN  GREAT  BRITAIN  BY  FLETCHER  AND   BON,  LTD.,   NORWICH. 


AUTHOR'S  PREFACE. 


AN  exceptionally  long  interval  of  seven  years  has  elapsed  since 
the  publication  of  the  last  edition  of  this  work,  whilst  it  has 
been  out  of  print  for  nearly  eighteen  months,  a  fact  which  is 
without  precedent  in  its  history.  The  interval  has  not  left  me 
a  younger  man,  and  I  must  confess  that  as  the  time  for  a  new 
edition  approached  I  have  found  myself,  at  the  age  of  four-score 
years,  less  and  less  equal  to  the  task.  So  large  and  so  constant 
is  the  work  now  being  done  in  the  domain  of  volumetric  analysis, 
that  the  need  for  reconsideration  of  old  methods  and  of  selection 
from  amongst  the  newer  methods  becomes  more  imperative  with 
each  succeeding  edition  of  a  book  of  this  character.  The  present, 
moreover,  being  the  tenth  edition,  I  was  particularly  anxious 
that  it  should  be  distinguished  by  the  most  thorough  and 
critical  revision  yet  attempted,  and,  in  the  result,  by  the 
greatest  possible  consonance  with  modern  practice.  To  this  end, 
I  placed  its  preparation  entirely  in  the  hands  of  my  son  and 
partner,  W.  Lincoln e  Sutton,  who  had  accumulated  a  large 
amount  of  material  in  anticipation,  and  of  Mr.  Alfred  E. 
Johnson,  author  of  the  well-known  "Analyst's  Laboratory  Com- 
panion" who  had  rendered  me  valuable  and  acknowledged 
assistance  in  the  course  of  preparing  the  ninth  edition.  I  feel 
that  I  cannot  pay  too  generous  a  tribute  to  the  devotion  of  both 
editors  to  the  task  they  undertook.  It  has  meant  twelve  months 
hard  labour  for  each  of  them  in  the  midst  of  exacting  professional 
duties.  I  wish  to  record  my  gratitude  especially  to  Mr.  Johnson, 
who,  without  the  hereditary  interest  of  his  co-editor,  has  worked 


VI  PREFACE. 

without  reservation  of  time  or  trouble.  He  undertook  the 
laborious  task  of  critically  revising  the  ninth  edition  line  by  line. 
I  must  admit  that  the  result  has  rather  damaged  my  conceit  as 
an  author,  for  it  is  obvious  that  many  possible  improvements  will 
reveal  themselves  under  a  meticulate  examination  in  a  book  which 
has  grown,  as  this  has,  by  a  process  of  accretion  tempered  by 
pruning  and  extended  through  nine  editions  over  a  period  of  forty 
years.  I  am  sensible  that  much  remains  to  be  done,  but  am 
sanguine  enough  to  be  looking  forward  already  to  the  next  and 
Jubilee  edition,  when  the  book  will  have  attained  its  fiftieth  year. 
A  fresh  opportunity  will  then  present  itself  for  moulding  this, 
the  least  perishable  work,  I  suppose,  of  my  life,  nearer  my  desire 
Meanwhile,  1  celebrate  my  jubilee  as  a  Fellow  of  the  Chemical 
Society  with  the  present  edition. 

FRANCIS   SUTTON. 

NORWICH, 

March,  1911. 


EDITORS'    PREFACE. 


IN  preparing  this  edition,  the  ninth  has  been  critically  revised 
line  by  line,  and  the  text  slightly  re-cast,  added  to,  or  considerably 
altered,  as  it  seemed  desirable  in  the  interests  of  clearness,  fulness 
of  treatment,  or  accuracy.  A  good  deal  of  obsolete  matter  has 
been  deleted.  For  the  sake  of  greater  adaptability  for  constant 
use  and  reference  in  the  laboratory,  which  has  always  been  a  valued 
feature  of  "  Button,"  a  new  type  has  been  selected,  all  references 
in  the  text  have  been  carried  to  the  foot  of  the  page,  and  the  page- 
headings  have  been  amplified.  For  the  same  reason,  we  have  en- 
deavoured by  compression  and  deletion  not  to  increase  the  size 
of  the  book,  in  spite  of  the  large  amount  of  new  matter  inserted, 
and  the  present  edition,  in  fact,  does  not  differ  from  the  last  by 
more  than  a  few  pages.  At  the  same  time,  great  care  has  been 
taken  not  to  alter  in  any  way  the  general  scope  and  original  features 
of  the  work. 

The  International  Atomic  Weights  for  1911  have  been  adopted 
throughout,  and  all  factors,  numerical  examples,  etc.,  recalculated 
accordingly.  Special  pains  have  been  taken  to  show  how  the 
numerical  results  of  analyses  have  been  arrived  at. 

In  Parts  I.  and  II.,  the  sections  on  the  pipette,  the  measuring 
flask,  and  weights  and  measures,  have  been  entirely  re- written, 
the  data  used  therein  having  been  kindly  supplied  by  Dr.  R.  J. 
Glazebrook,  F.R.S.,  the  Director  of  the  National  Physical 
Laboratory,  to  whom  we  gratefully  tender  our  sincere  thanks. 


Vlll  PREFACE. 

The  term  "normal  solution"  has  been  denned  in,  perhaps,  a  clearer 
manner,  and  fuller  practical  directions  have  been  given  for  the 
preparation  of  the  principal  normal  solutions  in  common  use. 
The  section  on  Indicators  has  been  somewhat  amplified  and  that 
dealing  with  the  titration  of  mixed  alkali  salts  has  been  partially 
re-cast  and  re-written  for  the  sake  of  greater  clearness.  The  most 
recent  procedure  for  the  technically  complete  analysis  of 
ammoniacal  liquors,  as  published  in  the  Annual  Report  for  1909 
of 'the  Chief  Inspector  under  the  Alkali  etc.  Works  Regulation 
Act,  has  been  substituted  for  that  given  in  the  last  edition.  In 
this  connexion  our  best  thanks  are  due  to  Mr.  Forbes  Carpenter, 
the  Chief  Alkali  Works  Inspector,  and  Mr.  Linder,  his  assistant. 
Many  other  sections  in  these  two  parts  have  been  considerably 
altered,  re-written,  or  added  to. 

The  introductory  portion  of  Part  III.  has  been  re- written  and 
the  subject  matter  considerably  amplified.  Part  IV.  is  short 
(pp.  141-147),  and  has  undergone  but  little  alteration. 

In  Part  V.  (pp.  148-420)  the  alterations  have  been  so  numerous 
and  important  that  attention  can  be  directed  only  to  a  few  of  them. 
The  section  on  ferrocyanides  has  been  entirely  re-written,  much 
expanded,  and  brought  up  to  date  ;  Knecht's  latest  process 
for  the  titration  of  iron  by  titanous  chloride  has  been  described  ; 
the  determination  of  sulphur  in  iron  and  steel  as  carried  out  at 
the  National  Physical  Laboratory  is  included  ;  under  Magnesium, 
Manganese,  Mercury  and  Nickel  several  new  processes  are  given. 
The  section  on  Dissolved  Oxygen  has  been  greatly  enlarged. 
Portions  of  the  section  on  Sugars  have  been  re-written,  and  Mr. 
A.  R.  Ling  has  kindly  contributed  an  account  of  his  most  recent 
methods  of  procedure  in  the  use  of  F  e  h  1  i  n  g '  s  solution .  Important 
new  matter  is  given  under  Titanium,  Vanadium,  and  Zinc.  The 
article  on  Acetone  has  been  entirely  re-written  and  thfc  Govern- 
ment Specification  inserted.  We  are  indebted  to  the  Director 
of  Artillery,  Royal  Gunpowder  Factory,  Waltham  Abbey,  for 
kindly  furnishing  the  latter.  Under  "  Oils,  Fats  and  Waxes  " 


PREFACE. 

numerous  portions  of  the  text  have  been  re-written,  worked 
examples  added,  and  a  description  of  the  Pol  en  ske  method,  with 
figure,  inserted.  Phenols  and  Cresols  have  also  received  attention. 

In  Part  VI.  Liebig's  method  for  determination  of  urea  has  been 
deleted  and  Gerrard's  apparatus  described.  The  Water  and 
Sewage  Section  has  been  most  thoroughly  revised  and  re-arranged, 
with  numerous  additions,  including  a  special  index  to  the  section. 

Beyond  numerical  and  typographical  revision,  little  change  has 
been  made  in  the  Gas  Analysis  section. 

For  the  loan  of  blocks  we  desire  to  thank  Messrs.  Macmillan 
(fig.  57a),  the  publishers  of  Lewkowitsch's  "Oils,  Fats  and 
Waxes" ;  Messrs.  Longmans  (fig.  55),  the  publishers  of  Knecht 
and  Hibbert's  "New  Reduction  Methods  in  Volumetric  Analysis" ; 
and  Messrs.  Churchill  (fig.  59),  publishers  of  Allen's  "Chemistry 

of  Urine." 

W.  L.  S. 

A.  E.  J. 


TABLE  OF  CONTENTS. 


MEMORANDA  

PART    I. 

GENERAL  PRINCIPLES 

The  Balance 

Volumetric  Analysis  without  Weights  .          .          .          * 
Volumetric  Analysis  withoiit  Burettes  .          .          .  -"•'.' 

The  Burette ^ 

The  Pipette 

The  Measuring  Flask  ........ 

On  the  Correct  Reading  of  Graduated  Instruments 

The  Calibration  of  Graduated  Apparatus 

Weights  and  Measures  used  in  Volumetric  Analysis        .          . 

Normal  Solutions \-  •- 

Direct  and  Indirect  Processes  of  Analysis      .          .  . 

PART    II. 

ALKALIMETRY 

Indicators  .          .          .          .          .          ... 

Preparation  of  Normal  Acid  and  Alkali  Solutions 
Adjustment  of  Standard  Solutions        ..... 

Titration  of  Alkali  Salts 

Direct  Determination  of  Sodium  ...          .          . 

Titration  of  Alkaline  Earths  and  their  Compounds 
Ammonia  .......... 

Kjeldahl's  Method 

ACIDIMETRY 

Acetic  Acid         .          .          .          .          .          .          .          .       '  . 

Boric  Acid  and  Borates 

Carbonic  Acid  and  Carbonates     ...... 

Citric  Acid          .          .          .          .          .          .          ... 

Formic  Acid       .          .          .          .          .          .          .          .          ^ 

Hydrofluoric  and  Hydrofluosilicic  Acids         .          .          .          . 

Oxalic  Acid         .......          * 

Phosphoric  Acid          .          .          .          .          .          .          .         . 

Fuming  Sulphuric  Acid 

Tartaric  Acid     ......  . 

PART    III. 

ANALYSIS  BY  OXIDATION  OR  REDUCTION. 
Potassium  Permanganate  and  Ferrous  Salts 
Potassium  Dichromate  and  Ferrous  Salts 
Iodine  and  Sodium  Thiosulphate . 
Decinormal  Iodine  Solution 

Analysis  of  Substances  by  Distillation  with  Hydrochloric  Acid 
Arsenious  Acid  and  Iodine *'. 


TABLE    OF    CONTENTS. 


XI 


PART    IV. 

ANALYSIS  BY  PRECIPITATION 

Decinormal  Silver  Solution  ... 

Indirect  Analyses  by  Decinormal  Silver  and  Potassium  Chromate 

Silver  and  Thiocyanic  Acid  ...... 

Precision  in  Colour  Reactions 


Page. 
141 
141 
143 
145 
146 


PART    V. 

Aluminium          .          .          .          .          .          .          .          .          .          .          .148 

Antimony            ...........  151 

Arsenic       .          .          .          .          .          .          .          .          .          .          .          .  155 

Barium      .          .          .          .          .          .          .          .          .                     .          .  165 

Bismuth    .          .          ...          .          ...          .          .          .166 

Bromine           "' '.  "     '•.. 168 

Cadmium         •    .                    171 

Calcium     •       •..,,, .          .          .  172 

Cerium 173 

Chlorine 175 

Chlorine  Gas  and  Bleach     .          .          .          .'         .          .          .          .          .177 

Chromium           ...........  183 

Cobalt .          .189 

Copper       .         V       .          .          .          .          .          .          .          .      •    .          .191 

Cyanogen        ;  _*"  . '       .          .          .          .          .          .          .          .          .          .  207 

Ferro-  and  Ferri-cyanides    .          .          .          .          .          .          .          .          .216 

Thiocyanates      .          .          .          .          .          .          .          .          .          .          .221 

Gold           .    ' 222 

Iodine        .          .          .          .          ...          .          .          .          .          .  224 

Iron  (Ferrous)    . 231 

Iron  (Ferric) 234 

Iron  Ores,  Iron  and  Steel    .          .                    .          .          .          .          .          .  239 

Lead           . 245 

Magnesium         .    ~   •  .          .          .          .          .          .          .          .          .          .  249 

Manganese          .          .          .          .          .......  250 

Mercury     .          .         .  •  -.  .    .  ..     *         ./•         ......  262 

Nickel        .          .          ,;    ..- -. .--.'  •-.-• 267 

Nitrates  and  Nitrites  .          .          .  .          .          .          .          .          .271 

Nitrites      .          .'  ^ 286 

Dissolved  Oxygen        ..........  290 

Hydrogen  Peroxide     ..........  305 

Sodium  Peroxide         ..........  306 

Phosphoric  Acid  and  Phosphates           .......  307 

Silver         .          . 316 

Sugars        .                    323 

Sulphur,  Sulphides  and  Sulphites          .......  339 

Sulphuretted  Hydrogen       .........  347 

Sulphuric  Acid  and  Sulphates      ........  349 

Persulphates       ...........  354 

Tannic  Acid 355 

Tin 364 

Titanium  .          .....          .          .          .          .          .          .          .  367 

Uranium    .          .          ..."      .* 368 

Vanadium.          .          .          ....          .          .                              .  369 

Zinc           /        .          .              370 

Acetone 333 

Aniline       .          .          .          .          .          .          .          .          .          .          .          .  385 

Azo-Dyes,  Nitro-  and  Nitroso-Compounds     .          .          .          .          .          .  387 

Carbon  Disulphide  and  Thiocarbonates          .                    ....  389 

Formaldehyde    .........                    .  390 

Glycerol 393 


Xll  TABLE    OF   CONTENTS. 

Page. 

Indigo .         .397 

Oils,  Fats  and  Waxes 400 

Phenols  and  Cresols    .          .          .          .          .          .          .          .          .          .  415 

Salicylic  Acid 418 

PART    VI. 

Urine  Analysis   .          .          .          .          .          .          .                   .         .  '"      .  421 

Water  and  Sewage  Analysis          .          .          .          .          ...  ,j     .  437 

Special  Index  to  Processes  of  Water  and  Sewage  Analysis      .          .          .  444 

PART    VII. 

VOLUMETRIC  ANALYSIS  OF  GASES 505 

Gases  Determined  Directly  .          .          .          .          .          .          ..518 

Gases  Determined  Indirectly         .          .          .':.'.          .          ...  526 

Improved  Gas  Apparatus    .          .          .     "  - ,          .          .          .          .          .  540 

Simpler  Methods  of  Gas  Analysis          .          .          .          ..         .          .          .  568 

The  Nitrometer  and  Gas- Volumeter      .          .          .          .    '.     .'        .          .  582 

ADDENDA  AND  CORRIGENDA    .       '.         .         .         .         .     '  ...        .         .611 

INDEX  ...  .         .  .  .614 

LIST    OF    TABLES. 

International  Atomic  Weights,  1911,  abridged            .          .          ....         .  xiv 

Correction  of  Volumes  of  Liquids  for  Temperature    .          .          ,     '-;.  .•        .  26 
List  of  Normal  Solutions         .          .          .          .          .          .          .    -      .          .30 

Equivalents  to  1  c.c.  Normal  Potash  in  Butyric  Acid,  etc.       •-'.'.        .          .  39 

Glaser' s  Classification  of  Indicators.     .          .          .          .          .          .          .  45 

Strength  of  Sulphuric  Acid  by  Sp.  Gr .          .  51 

Normal  Factors  for  Alkalies,  Alkaline  Earths  and  Acids    ....  59 

Determination  of  Organic  Salts  of  Alkalies  by  Titration  of  residue  left 

after  ignition  ...........  68 

The  amount  of  Ammonium  Sulphate  produced  by  Gas  Liquor  ...  79 

Factors  for  use  with  Decinormal  Potassium  Permanganate          .          . .        .  126 

Determination  of  Sugars  by  Fehling-Pavy  Method        .          .          .          .  336 

Tannin  in  various  Substances           .          .          .          .          ...          .  358 

Saponification  or  Kottstorfer  Value  of  Oils  and  Fats  .         .....  403 

See  also  under  Addenda  and  Corrigenda          .         .         .--.          .          .  611 

Typical  Analyses  of  Water  and  Sewage    .          .        ..          .  "       .         .          .  474 

Analyses  of  Sewage  Effluents           .          .          ...        ..."        .          .  496 

Density  and  Volume  of  Mercury  and  of  Water          ....         .          .  517 

Tables  for  Water  Analysis       .          .          .          .         %          .         ,          .          .  594 

Tables  for  Gas  Analysis .          ...  601 

TABLE  OF  FACTORS  AND  THEIR  LOGARITHMS  FOR  VOLUMETRIC  ANALYSIS  .  608 


xin 

MEMORANDA. 


WEIGHTS   AND    MEASURES. 

(See  p.  23.) 
1  metre  39-370113  inches. 

3-280843  feet. 

1  gram  (gm.)  15-43236  grains  (grs.). 

1  kilogram  2-2046  Ib.  avoirdupois. 

The  standard  litre  is  the  volume  of  a  kilogram  of  pure  water 
at  4°  C.  under  standard  barometric  pressure. 

1  litre  1000-028  cubic  centimetres. 

1-75980  pints. 
0-219975  gallon. 
1  litre  of  water  at  15°  C.  weighs  999-13  grams. 

TEMPERATURE. 

The  usual  standard  temperature  for  graduated  vessels  and 
standard  solutions  used  in  volumetric  analysis  is  15°  C.  (  =  59°  F.). 
For  special  purposes,  however,  other  temperatures  are  also  in 
common  use  (see  p.  25). 

NORMAL    SOLUTIONS. 

A  normal  solution  of  a  reagent  is  one  that  contains  in  a  litre 
that  proportion  of  its  molecular  weight  in  grams  which  corresponds 
to  one  gram  of  available  hydrogen  or  its  equivalent  (see  p.  28). 
Decinormal  (N/10)  and  centinormal  (N/10o)  solutions  are  respectively 
of  one-tenth  and  one-hundredth  of  this  strength. 

By  means  of  a  standard  solution,  a  given  volume  of  which  has 
been  proved  to  be  equivalent  to  a  known  amount  of  a  certain 
substance,  the  quantity  of  such  substance  contained  in  a  given 
quantity  of  another  solid  or  liquid  body  can  be  determined.  This 
process  is  termed  titration. 

ABBREVIATIONS    USED. 

Ber.  =   Berichte  der  deutschen  chemischen  Gesellschaft. 

C.  N.  =   Chemical  News. 

J.  A.  C.  S.  =  Journal  of  the  American  Chemical  Society. 

J*  C.  S.  =   Journal  of  the  Chemical  Society. 

J.  S.  C.  I.  =   Journal  of  the  Society  of  Chemical  Industry. 

Z.  a.  C.  =   Zeitschrift  fur  analytische  Chemie. 

Z.f.angew.C.  —  ,,  ,,    angewandte       ,, 


XIV 


ABRIDGED    LIST   OF   THE 

INTERNATIONAL    ATOMIC    WEIGHTS    FOR    1911 
(used  throughout  this  work). 


0=16. 

Aluminium Al          27'1 

Antimony    Sb  120-2 

Arsenic As         74-96 

Barium Ba  137-37 

Bismuth Bi  208 

Boron B  11 

Bromine Br         79-92 

Cadmium Cd  112-4 

Calcium Ca         40-09 

Carbon C  12 

Cerium Ce  140-25 

Chlorine Cl          35-46 

Chromium Cr          52 

Cobalt Co         58-97 

Copper Cu         63-57 

Fluorine F  19 

Gold Au  197-2 

Hydrogen H  1-008 

Iodine I  126-92 

Iron Fe         55-85 

Lead Pb  207-1 

Lithium.  ..Li  6-94 


Magnesium Mg  24-32 

Manganese Mn  54-93 

Mercury Hg  200 

Molybdenum Mo  96 

Nickel Ni  58-68 

Nitrogen N  14-01 

Oxygen O  16 

Palladium Pd  106-7 

Phosphorus P  31-04 

Platinum Pt  195-2 

Potassium K  39-1 

Silicon Si  28-3 

Silver Ag  107-88 

Sodium Na  23 

Strontium Sr  87-63 

Sulphur S  32-07 

Tin Sn  119 

Titanium Ti  48-1 

Tungsten W  184 

Uranium U  238-5 

Vanadium V  51*06 

Zinc..  ..Zn  65-37 


VOLUMETRIC  ANALYSIS 


OF 


LIQUIDS  AND  SOLIDS. 


PART    I. 
GENERAL    PRINCIPLES. 

QUANTITATIVE  analysis  by  weight,  or  gravimetric  analysis, 
consists  in  separating  out  the  constituents  of  any  compound,  either 
in  a  pure  state  or  in  the  form  of  some  new  substance  of  known  com- 
position, and  accurately  weighing  the  products.  Such  operations 
are  frequently  very  complicated,  and  occupy  a  long  time,  besides 
requiring  in  many  cases  elaborate  apparatus,  and  the  exercise  of 
much  care  and  experimental  knowledge.  Volumetric  processes  on 
the  other  hand,  are,  as  a  rule,  quickly  performed  ;  in  most  cases  are 
susceptible  of  extreme  accuracy,  and  need  much  simpler  apparatus. 
The  leading  principle  of  the  method  consists  in  submitting  the 
substance  to  be  determined  to  certain  characteristic  reactions, 
employing  for  such  reactions  solutions  of  known  strength,  and 
from  the  volume  of  solution  necessary  for  the  production  of  such 
reaction  calculating  the  weight  of  the  substance  to  be  determined 
by  aid  of  the  known  laws  of  chemical  equivalence. 

Volumetric  analysis,  or  quantitative  chemical  analysis  by  measure, 
in  the  case  of  liquids  and  solids,  consequently  depends  upon  the 
following  conditions  for  its  successful  practice  : — 

1.  A  solution  of  the  reagent,  the  chemical  value  of  which  is 
accurately  known,  called  the  "  standard  solution." 

2.  A   graduated   vessel   from   which   portions    of    it   may   be 
accurately  delivered,  called  the  "  burette." 

3.  The  decomposition  produced  by  the  standard  solution  with 
any  given  substance  must  either  in  itself  or  by  an  indicator  be  such, 
that  its  termination  is  unmistakable,  to  the  eye,  and  thereby  the 


2  GENERAL   PRINCIPLES. 

quantity  of  the  substance  with  which  it  has  combined  accurately 
calculated. 

Suppose,  for  instance,  that  it  is  desired  to  know  the  quantity  of 
pure  silver  contained  in  a  shilling.  The  coin  is  first  dissolved  in 
nitric  acid,  by  which  means  a  bluish  solution,  containing  silver, 
copper,  and  probably  other  metals,  is  obtained.  It  is  a  known  fact 
that  chlorine  combines  with  silver  in  the  presence  of  other  metals 
to  form  silver  chloride,  which  is  insoluble  in  nitric  acid.  The 
proportions  in  which  the  combination  takes  place  are  35' 46  of 
chlorine  to  every  107' 88  of  silver  ;  consequently,  if  a  standard 
solution  of  pure  sodium  chloride  is  prepared  by  dissolving  in  water 
such  a  weight  of  the  salt  as  will  be  equivalent  to  35' 46  grains  of 
chlorine  (  =  58' 46  grains  NaCl)  and  diluting  to  the  measure  of 
1000  grains,  every  single  grain  measure  of  this  solution  will  com- 
bine with  0' 10788  grain  of  pure  silver  to  form  silver  chloride,  which 
is  precipitated  to  the  bottom  of  the  vessel  in  which  the  mixture  is 
made.  In  the  process  of  adding  the  salt  solution  to  the  silver, 
drop  by  drop,  a  point  is  at  last  reached  when  the  precipitate  ceases 
to  form.  Here  the  process  must  stop.  On  looking  carefully  at 
the  graduated  vessel  from  which  the  standard  solution  has  been 
used,  the  operator  sees  at  once  the  number  of  grain  measures  which 
has  been  necessary  to  produce  complete  decomposition.  For 
example,  suppose  the  quantity  used  was  520  grain  measures  ;  all 
that  is  necessary  to  be  done  is  to  multiply  520  by  the  coefficient  for 
each  grain  measure,  viz.  0' 10788,  which  shows  the  amount  of  pure 
silver  present  to  be  56*098  grains. 

This  method  of  determining  the  quantity  of  silver  in  any  given 
solution  occupies  scarcely  a  quarter  of  an  hour,  whereas  the 
determination  by  weighing  could  not  be  done  in  half  a  day,  and 
even  then  not  so  accurately  as  by  the  volumetric  method.  It  must 
be  understood  that  there  are  certain  necessary  precautions  in  con- 
ducting the  above  process  which  have  not  been  described  ;  those 
will  be  found  in  their  proper  place  ;  but  from  this  example  it  will  at 
once  be  seen  that  the  saving  of  time  and  trouble,  as  compared  with 
the  older  methods  of  analysis,  is  immense  ;  besides  which,  in  the 
majority  of  instances  in  which  it  can  be  applied,  it  is  equally 
accurate,  and  in  many  cases  much  more  so. 

The  only  conditions  on  which  the  volumetric  system  of  analysis 
can  be  carried  on  successfully  are  that  great  care  is  taken  with 
respect  to  the  graduation  of  the  measuring  instruments,  and  their 
agreement  with  each  other,  the  strength  and  purity  of  the  standard 
solutions,  and  the  absence  of  other  matters  which  would  interfere 
with  the  accurate  determination  of  the  particular  substance  sought. 

The  fundamental  distinction  between  gravimetric  and  volumetric 
analysis  is  that,  in  the  former  method,  the  substance  to  be 
determined  must  be  completely  isolated  in  the  purest  possible  state 
or  combination,  necessitating  in  many  instances  very  patient  and 
discriminating  labour ;  whereas,  ^in  volumetric  processes,  such 
complete  separation  is  very  seldom  required,  the  processes  being  so 


CLASSIFICATION    OF   METHODS.  .5 

contrived  as  to  admit  of  the  presence  of  half  a  dozen  or  more 
other  substances  which  have  no  effect  upon  the  particular  chemical 
reaction  required. 

The  process  just  described  for  instance,  the  determination  of 
silver  in  coin,  is  a  case  in  point.  The  alloy  consists  of  silver  and 
copper,  with  small  proportions  of  lead,  antimony,  tin,  gold,  etc. 
None  of  these  metals  affect  the  amount  of  salt  solution  which  is 
chemically  required  to  precipitate  the  silver,  whereas,  if  the  metal 
had  to  be  determined  by  weight  it  would  be  necessary  first  to  filter 
the  nitric  acid  solution  to  free  it  from  insoluble  tin,  gold,  etc.  ;  then 
precipitate  with  a  slight  excess  of  sodium  chloride  ;  then  to  bring 
the  precipitate  upon  a  filter,  and  wash  repeatedly  with  pure  water 
until  every  trace  of  copper,  salt,  etc.,  is  removed.  The  pure  silver 
chloride  is  then  carefully  dried,  ignited  separately  from  the  filter, 
and  weighed  ;  the  filter  burnt,  the  residue  as  reduced  metallic 
silver  and  filter  ash  allowed  for,  and  thus  finally  the  amount  of 
silver  is  found  by  the  balance  with  ordinary  weights. 

On  the  other  hand  the  volumetric  process  has  been  purely 
chemical,  the  burette  or  measuring  instrument  has  taken  the  place 
of  the  balance,  and  theoretical  or  atomic  weights  have  supplanted 
ordinary  'weights. 

The  end  of  the  operation  in  this  method  of  analysis  is  in  all  cases 
made  apparent  to  the  eye.  In  alkalimetry  it  is  the  change  of  colour 
produced  in  litmus,  turmeric,  or  other  sensitive  colouring  matter. 
The  formation  of  a  permanent  precipitate,  as  in  the  determination  of 
cyanogen.  A  precipitate  ceasing  to  form,  as  in  chlorine  and  silver 
determination.  The  appearance  of  a  distinct  colour,  as  in  iron 
analysis  by  permanganate  solution,  and  so  on. 

I  have  adopted  the  classification  of  methods  used  by  Mohr  and 
others,  namely  : 

1.  Where  the  determination  of  the  substance  is  effected  by 
saturation  with  another  substance  of  opposite  properties — generally 
understood  to  include  acids  and  alkalies  and  alkaline  earths. 

2.  Where   the   determination   of   a   substance   is    effected   by 
a  reducing  or  oxidizing  agent  of  known  power,  including  most 
metals,  with  their  oxides  and  salts.     The  principal  oxidizing  agents^- 
are  potassium  permanganate,  potassium  dichromate,  and  iodine  ; 
and    the    corresponding   reducing    agents,    ferrous    and    stannous 
compounds,  and  sodium  thiosulphate. 

3.  Where  the  determination  of  a  substance  is  effected  by  pre- 
cipitating it  in  some  insoluble  and  definite  combination,  an  example 
of  which  occurs  in  the  determination  of  silver  described  above. 

This  classification  does  not  completely  include  all  the  volumetric 
processes  that  may  be  used,  but  it  divides  them  into  convenient 
sections  for  describing  the  peculiarity  of  the  reagents  used,  and 
their  preparation.  If  strictly  followed  out,  it  would  in  some  cases 
necessitate  the  registration  of  the  body  to  be  determined  under 
two  or  three  heads.  Copper,  for  instance,  can  be  determined 
residually  by  permanganate ;  it  can  also  be  determined  by 

B  2 


4  CHOICE    OF   METHODS. 

precipitation  with  sodium  sulphide.  The  determination  of  the 
same  metal  by  potassium  cyanide,  on  the  other  hand,  would  not 
come  under  any  of  the  above  heads. 

It  will  be  found,  therefore,  that  liberties  have  been  taken  with  the 
arrangement ;  and  for  convenient  reference  all  analytical  processes 
applicable  to  a  given  body  are  included  under  its  name. 

It  may  be  a  matter  of  surprise  to  some  that  several  distinct 
volumetric  methods  for  one  and  the  same  substance  are  given  ;  but 
a  little  consideration  will  show  that  in  many  instances  greater 
convenience,  and  also  accuracy,  may  be  gained  in  this  way.  The 
operator  may  not  have  one  particular  reagent  at  command,  or  he 
may  have  to  deal  with  such  a  mixture  of  substances  as  to  preclude 
the  use  of  some  one  method ;  whereas  another  method  may  be 
quite  free  from  such  objection.  The  choice  in  such  cases  of  course 
requires  judgment,  and  it  is  of  the  greatest  importance  that  the 
operator  should  be  acquainted  with  the  qualitative  composition  of 
the  matters  with  which  he  is  dealing,  and  that  he  should  ask  himself 
at  every  step  why  such  and  such  a  thing  is  to  be  done. 

It  will  be  apparent  from  the  foregoing  description  of  the  volu- 
metric system  that  it  may  be  successfully  used  in  many  instances 
by  those  who  have  never  been  thoroughly  trained  as  analytical 
chemists  ;  but  we  can  never  look  for  the  scientific  development  of 
the  system  in  such  hands  as  these. 

In  the  preparation  of  this  work  an  endeavour  has  been  made  to 
describe  all  the  operations  and  chemical  reactions  as  simply  as 
possible,  all  the  necessary  calculations  being  made  as  far  as  possible 
without  the  aid  of  long  mathematical  formulae  and  requiring 
usually  nothing  further  than  a  knowledge  of  the  ordinary  rules  of 
arithmetic  and  occasionally  of  elementary  algebra. 


APPARATUS. 


THE    INSTRUMENTS    AND    APPARATUS. 
THE    BALANCE. 

STRICTLY  speaking,  it  is  necessary  to  have  two  balances  in  order 
to  carry  out  completely  the  volumetric  system.  One  to  carry  about 
a  kilogram  in  each  pan  and  to  turn  when  fully  loaded  with  about 
5  milligrams  ;  the  other  to  carry  about  50  grams  and  to  turn  easily 
and  quickly,  when  fully  loaded,  with  one-  or  two-tenths  of  a  milli- 
gram. The  former  instrument  is  used  for  weighing  large  amounts 
of  pure  reagents  in  the  preparation  of  standard  solutions,  and  for 
making  the  necessary  weighings  when  graduating  or  testing 
measuring  flasks.  The  latter  instrument,  which  must  be  of  much 
lighter  construction,  serves  for  weighing  smaU  quantities  of 
substances  to  be  tested,  many  of  which,  being  hygroscopic,  need 
weighing  quickly  as  well  as  accurately,  also  for  the  delicate  weighings 
required  when  testing  the  accuracy  of  pipettes  and  burettes. 

For  all  technical  purposes,  however,  a  moderate-sized  balance  of 
medium  delicacy  is  quite  sufficient,  especiaUy  if  rather  large 
quantities  of  substances  are  weighed  and  brought  into  solution — 
then  further  subdivided  by  means  of  measuring  flasks  and  pipettes. 

The  operator  also  requires,  besides  the  balance  and  graduated 
instruments,  a  few  beakers,  porcelain  basins,  flasks,  funnels,  stirring 
rods,  etc.,  as  in  gravimetric  analysis.  Above  all,  he  must  be 
practically  familiar  with  proper  methods  of  filtration,  washing  of 
precipitates,  and  the  application  of  heat. 

VOLUMETRIC    ANALYSIS    WITHOUT    WEIGHTS. 

THIS  is  more  a  matter  of  curiosity  than  of  value  ;  but,  neverthe- 
less, one  can  imagine  circumstances  in  which  it  might  be  useful. 
In  carrying  it  out,  it  is  necessary  only  to  have  (1)  a  correct  balance, 
(2)  a  pure  specimen  of  substance  to  use  as  a  weight,  (3)  an  accurate 
burette  filled  with  the  appropriate  solution.  It  is  not  necessary 
that  the  strength  of  this  should  be  known ;  but  the  state  of 
concentration  should  be  such  as  to  permit  the  necessary  reaction  to 
occur  under  the  most  favourable  circumstances. 

If  a  perfectly  pure  specimen  of  substance,  say  calcium  carbonate, 
be  put  into  one  scale  of  the  balance,  and  be  counterpoised  with  an 
impure  specimen  of  the  same  substance,  and  both  titrated  with  the 
same  acid,  and  the  number  of  c.c.  used  for  the  pure  substance  be 
called  100,  the  number  of  c.c.  used  for  the  impure  substance  will 
correspond  to  the  percentage  of  pure  calcium  carbonate  in  the 
specimen  examined. 

The  application  of  the  process  is,  of  course,  limited  to  the  use  of 
such  substances  as  are  to  be  had  pure,  and  whose  weight  is  not 
variable  by  exposure  ;  but  where  even  a  pure  substance  of  one  kind 


6  VOLUMETRIC   ANALYSIS    WITHOUT   WEIGHTS 

cannot  be  had  as  a  weight,  one  of  another  kind  may  be  used  as 
a  substitute,  and  the  required  result  obtained  by  calculation.  For 
instance,  it  is  required  to  ascertain  the  purity  of  a  specimen  of 
sodium  carbonate,  and  only  pure  calcium  carbonate  is  at  hand  to 
use  as  a  weight ;  equal  weights  of  the  two  are  taken,  and  the  impure 
specimen  titrated  with  acid.  To  arrive  at  the  required  result,  it  is 
necessary  to  find  a  coefficient  or  factor  by  which  to  convert  the 
number  of  c.c.  required  by  the  sodium  carbonate,  weighed  on  the 
calcium,  into  that  which  should  be  required  if  weighed  on  the 
sodium,  basis.  A  consideration  of  the  relative  molecular  weights 
of  the  two  bodies  will  give  the  factor  thus — 

Calcium  carbonate  100 '09 


Sodium  carbonate      106-' 


=0-9443 


If,  therefore,  the  c.c.  used  are  multiplied  by  this  number,  the 
percentage  of  pure  sodium  carbonate  will  be  obtained.  On  this 
principle,  and  with  the  exercise  of  a  little  ingenuity,  the  analysis 
of  a  number  of  substances  may  be  carried  out. 

L.  de  Koningh  has  communicated  to  me  a  similar  method 
devised  by  himself  and  Peacock,  in  which  the  same  end  is  attained 
without  the  aid  of  a  pure  substance  as  standard,  thus  :  Say 
a  specimen  of  impure  common  salt  is  to  be  examined.  A  moderate 
portion  is  put  on  the  balance  and  counterpoised  with  silver  nitrate  ; 
the  latter  is  then  dissolved  in  water,  made  up  to  100  c.c.  and  placed 
in  a  burette.  The  salt  is  dissolved  in  water,  a  few  drops  of  potassium 
chromate  added  and  titrated  with  the  silver  solution,  of  which  10  c.c. 
is  required  ;  the  salt  is  therefore  equal  to  10  per  cent,  of  its  weight 
of  silver  nitrate,  then — 

16-99  :  58-46  :  :  10  =  3'44  %  NaCl 

Or,  in  the  case  of  an  impure  soda  ash,  an  equal  weight  of  oxalic 
acid  is  taken  and  made  up  to  100  c.c.  ;  the  soda  requires,  say,  50  c.c. 
for  saturation,  or  50  per  cent.,  then— 

126  :  106  :  :  50-42  %  Na2CO3 

It  may  happen  that,  in  some  cases,  more  than  one  portion  of  the 
reagent  is  required  to  decompose  the  substance  titrated,  and  to 
provide  against  this  two  or  more  lots  should  be  weighed  in  the  first 
instance. 

VOLUMETRIC    ANALYSIS    WITHOUT    BURETTES    OR    OTHER 
GRADUATED    INSTRUMENTS. 

THIS  operation  consists  in  weighing  the  standard  solutions  on 
the  balance  instead  of  measuring  them.  The  influence  of  variation 
in  temperature  is,  of  course,  here  of  no  consequence.  The  chief 
requisite  is  a  delicate  flask,  fitted  with  a  tube  and  blowing  ball,  as 
in  the  burette  fig.  7,  or  an  instrument  known  as  Schuster's 
alkalimeter  may  be  used.  A  special  burette  has  been  devised  for 


AND    WITHOUT   MEASURES. 


this  purpose  by  Gas  am  a  j  or*.  The  method  is  capable  of  very 
accurate  results,  if  care  be  taken  in  preparing  the  standard  solutions 
and  avoiding  any  loss  in  pouring  the  liquid  from  the  vessel  in  which 
it  is  weighed.  It  occupies  much  more  time  than  the  usual  processes 
of  volumetric  analysis,  but  at  great  extremes  of  temperature  it  is 
far  more  accurate. 

THE    BURETTE. 

THIS  instrument  is  used  for  the  delivery  of  an  accurately  measured 
quantity  of  any  particular  standard  solution.  It  invariably 
consists  of  a  long  glass  tube  of  even  bore,  throughout  the  length  of 
which  are  engraved,  by  means  of  hydrofluoric  acid,  certain  divisions 
corresponding  to  a  known  volume  of  fluid. 


Fig.  1.  Fig.  2. 

It  may  be  obtained  in  a  great  many  forms,  under  the  names  of 

*  c.  N.  86,  98. 


s 


THE    BURETTE. 


their  respective  inventors,  such  as  Mphr,  GayLussac,  Bink,  etc., 
but  as  some  of  these  possess  a  decided  superiority  over  others  it  is 
not  quite  a  matter  of  indifference  which  is  used,  and  therefore 
a  slight  description  of  them  may  not  be  out  of  place  here.  The 
burette,  with  india-rubber  tube  and  clip,  contrived  by  Mohr,  is 
shown  in  figs.  1  and  2,  and,  with  glass  stop-cock,  in  fig  3.  This 
latter  form  of  instrument  is  now  made  and  sold  at  such  a  moderate 
price  that  it  has  largely  displaced  the  original  form  designed  by 
Mohr. 

A  further  improvement  in  modern  graduated  instruments  applied 
to  burettes,  thermometers,  etc.,  is  a  strip  of  milk  glass  in  the  tube, 
behind  the  graduation  marks  and  figures,  which  are  filled  with 
black  varnish  to  render  them  conspicuous. 


Fig.  3.  Fig.  4. 

The  advantages  possessed  by  Mohr's  burette  are  as  follows: 
Its  fixed  upright  position  in  a  stand  enables  the  operator  at  once 
to  read  off  the  volume  of  a  standard  solution  used  ;  the  quantity 
of  liquid  to  be  delivered  can  be  regulated  to  the  greatest  nicety  ; 
and,  not  being  touched  by  the  hand,  the  volume  of  the  liquid  cannot 
be  increased  by  the  heat  of  the  body,  as  is  often  the  case  with 


THE    BURETTE. 


9 


Bink^'sorGayLussac's  burette.  The  principal  disadvantage  of 
these  two  latter  forms,  however,  is  that  a  correct  reading  in  each 
case  can  only  be  obtained  by  placing  the  instrument  in  an  upright 
position  and  allowing  the  fluid  to  find  its  proper  level.  The 
preference,  therefore,  should  unhesitatingly  be  given  to  Mohr's 
burette.  The  tap  burette  may  be  used  not  only  for  solutions 
affected  by  the  rubber  tube,  but  for  all  other  solutions,  and  may 
also  be  arranged  so  as  to  deh'ver  the  liquid  in  drops,  leaving  both 
the  hands  of  the  operator  disengaged.  A  new  arrangement  is  shown 
in  fig.  4,  the  tap  being  placed  obliquely  through  the  jet,  so  as  to 
avoid  its  dropping  out  of  place  ;  the  floats  shown  are  very  small 


thermometers.  Owing  to  the  action  of  caustic  alkalies  upon  glass, 
tap  burettes  do  not  answer  well  for  strong  solutions  of  potash  or 
soda,  unless  emptied  and  washed  immediately  after  use.  A  very 
good  modification  of  this  burette,  as  usually  made,  is  to  have  the 
top  funnel-shaped.  This  not  only  admits  of  more  easy  filling,  but 
enables  the  burette  to  be  hung  on  a  stand  by  the  funnel  without 
other  support  and  also  to  be  tilted  from  the  vertical  when  titrating 


10 


THE   BURETTE. 


hot  solutions.  When  not  in  use,  the  dust  may  be  kept  out  of  such 
a  burette  by  a  greased  glass  plate.  Ordinary  burettes  should  be 
covered  with  an  inverted  test-tube  when  not  in  use.  Two 
convenient  forms  of  stand  for  Mohr's  burettes  are  shown  in  figs.  5 
and  6.  In  the  former  the  arms  carrying  the  burettes  revolve. 

Special  care  should  always  be  taken  with  Mohr's  form  of  burette 
to  fill  the  delivery  point  of  the  instrument  and  the  intervening 
rubber  tube  with  the  liquid,  before  commencing  a  titration.  This 
is  easily  done  by  filling  the  burette  well  above  the  0  mark,  then 
rapidly  opening  the  clip  wide  to  expel  the  air  bubbles.  When  this 
is  done,  the  excess  of  liquid  may  be  quietly  run  out  to  the  mark. 
In  the  tap  burette  the  air  space  is  smaller  than  with  the  rubber 
tube,  but  the  same  method  should  be  invariably  adopted.  Glass 
taps  should  be  occasionally  smeared  with  a  small  quantity  of  vaseline 
as  lubricant.  A  thin  ring  of  india-rubber  tubing  stretched  over  the 
projecting  narrow  end  of  the  tap  is  useful  for  keeping  it  in  position. 
We  are  indebted  to  Mohr  for  another  form  of  instrument  to 
avoid  the  contact  of  permanganate  and  india-rubber,  viz.,  the  foot 
burette,  with  elastic  ball,  shown  in  fig.  7. 

The  flow  of  liquid  from  the  exit  tube  can  be  regulated  to  a  great 
nicety  by  pressure  upon  the  ball,  which  should  be  large,  and  have 
two  openings, — one  cemented  to  the  tube  with  marine  glue,  and 
the  other  at  the  side,  over  which  the  thumb  is  placed  when  pressed, 
and  on  the  removal  of  which  it  refills  with  air. 

Gay  Lussac's  burette,  sup- 
ported in  a  wooden  foot,  may  be 
used  instead  of  the  above  form 
by  inserting  into  the  open  end  a 
good  fitting  cork,  through  which 
a  small  tube  bent  at  right- angles 
is  passed.  If  the  burette  is 
held  in  the  right  hand,  slightly 
inclined  towards  the  beaker  or 
flask  into  which  the  fluid  is  to  be 
measured,  and  the  mouth  applied 
to  the  tube,  any  portion  of  the 
solution  may  be  emptied  out  by 
the  pressure  of  the  breath,  and 
the  disadvantage  of  holding 
the  instrument  in  a  horizontal 
position,  to  the  great  danger  of 
spilling  the  contents,  is  avoided  ; 
at  the  same  time  the  beaker  or 
flask  can  be  held  in  the  left 
hand  and  shaken  so  as  to  mix 
the  fluids,  and  by  this  means 
the  end  of  the  operation  be 
more  accurately  determined  (see 
fig.  8). 


Fig.  7. 


Fig.  8. 


THE   BURETTE. 


11 


There  is  an  arrangement  of  Mohr's  burette  which  is  ex- 
tremely serviceable  when  a  series  of  titrations  of  the  same  character 
have  to  be  made,  such  as  in  alkali  works,  assay  offices,  etc.  It 
consists  in  having  a  T  piece  of  glass  tube  inserted  between  the  lower 
end  of  the  burette  and  the  spring  clip,  communicating  with  a  reservoir 
of  the  standard  solution  placed  above,  so  that  the  burette  may  be 
filled  by  a  siphon  as  often  as  emptied,  and  in  so  gradual  a  manner 
that  no  air  bubbles  are  formed,  as  when  filling  it  with  a  funnel  or 
pouring  in  liquid  from  a  bottle.  This  arrangement  has  the 
additional  advantage  of  preventing  evaporation  of  the  standard 
solution  either  in  the  burette  or  the  reservoir,  and  also  keeps  out 
dust. 

Figs.  9  and  11  show  this  arrangement  in  detail.  Connections  of 
this  kind  may  now  be  had  with  glass  stop-cocks,  either  of  the  simple 
form  or  the  patent  two-way  cock,  made  by  Greiner  and 
Friedrichs,  and  supplied  by  most  apparatus  dealers  (fig.  10). 


Fig.  9. 


Fig.  10. 


It  sometimes  happens  that  a  solution  requires  titration  at  a  hot  or 
even  boiling  temperature,  such  as  the  determination  of  sugar  by 


12 


GAY   LUSSAC'S   BURETTE. 


copper  solution:  here  the  ordinary  arrangement  of  Moh'r's  burette 
will  not  be  available,  since  the  steam  rising  from  the  liquid  heats 
the  burette  and  alters  the  volume  of  fluid.  This  may  be  avoided 
either  by  using  a  special  burette,  in  which  the  lower  end  is  extended 
at  a  right-angle  with  a  stop-cock,  or  by  attaching  to  an  ordinary 
burette  a  much  longer  piece  of  india-rubber  tube,  so  that  the  burette 
stands  at  the  side  of  the  capsule  or  beaker  being  heated,  and  the 
elastic  tube  is  brought  over  its  edge,  the  pinch-cock  being  fixed 
midway.  No  heat  can  then  reach  the  body  of  fluid  in  the  burette, 
since  there  can  be  no  conduction  past  the  pinch-cock.  A  burette 


Fig.  11.  Fig.  12. 

with  funnel  neck  as  described  on  page  9  may  also  be  used  for  this 
purpose. 

Gay  Lussac's  burette  is  shown  in  figs.  8  and  12.     By  using  it  in 


PINCH-COCKS. 


13 


the  following  manner  its  inherent  disadvantages  may  be  overcome  to 
a  great  extent.  Having  fixed  the  burette  into  the  foot  securely, 
and  filled  it,  take  it  up  by  the  foot,  and  resting  the  upper  end  upon 
the  edge  of  the  beaker  containing  the  solution  to  be  titrated  drop 
the  test  fluid  from  the  burette,  meanwhile  stirring  the  contents  of 
the  beaker  with  a  glass  rod ;  by  a  slight  elevation  or  depression  the 
flow  of  test  liquid  is  regulated  until  the  end  of  the  operation  is 
secured.  In  this  way  the  annoyances  which  arise  from  alternately 
placing  the  instrument  in  an  upright  and  a  horizontal  position  are 
avoided. 

Bink's  burette  is  well  known,  and  need  not  be  described  ;  it  is 
the  least  recommendable  of  all  forms,  except  for  very  rough 
estimations. 

It  is  convenient  to  have  burettes  graduated  to  contain  from  30  to 
50  c.c.  in  TV  c.c.  and  100  or  110  c.c.  in  ^  or  J  c.c. 

The  pinch-cock  generally  used  in  Mohr's  burette  is  shown  in 
fig.  1.  These  are  made  of  brass  and  are  now  generally  nickel-plated 
to  prevent  corrosion.  Another  form  is  made  of  one  piece  of  steel 
wire,  as  devised  by  Hart;  the  wire  is  softened  by  heating,  and 
coiled  round  as  shown  in  fig.  13.  When  the  proper  shape  has  been 
attained,  the  clip  is  hardened  and  tempered  so  as  to  convert  it 
into  a  spring. 

Another  pinch-cock  is  shown  in  fig.  13.  It  may  be  made  of  hard 
wood,  horn,  or,  preferably,  of  flat  glass  rod.  The  levers  should  be 
long.  A  small  piece  of  cork,  of  the  same  thickness  as  the  elastic 
tube  of  the  burette  when  pressed  close,  should  be  fastened  at  the 
angles  of  the  levers  as  shown  in  the  engraving. " 


Fig.^3. 

The  use  of  any  kind  of  pinch-cock  may  be  avoided,  and  a  very 
delicate  action  obtained,  by  simply  inserting  a  not  too  tightly  fitting 
piece  of  solid  glass  rod  into  the  elastic  tube  between  the  end  of  the 


14 


THE    PIPETTE. 


burette  and  the  jet.  A  firm  squeeze  being  given  by  the  finger  and 
thumb  to  the  elastic  tube  surrounding  the  rod,  a  small  canal  is 
opened,  and  thus  the  liquid  escapes,  and  of  course  can  be  controlled 
by  the  operator  at  will  (see  fig.  14). 

THE    PIPETTE. 

The  pipettes  used  in  volumetric  analysis  are  of  two  kinds  : 
(1)  whole  pipettes,  which  have  but  one  mark  and  deliver  a  fixed 
quantity  marked  on  the  measure  ;  (2)  graduated  pipettes,  the  stems 


50CC 


10  CC 


Fig.  14. 


Fig.  15. 


Fig.  16. 


of  which  are  graduated  to  deliver  various  quantities  at  the  discretion 
of  the  analyst.  In  using  the  former,  they  are  first  filled  from  the 
jet  to  about  1  cm.  above  the  mark,  then  allowed  to  run  down  just 
to  the  mark.  Any  drops  adhering  to  the  jet  are  removed.  The 
liquid  is  then  allowed  to  run  out  into  the  vessel  where  it  is  required, 


THE    GRADUATED    FLASK.  15 

the  point  of  the  jet  touching  the  wall  of  the  vessel.  After  the 
continuous  outflow  has  ceased,  the  pipette  is  allowed  to  drain  for 
15  seconds  and  the  jet  is  then  stroked  off  the  wall  of  the  vessel. 
The  "  Limits  of  Error  "  allowed  in  pipettes  standardized  at  the 
National  Physical  Laboratory  are  as  follows  :— 

Limits  of  Error  in  c.c. 
For  c.c.  inclusive     2         10         30         75         200 

c.c.  -01      '02        "03        '05          'I 

The  standard  temperature  is  15°  C. 

In  all  pipettes,  the  upper  end  is  narrowed  to  about  J  inch,  so  that 
the  pressure  of  the  finger  is  sufficient  to  arrest  the  flow  at  any  point. 

Pipettes  are  invariably  filled  by  sucking  the  upper  end  with  the 
mouth,  unless  the  liquid  is  volatile  or  highly  poisonous,  in  which 
case  it  is  best  to  use  some  other  kind  of  measurement.  Beginners 
invariably  find  a  difficulty  in  quickly  filling  the  pipette  above  the 
mark,  and  stopping  the  fluid  at  the  exact  point.  Practice  with  pure 
water  is  the  only  method  of  overcoming  this. 

Fig.  15  shows  two  whole  pipettes,  one  of  small  and  the  other  of 
large  capacity,  and  also  a  graduated  pipette  of  medium  size.  It 
must  be  borne  in  mind  that  the  pipette  graduated  throughout  the 
stem  is  not  a  reliable  instrument  for  accurate  titration,  owing  to 
the  difficulty  of  stopping  the  flow  of  liquid  at  any  given  point  and 
reading  off  the  exact  measurement.  Its  chief  use  is  in  the  approxi- 
mate determination  of  the  strength  of  any  standard  solution  in 
the  course  of  preparation. 

Fig.  16  shows  a  very  useful  form  of  pipette  for  measuring  strong 
acids  or  alkalies,  etc.,  the  bulb  preventing  the  entrance  of  any  liquid 
into  the  mouth. 

THE    MEASURING    FLASK. 

MEASURING  flasks  serve  to  make  up  standard  solutions  to  a  given 
volume,  and  also  enable  the  analyst,  with  the  aid  of  pipettes,  to 
obtain  aliquot  portions  of  a  substance  to  be  tested.  They  should 
be  as  narrow  in  the  neck  as  is  compatible  with  easy  filling  and 
emptying,  and  the  mark  should  be  situated  below  the  middle  of 
the  neck,  so  as  to  allow  room  for  thoroughly  mixing  the  contents 
by  shaking. 

Measuring  flasks  are  made  either  to  contain  or  to  deliver  the 
quantities  marked  on  them,  and  the  temperature  at  which  they  have 
been  standardized  should  invariably  be  marked  on  also.  Ordinary 
flasks  with  one  mark  are  always  taken  to  contain  the  amount 
specified.  Vessels  standardized  at  the  National  Physical 
Laboratory,  Teddington,  are  marked  with  the  letter  D  when  they 
are  meant  to  deliver  ;  if  meant  for  both  content  and  delivery  the 
letter  D  is  placed  above  the  upper,  and  the  letter  C  below  the 
lower,  mark.  The  standard  temperature  is  15°  C.  Thus,  a 
standardized  flask  marked  "  1  litre  15°  C."  is  such  that,  at  a 
temperature  of  15°  C.,  the  volume  of  the  contents  of  the  flask  is  the 


16 


THE   GRADUATED    FLASK. 


same  as  that  of  a  kilogram  of  water  at  a  temperature  of  4°  C. 
Since,  however,  the  density  of  water  at  15°  C.  is  0 '999 13  grams  per 
c.cm.,  the  weight  of  water  at  15°  C.  which  fills  the  flask  up  to  the 
mark  is  999*13  grams,  not  1000  grams. 


Fig.  17. 


Fig.  18. 


Volumetric  measures  standardized  at  Charlottenburg  are  marked 
with  the  letters  A  and  E  to  indicate  "  deliver  "  and  "  contain  " 
respectively,  these  being  the  initial  letters  of  the  words  "  Ausguss  " 
and  "  Einguss,"  signifying  pouring  out  and  pouring  in. 

As  examples  of  the  "  Limits  of  Error  "  observed  at  the  National 
Physical  Laboratory  the  following  may  be  given  : — 

Measuring  Flasks. 
Limits  of  Error  in  c.c. 
For  c.c.  50       100  to  250     300  to  500     550  to  1000 

/to  contain    '05  '1  '15  '3 

c'c<\ to  deliver       -1  -2  -3  -6 

The  German  limits  are  practically  the  same. 
Measuring  flasks  are  ordinarily  made  of  50,  70,  100,  200,  250, 
500,  1000  and  2000  c.c.  capacity.  Flasks  for  delivery  should  be 
gradually  tilted  till  nearly  vertical,  drained  for  one  minute,  and 
the  last  drop  removed  by  touching  the  side  of  the  vessel  into  which 
they  are  being  emptied. 

W.  B.  Giles  has  described  a  modified  flask*,  shown  in  fig.  18. 
It  is  handy  in  making  up  standard  solutions  where  the  reagent 
cannot  be  weighed  in  an  absolutely  pure  state,  for  instance,  sulphuric 

*  C.  N.  69,  99. 


2000 

•5 
1-0 


THE    GRADUATED    CYLINDER. 


17 


s 


acid,  ammonium  thiocyanate,  or  uranic  salts.  Such  a  quantity, 
however,  is  taken  as  will  give  a  solution  about  a  ninth  or  tenth 
too  strong,  and  the  measure  is  made  up  to  1100  c.c.  The  real 
strength  is  then  taken  by  two  titrations  on  25  or  30  c.c.  with 

a  known  standard,  so  that  its 
exact  working  strength  is  known  ; 
the  remainder  of  the  100  c.c.  is 
then  removed  down  to  the  1000 
c.c.  mark,  and  a  slight  calculation 
will  show  how  much  water  has 
to  be  added  to  the  1000  c.c.  to 
make  a  correct  solution.  If  only 
a  litre  is  made  up,  an  unknown 
volume  is  left  in  the  flask,  and  it 
must  be  transferred  to  a  measuring 
cylinder,  where,  owing  to  the 
large  diameter  of  the  vessel,  the 
graduation  can  never  be  so  accu- 
rate as  in  the  narrow  neck  of  the 
flask.  Should  the  solution  prove 
to  be  only  about  a  tenth  too 
strong,  the  necessary  dilution  may 
be  made  in  the  flask  itself  ;  but 
if  stronger  than  this,  the  flask 
must  be  emptied  into  the  store 
bottle  and  rinsed  out  with  the 
measured  quantity  of  water  re- 
quired, which  is  then  drained 
into  the  store  bottle,  and  the 
whole  carefully  mixed. 

In  addition  to  the  measuring 
flasks  it  is  necessary  to  have 
graduated  vessels  of  cylindrical 
form  for  the  purpose  of  preparing 
standard  solutions,  etc. 

Fig.  19  shows  a  stoppered 
cylinder  for  this  purpose, 
generally  called  a  test  mixer. 
Wide-mouthed  open  cylinders, 
with  spouts,  of  various  sizes 
and  graduated  like  fig.  19,  are 
also  used. 

ON    THE    CORRECT 

READING    OF    GRADUATED 

INSTRUMENTS. 

-p.    2  IN  conseq'uence  of  capillarity 

the  surface  of  liquids  in  narrow 
tubes  is  always  curved.     Where  the  liquid  wets  the  tube,  its  surface 


18 


READING    OF   BURETTES. 


takes  the  form  of  a  meniscus  which  is  concave,  as  shown  in  fig.  20. 

In  reading  the  heights  of  such  liquids  in  tubes,  the  point  where 

a  graduation   mark  coincides  with  the  bottom  of  the  curve  is 

taken. 

The  eye  may  be  assisted  materially  in  reading  the  divisions  on 

a  graduated  tube  by  using  a  piece  of  white  paper  or  opal  glass  held 

at  an  angle  of  30°  or  40°  from  the  burette  and  near  the  surface  of  the 
liquid,  or  a  small  card,  the  lower  half  of  which  is 
blackened,  the  upper  remaining  white.  If  the  line  of 
division  between  the  black  and  white  be  held  about  an 
eighth  of  an  inch  below  the  surface  of  the  liquid,  and  the 
eye  brought  on  a  level  with  it,  the  meniscus  can  then  be 
seen  by  transmitted  light,  bounded  below  by  a  sharply 
defined  black  line.  A  card  of  this  kind,  sliding  up  and 
down  a  support,  'is  of  great  use  in  verifying  the  graduation 
of  the  burettes  or  pipettes  with  a  cathetometer.  Another 
good  method  is  to  use  a  piece  of  mirror,  upon  which  are 
gummed  two  strips  of  black  paper,  half  an  inch  apart ; 
apply  it  in  contact  with  the  burette  so  that  the  eye  can 
be  reflected  in  the  open  space.  The  operator  may  con- 
sult with  advantage  the  directions  for  calibration  on  the 
following  page,  and  details  of  graduating  and  verifying 
measuring  ^instruments  for  the  analysis  of  gases  as 
described  in  Part  7.  In  taking  the  readings  of  burettes, 

pipettes,  and  flasks,  the  graduation  mark  should  coincide  as  nearly 

as  possible  with  the  level  of  the  operator's  eye. 


Fig.  21. 


Erdmann's  Float. — This  useful  little  instrument  to 
accompany  Mohr's  burette,  gives  the  most  accurate  reading 
that  can  be  obtained  ;  one  of  its  forms  is  shown  in  fig.  21, 
another,  containing  a  thermometer,  is  shown  in  fig.  4.  The 
latest  form  is  shown  in  fig.  22,  where  the  ring-mark  is  made 
within  the  bulb,  as  indeed  it  is  best  to  be  in  all  cases.  A 
special  form  for  use  with  dark-coloured  solutions  like  iodine, 
permanganate,  etc.,  is  to  have  two  bulbs  with  the  ring-mark 
in  the  upper  bulb,  and  the  instrument  is  so  weighted  that  the 
Fi  *22  uPPer  bulb  stands  out  of  the  liquid,  and  of  course 
'  may  then  be  read  off  as  easily  as  if  the  liquid 
were  transparent.  The  instrument  consists  essentially  of 
an  elongated  glass  tube,  rather  smaller  in  diameter  than 
the  burette  itself,  and  weighted  at  the  lower  end  with 
a  globule  of  mercury.  The  actual  height  of  the  liquid 
in  the  burette  is  not  regarded,  because  if  the  operator 
begins  with  the  line  on  the  float  opposite  the  0  graduation 
mark  on  the  burette  the  same  proportional  division  is 
always  maintained. 

1    It  is  essential  that  the  float  should  move  up  and  down  in 
the  burette  without  wavering,  and  the  line  upon  it  should     Fj 
always  be  parallel  to  the  graduations  of  the  burette. 


CALIBRATION.  19 

Filter  for  ascertaining  the  end-reaction  in  certain  processes.— 

This  is  shown  in  fig.  23,  and  the  instrument  is  known  as  Beale's 
filter.  It  serves  well  for  taking  a  few  drops  of  clear  solution  from 
any  liquid  in  which  a  precipitate  will  not  settle  readily.  To  use 
it,  a  piece  of  filter  paper  is  tied  over  the  lower  end,  and  over  that 
a  piece  of  fine  muslin  to  keep  the  paper  from  being  broken.  When 
dipped  into  a  muddy  mixture,  the  clear  fluid  rises  and  may  be 
poured  out  of  the  little  spout  for  testing.  If  the  process  in  hand 
is  not  completed,  the  contents  are  washed  back  to  the  bulk,  and 
the  operation  repeated  as  often  as  may  be  required. 


THE    CALIBRATION    OF    GRADUATED    APPARATUS. 

IT  is  obvious  that  in  the  practice  of  volumetric  analysis  the 
absolute  correctness  of  the  graduations  of  the  vessels  used  to  a  given 
standard  is  not  necessary  so  long  as  they  agree  with  one  another. 
In  the  present  day  there  are  many  makers  of  instruments,  some 
using  the  litre  of  1000  grams  of  distilled  water  at  4°  C.,  others  at 
15*5°  C.,  and  others  again  at  17'5°  C.  In  these  circumstances  it  is 
conceivable  that  operators  may  purchase,  from  time  to  time, 
a  mixture  of  instruments  of  a  heterogeneous  character.  The  German 
Imperial  Standard  Commission  have  now  made  it  legal  only  to  use 
for  official  purposes  the  litre  and  its  divisions,  containing  1000  grams 
of  pure  water  at  4°  C.  (p.  23).  These  instruments  for  use  in  that 
country  are  all  stamped  in  the  same  way  as  commercial  measures 
are  stamped  by  law  in  this  country.  If,  then,  instruments  are  sent 
abroad,  they  will  not  agree  with  the  bulk  of  those  hitherto  used. 
On  this  account,  as  well  as  for  general  accuracy,  it  is  necessary  to 
calibrate  or  measure  the  divisions  upon  the  various  instruments  by 
actual  experiment,  carried  on  in  a  room  kept  at  the  temperature 
of  15°  C. 

Flasks. — The  shortest  way  to  get  at  the  true  contents  of  a  litre 
flask,  or  to  correct  it  for  a  given  temperature  by  making  a  fresh  mark, 
is  to  weigh  the  contents  by  substitution,  which  is  done  as  follows  : — 

The  flask  is  cleaned  and  dried,  by  first  rinsing  with  alcohol,  then 
ether,  and  the  latter  blown  out  with,  a  bellows  or  driven  off  by 
warming.  When  cool,  it  is  placed  on  a  sufficiently  large  and 
sensitive  balance,  together  with  a  kilogram  weight,  side  by  side — 
a  shallow  metal  tray  is  placed  on  the  other  pan,  and  sufficient  shot 
added  to  exactly  balance  the  flask  and  weight ;  both  the  latter  are 
then  removed,  leaving  the  shot  on  the  other  pan.  The  flask  is  then 
placed  level,  and  distilled  water  at  15°  C.  poured  in  up  to  the  mark  ; 
the  moisture  in  the  neck  is  removed  after  a  few  minutes  by  filter 
paper  and  the  flask  placed  on  the  empty  pan.  If  the  two  pans  are 
in  equilibrium  the  mark  is  correct  ;  if  not,  water  must  be  added  or 
removed  with  a  small  pipette,  and  the  mark  altered.  Smaller 
flasks  are  calibrated  in  the  same  way. 

C  2 


20  CALIBRATION. 

To  calibrate  a  flask  for  delivering  an  exact  litre  or  less,  some  water 
is  poured  into  the  empty  flask,  which  is  drained  for  half  a  minute, 
and  weighed  with  its  stopper  ;  it  is  then  filled  to  the  neck  with  pure 
water,  and  closed  by  the  glass  .or  rubber  stopper,  to  prevent 
evaporation,  and  water  added  or  removed  as  before.  A  nick  is  then 
made  with  a  diamond,  or  sharp  file,  opposite  the  lowest  part  of  the 
meniscus,  which  may  be  extended  to  a  proper  mark  after  the  flask, 
is  emptied.  Such  a  flask,  when  correctly  marked,  will  deliver  the 
volume  required  at  the  given  temperature,  after  the  contents  have 
been  poured  out  and  drained  for  half  a  minute. 

Burettes. — After  firmly  fixing  in  its  stand,  filling  with  pure  water 
at  15°  C.,  and  getting  rid  of  the  air  bubbles  in  the  tap  or  jet,  the 
exact  level  at  the  0  mark  is  made  preferably  with  an  Erdmann 
float ;  successive  quantities  of  5  or  10  c.c.  are  then  run  into  a  small 
dry  tared  beaker  and  rapidly  weighed.  If  great  accuracy  is  required 
a  closed  vessel  ought  to  be  employed,  but  this  necessitates  the  drying 
after  each  weighing  ;  a  very  small  beaker  can  be  easily  wiped  dry, 
and  rapid  weighings  made  without  any  sensible  loss  of  accuracy.  If 
the  weighings  have  shown  reasonable  accuracy,  say  within  a  milli- 
gram or  so  for  each  c.c.,  it  will  be  sufficiently  correct ;  if  otherwise, 
a  table  must  be  constructed  showing  the  correct  contents  at  any 
given  point. 

An  excellent  method  of  calibrating  tap  burettes  is  described  by 
Carnegie,*  which  saves  the  labour  involved  in  the  separate 
weighings  just  described,  but  does  not  give  the  weight  contents. 
A  small  column  of  CS2,  saturated  with  water,  and  tinted  with 
iodine,  is  used  to  measure  the  spaces  between  the  graduation  marks 
of  the  instrument.  The  burette  is  connected  by  rubber  tube  with 
a  reservoir  of  water  like  that  used  for  mercury  in  gas  apparatus, 
and  by  the  pressure  of  the  water  in  this  reservoir  5  c.c.  or  so  of  the 
CS2  may  be  moved  from  the  bottom  upwards,  throughout  the  whole 
length  of  the  instrument,  so  as  to  compare  portions  of  the  scale 
throughout.  It  is  essential  that  the  measurement  takes  place  from 
the  bottom,  which  is  done  by  allowing  water  to  flow  in  up  to  the 
lower  mark  of  the  burette,  then  gently  running  in  the  portion  of  CS2 
from  a  long  fine  pipette  ;  when  settled,  and  the  meniscus  observed, 
a  cautious  opening  of  the  tap  will  allow  of  the  movement  of  the 
column,  through  the  various  divisions,  up  to  the  top. 

Pipettes. — With  the  instrument  made  to  deliver  one  quantity  only 
it  is  generally  sufficient  to  fill  it  by  suction  above  the  mark,  then 
gently  release  the  pressure  of  the  finger  until  the  exact  mark  is 
reached.  The  contents  are  then  run  into  a  dry  tared  beaker,  drained 
for  15  seconds  in  contact  with  the  sides  of  the  beaker,  and  the 
beaker  quickly  weighed.  If  not  fairly  correct,  trials  must  be  made 
by  placing  a  thin  strip  of  gummed  paper  on  the  stem,  and  marking 

*  c.  N.  64,  42. 


PRESERVATION    OF    STANDARD    SOLUTIONS. 


21 


the  height  of  each  trial  until  the  correct  weight  is  found,  when 
a  permanent  mark  may  be  made. 

Graduated  pipettes  are  best  calibrated  by  filling  them  above  the 
mark,  fixing  them  in  a  stand  like  a  burette,  closing  the  top  with 
a  stout  piece  of  rubber  tube,  clamped  with  a  strong  clip,  then,  after 
adjusting  the  level,  drawing  off  in  quantities  of  5  c.c.  or  so,  and 
weighing  in  the  same  way  as  directed  for  burettes. 

Cylinders. — The  only  method  of  calibrating  these  vessels  is  to 
measure  into  them  repeatedly  various  volumes  of  water  from 
delivery  pipettes  of  proved  accuracy,  taking  precautions  as  to  level, 
meniscus,  and  the  proper  drainage  of  the  pipette  after  each 
delivery.* 

Preservation  of  Solutions. — There  are  test  solutions  which,  in 
consequence  of  their  proneness  to  decomposition,  cannot  be  kept  at 


Fig.  24. 


Fig.  25. 


any  particular  strength  for  a  length  of  time  ;  consequently  they  must 
be  titrated  on  every  occasion  before  being  used.  Stannous  chloride 
and  sulphurous  acid  are  examples  of  such  solutions.  Special  vessels 

*  An  excellent  method  of  calibration  for  volumetric  instruments  is  given  by  M  o  r  s  e 
and  Blalock  (Amer.  Chem.  Journ.  16,  479). 


22 


PRESERVATION    OF    STANDARD    SOLUTIONS. 


have  been  devised  for  keeping  solutions  liable  to  alter  in  strength 
by  access  of  air,  as  shown  in  figs.  24  and  25. 

Fig.  24  is  especially  applicable  to  caustic  alkali  solutions,  the 
tube  passing  through  the  caoutchouc  stopper  being  filled  with  dry 
soda-lime,  resting  on  cotton  wool. 

Fig.  25,  designed  by  Mohr,  is  a  considerable  improvement  upon 
this,  since  it  allows  of  the  burette  being  filled  with  the  solution  from 
the  store  bottle  quietly,  and  without  any  access  of  air  whatever. 
The  vessel  can  be  used  for  caustic  alkalies,  baryta,  stannous  chloride, 
permanganate,  and  sulphurous  acid,  or  any  other  liquid  liable  to 
undergo  change  by  absorbing  oxygen.  Rubber  stoppers  should  be 
used  for  these  bottles  ;  and  a  thin  layer  of  white  mineral  oil  is  poured 
on  the  top  of  the  solution,  where,  owing  to  its  low  density,  it  always 
floats,  placing  an  impermeable  division  between  the  air  and  the 
solution  ;  and  as  this  oil  is  not  affected  by  these  solutions  in  their 
diluted  state,  this  form  is  of  great  advantage.  Fig.  25  can  be 
improved  by  having  a  two-holed  rubber  stopper — one  hole  is  used 
for  a  tapped  funnel,  through  which  the  bottle  is  filled,  the  other 
hole  contains  a  small  tapped  tube,  which  is  opened  when  drawing 
the  solution  out  or  when  filling  the  bottle.  Solutions  not  affected 
chemically  by  contact  with  air  should  be  kept  in  bottles,  the  corks 
or  stoppers  of  which  keep  them  perfectly  closed,  and  tied  over  with 
india-rubber  or  bladder  to  prevent  evapora- 
tion, and  should  further  be  always  shaken 
before  use,  when  they  are  not  quite  full. 
The  influence  of  bright  light  upon  some 
solutions  is  very  detrimental  to  their  chemical 
stability  ;  hence  it  is  advisable  to  preserve 
some  solutions  not  in  immediate  use  in  the 
dark,  and  at  a  temperature  not  exceeding 
15  or  16°  C. 

The  apparatus  devised  by  J.  C.  Chorley, 
and  shown  in  fig.  26,  will  be  found  useful  for 
preserving  and  delivering  known  volumes  of 
such  solutions  as  alcoholic  potash,  which  are 
liable  to  alteration  by  exposure  to  air.  The 
wash  bottle  inserted  in  the  cork  of  the  large 
store  bottle  contains  a  solution  of  caustic 
soda,  and  serves  to  wash  all  air  entering  the 
large  bottle.  By  means  of  the  three-way 
stop-cock  at  the  bottom  of  the  apparatus  the 
solution  is  allowed  to  fill  the  pipette  and 
overflow  into  its  upper  chamber,  the  excess 
being  caught  in  the  small  side  bulb  and 
reservoir  ;  this  solution  serves  to  wash  all  air  Fig.  26. 

entering  the  pipette  when  the  stop-cock  is 

turned  to  deliver  the  solution,  which  is  run  off  to  a  mark  just  above 
the  tap.  When  full,  the  side  reservoir  may  be  emptied  by  with- 
drawing the  small  ground  stopper. 


METRIC   SYSTEM.  23 

ON    THE    SYSTEM    OF    WEIGHTS    AND    MEASURES    TO 
BE    ADOPTED    IN    VOLUMETRIC    ANALYSIS.* 

IT  is  much  to  be  regretted  that  the  metric  system  of  weights  and 
measures  used  on  the  Continent  is  not  universally  adopted,  for  both 
scientific  and  general  purposes,  throughout  the  civilized  world. 
The  two  great  advantages  of  the  metric  system  are  that  it  is,  first, 
a  decimal  system  and,  secondly,  a  simply-related  system.  A  short 
description  of  the  origin  and  development  of  this  system  may  not 
here  be  out  of  place. 

The  Metric  System  is  founded  on  the  metre,  which  was  originally 
intended  to  be  one-ten-millionth  part  of  a  quadrant  of  the  meridian 
through  Paris,  in  other  words,  one  forty-millionth  part  of  the 
circumference  of  the  earth.  Subsequent  measurements,  however, 
have  shown  that  the  metre  does  not  exactly  represent  this,  hence 
the  unit  becomes  an  arbitrary  measure  after  all. 

All  the  multiples  of  units  in  the  Metric  System  are  indicated  by 
Greek  prefixes,  all  fractions  or  submultiples  by  Latin  prefixes, 
thus : — 

kilo     =a  thousand  times          milli   =a  thousandth  part,  or  *001 
hecto  =a  hundred  times  centi  =a  hundredth  part,    or   *01 

deca    =ten  times  deci    =a  tenth  part,  or  *1 

the  unit  to  which  of  the  unit  to  which  it  is 

it  is  prefixed.  prefixed. 

For  example,  a  hectogram  =  100  grams 

a  millimetre  =  one- thousandth    part  of    a   metre,  or 
•001  metre. 

The  decimals  of  a  metre  are  abbreviated  thus  : — 
Millimetre^    =mm. 
Centimetre,s  =cm. 
Decimetre,s    =dm. 

The  unit  of  surface  is  the  are,  which  is  a  square  whose  side  is 
10  metres.  Consequently  it  equals  100  square  metres  and  is 
indicated  thus  : — 1  are  =  100  m2. 

The  unit  of  volume  is  a  cube  whose  side  measures  one  decimetre 
and  it  is  called  the  litre. 

1  litre  =1  cubic  decimetre  (or  dm3.) 

=  1000  cubic  centimetres  (c.c.,  c.cm.,  or  cm3.). 

The  standard  of  weight  (strictly  speaking,  of  mass)  in  the  metric- 
system  is  the  kilogram,  which  was  constructed  to  represent  the 
weight  of  a  cubic  decimetre  of  pure  distilled  water  at  its  point  of 
maximum  density,  reckoned  at  4°  C.  In  this  way  a  most  intimate 
and  useful  relationship  between  volumes  and  weights  is  obtained. 
Thus,  at  4°  C.,  a  cubic  decimetre  of  water  weighs  1000  grams  and 
conversely  1000  grams  of  water  occupy  1  litre.  Similarly  a  litre 

*  The  author  is  greatly  indebted  to  Dr.  R.  T.  G  laze  brook,  F.R.S.,  Director  of 
the  National  Physical  Laboratory,  for  kindly  supplying  data  used  both  in  this  section 
and  in  those  dealing  with  the  pipette  and  the  measuring  flask. 


24  THE   LITHE. 

of  alcohol  of  sp.  gr.  0*9198  at  4°  C.  weighs  1000  x -9198-919-8 
grams.  For  scientific  purposes  the  kilogram  is  usually  too  large 
a  unit,  hence  for  practical  use  the  weight  made  use  of  is  the  gram 
and  its  subdivisions. 

Further  investigations  having  shown  that  the  above  relations  do 
not  strictly  hold  good,  the  litre  is  now  defined  as  follows  :— 

The  standard  litre  is  the  volume  of  a  kilogram  of  pure  water  at 

4°  a 

The  value  of  the  litre  in  terms  of  the  cubic  centimetre  has  been 
the  subject  of  numerous  experiments.  The  best  determination 
gives  :— 

1  litre  =  1000*028  cubic  centimetres. 

This  is  based  on  very  exact  measurements  made  during  the  last 
few  years,  as  follows  : — 

Method.  Experimenter.  Result. 

Mechanical  contact  Guillaume  1000*029 

Interference  by  reflection  Chappuis  1000*027 

„  „    transmission  Mace  de  Lepinay 

Benoit  1000-028 

Buisson 

Hence,  one  decilitre  =100*0028  cubic  centimetres 
one  millilitre   =1-000028     „ 

Thus  the  difference  between  the  two  is  practically  negligible, 
and  in  all  but  the  most  refined  experiments  the  volume  of  one  cubic 
centimetre  may  be  treated  as  one-thousandth  part  of  that  of  the 
litre.  Now  since  in  chemical  laboratories  it  is  not  customary  to  work 
at  a  temperature  of  4°  C.,  Mohr  introduced  another  standard, 
known  as  Mohr '  s  litre,  which  is  the  volume  occupied  by  1000  grams 
of  water  at  the  temperature  of  16°  C.,  this  being  considered  about 
the  average  temperature  of  a  working  laboratory.  On  this  system 
the  cubic  centimetre  should  contain  1  gram  of  distilled  water  at 
16°  C.,  and  a  10  c.c.  pipette,  for  example,  should  deliver  10  grams 
of  distilled  water  at  16°  C.  1000  true  c.c.  contain  almost  exactly 
999  grams  of  water  at  16°  C. 

From  these  considerations  it  becomes  evident  that  it  is  of  the 
first  importance  to  the  analyst  that  he  takes  care  to  work  with 
a  complete  set  of  volumetric  measures  all  graduated  on  one  plan 
or  the  other. 

Measures  marked,  e.g.,  "  25  c.c.  15°  C  "  should  contain  or  deliver, 
as  the  case  may  be,  25  true  c.c.  when  the  instrument  is  at  the 
temperature  of  15°  C.  On  the  other  hand,  a  flask  marked  "  1000 
grams  16°  C."  should,  of  course,  contain  1000  grams  of  distilled 
water  at  the  temperature  of  16°  C.,  i.e.,  a  Mohr's  litre.  Vessels 
graduated  according  to  Mohr's  system  should  bear  the  word 
'•  Gramme"  or  the  letters  "Grm"  together  with  the  temperature. 
It  should  be  noted  that  the  German  Kaiserliche  Normal-Eichungs 
Kommission  no  longer  employs  Mohr's  unit. 


INFLUENCE  OF  TEMPERATURE.  25 

The  usual  standard  temperature  for  volumetric  vessels  is  15°  C., 
which  means  that  a  vessel  contains  its  nominal  content  of  water 
when  the  vessel  is  at  15°  C.  This  is  usually  judged  by  the  water 
being  steady  at  15°  C.,  and  for  exactness  the  surrounding  air  should 
be  at  15°  C.  also.  When  the  vessel  is  at  a  different  temperature, 
its  volume  alters  in  accordance  with  the  coefficient  of  cubical 
expansion  of  the  glass.  Various  other  temperatures  in  addition 
to  the  standard  15°  C.  are,  however,  in  common  use,  as  for  instance 
20°  C.  for  vessels  employed  in  connection  with  polari meters  or 
viscosimeters,  80°  or  82°  F.  for  vessels  intended  for  tropical  climates, 
and  so  on. 

The  British  equivalents  of  the  principal  metric  units  are  as 
follows  : — 

1  metre        =  39'370113  inches. 

1  are  =119*59921  square  yards. 

1  litre  =     1-75980  pints. 

=     0-219975  gallons. 

1  kilogram  =     2'2046223  Ib.  avoirdupois. 

1  gram         =   15-43236  grains. 

Variations  of  Temperature. — In  the  preparation  of  standard 
solutions  one  thing  must  especially  be  borne  in  mind,  namely,  that 
saline  substances  on  being  dissolved  in  water  have  a  considerable 
effect  upon  the  volume  of  the  resulting  liquid.  The  same  is  also 
the  case  in  mixing  solutions  of  various  salts  or  acids  with  each 
other.* 

In  the  preparation  of  strong  solutions  the  contraction  in  volume 
is  as  a  rule  considerable.  Hence,  in  preparing  such  solutions  for 
volumetric  analysis,  or  in  diluting  such  solutions  to  a  given  volume 
for  the  purpose  of  removing  aliquot  portions  subsequently  for 
examination,  sufficient  time  must  be  given  for  liquids  to  acquire 
their  constant  volume  at  the  standard  temperature.  If  the  strength 
of  a  standard  solution  is  known  for  one  temperature,  the  strength 
corresponding  to  another  temperature  can  only  be  calculated  if  the 
rate  of  expansion  by  heat  of  the  liquid  is  known.  The  variation 
cannot  be  estimated  by  the  known  rule  of  expansion  of  distilled 
water  ;  for  Gerlach  has  shown  that  even  weak  solutions  of  acids 
and  salts  expand  far  more  than  water  for  certain  increments  of 
temperature.  The  rate  of  expansion  for  pure  water  is  known,  and 
may  be  used  for  the  purpose  of  verifying  the  graduation  of  instru- 
ments where  extreme  accuracy  is  required.  The  following  short 
table  furnishes  the  data  for  correction. 

The  weight  of  1000  c.c.  of  water  at  t°  C.,  when  determined  by 
means  of  brass  weights  in  air  of  t°  C.,  and  at  760  m.m.  pressure  is 
equal  to  1000— #  gm. 

>•    Slight    variations    of    atmospheric    pressure    may    be    entirely 
disregarded. 

*See  Gerlach,  "  Speciflsche  Gewichte  der  Salzlosungen ; "  also  Gerlach, 
"  Sp.  Gewichte  von  wasserigen  Losungeii,"  Z.  a.  C.  8,  245. 


26 


CORRECTION    FOR   TEMPERATURE. 


1 

t° 

10 

11 

12 

13 

14 

15 

16 

17 

18 

19 

X 

! 

1-34 

1-43 

1-52 

1-63 

1-76 

1-89 

2-04 

2-2 

2-37  2-55 

1 
«° 

20 

21  ! 

22 

23 

24 

25 

26 

27 

28 

29 

30 

x 

2-74  2-95 

3-17 

3-39 

3-63 

3-88 

4-13 

4-39 

4-67 

4-94 

5-24 

x  is  the  quantity  to  be  subtracted  from  1000  to  obtain  the  weight 
of  1000  c.c.  of  water  at  the  temperature  t°.  Thus  at  20°  2- 74  must 
be  deducted  from  1000  =  997'26. 

Bearing  the  foregoing  remarks  in  mind,  therefore,  the  safest  plan 
in  the  operations  of  volumetric  analysis,  so  far  as  measurement  is 
concerned,  is  to  use  solutions  as  dilute  as  possible.  Absolute 
accuracy  in  determining  the  strength  of  standard  solutions  can  only 
be  secured  by  the  process  of  weighing,  the  ratio  of  the  weight  of 
the  solution  to  the  weight-  of  active  substance  in  it  being  independent 
of  temperature. 

Casamajor*  has  made  use  of  the  data  given  by  Matthiessen 

'  in  his  researches  on  the  expansion  of  glass,  water,  and  mercury,  to 

construct  a  table  of  corrections  to  be  used  when  using  any  weak 

standard  solution  at  a  different  temperature  from  that  at  which  it 

was  originally  standardized. 

The  expansion  of  water  is  different  at  different  temperatures  ;  the 
expansion  of  glass  is  known  to  be  constant  for  all  temperatures  up 
to  100°  C.  The  correction  of  volume,  therefore,  in  glass  burettes 
must  be  the  known  expansion  of  each  c.c.  of  water  for  every  1°  C., 
less  the  known  expansion  of  glass  for  the  same  temperature. 

It  is  not  necessary  here  to  reproduce  the  entire  paper  of 
Casamajor,  but  the  results  are  shortly  given  in  the  following 
table. 

The  normal  temperature  is  15°  C.  ;  and  the  figures  given  are  the 
relative  contractions  below,  and  expansions  above,  15°  C 


Deg.  C. 

7  - '0006 12 
8- -000590 
9- -000550 
10  - -000492 
11--000420 
12  - -000334 
13--000236 
14- -000124 
15  Normal 
16+ -000147 
17+-000305 
18+ -000473 
19+ -000652 
20+ -000841 
21  + -001039 
22 +-00 1246 
23+ -001462 


)eg.  C. 
24 +'00 1686 
25+ -001919 
26+ -002 159 
27  + -002405 
28+ -002657 
29+ -0029 13 
30+ -003 179 
31  + -003453 
32+ -003739 
33+ -004035 

34  +  -004342 

35  +  -004660 
36+ -004987 
37  + -005323 
38+ -005667 
39+ -006040 
40+ -006382 


*  C.  N.  35,  160. 


THE    GRAIN    SYSTEM.  27 

By  means  of  these  numbers  it  is  easy  to  calculate  the  volume  of 
liquid  at  15°  C.  corresponding  to  any  volume  observed  at  any 
temperature.  If  35  c.c.  of  solution  has  been  used  at  37°  C.,  the 
table  shows  that  1  c.c.  of  water  in  passing  from  15°  to  37°  is  in- 
creased to  1-005323  c.c.  ;  therefore,  by  dividing  35  c.c.  by  1*005323 
is  obtained  the  quotient  34  819  c.c.,  which  represents  the  volume 
at  15°  corresponding  to  35  c.c.  at  37°.  The  operation  can  be 
simplified  by  obtaining  the  factor,  thus  : 

=0-994705 


1-005323 
and  35x0-9947=34-82 

A  table  can  thus  be  easily  constructed  which  would  show  the  factor 
for  each  degree  of  temperature. 

These  corrections  are  useless  for  concentrated  solutions,  such  as 
normal  alkalies  or  acids  ;  with  great  variations  of  temperature  these 
solutions  should  be  used  by  weight. 

Instruments  graduated  on  the  Grain  System. — Burettes,  pipettes, 
and  flasks  may  also  be  graduated  in  grains,  in  \vhich  case  it  is  best 
to  take  10,000  grains  as  the  standard  of  measurement.  In  order 
to  lessen  the  number  of  figures  used  in  the  grain  system,  so  far  as 
liquid  measures  are  concerned,  I  propose  that  ten  fluid  grains  be 
called  a  decem,  or  for  shortness  dm.  This  term  corresponds  to  the 
cubic  centimetre,  bearing  the  same  proportion  to  the  10,000  grain 
measure  as  the  cubic  centimetre  does  to  the  litre,  namely,  the  one- 
thousandth  part.  The  use  of  a  term  like  this  wih1  serve  to  reduce 
the  number  of  figures  which  are  unavoidably  introduced  by  the  use 
of  a  small  unit  like  the  grain. 

Its  utility  is  principally  apparent  in  the  analysis  for  percentages, 
particulars  of  which  will  be  found  hereafter. 

The  1000  grain  burette  or  pipette  will  therefore  contain  100 
decems,  the  10,000  gr.  measure  1000  dm.,  and  so  on. 

The  capacities  of  the  various  instruments  graduated  on  the  grain 
system  may  be  as  follows  : — 

Flasks  :  10,000,  5000,  2500,  and  1000  grs.  =  1000,  500,  250,  and 
100  dm.  Burettes  :  300  grs.  in  1-gr.  divisions,  for  very  delicate 
purposes  =  30  dm.  in  ^  ;  600  grains  in  2-gr.  divisions,  or  |  dm.  ; 
1100  grs.  in  5-gr.  divisions,  or  J  dm.  ;  1100  grs.  in  10-gr.  divisions,  or 
1  dm.  The  burettes  are  graduated  above  the  500  or  1000  grs.  in 
order  to  allow  of  analysis  for  percentages  by  the  residual  method. 
Whole  pipettes  to  deliver  10,  20,  50,  100,  200,  500,  and  1000  grs. ; 
graduated  ditto,  100  grs.  in  -^  dm.  ;  500  grs.  in  \  dm.  ;  1000  grs.  in 
1  dm. 

Those  who  may  desire  to  use  the  decimal  systems  constructed  on 
the  gallon  measure  (=70,000  grains)  will  bear  in  mind  that  the 
"  septem  "  of  Griffin,  or  the  "  decimillem  "  of  Acland  are  each 
equal  to  7  grs.  ;  and  therefore  bear  the  same  relation  to  the  pound 
(  =  7000  grs.)  as  the  cubic  centimetre  does  to  the  litre,  or  the 


28  NORMAL   SOLUTIONS. 

decem  to  10,000  grs.  An  entirely  different  set  of  tables  foi 
calculations,  etc.,  is  required  for  these  systems  ;  but  the  analyst 
may  readily  construct  them  when  once  the  principles  contained  in 
this  treatise  are  understood. 


VOLUMETRIC  ANALYSIS  BASED  ON  THE  SYSTEM  OF  CHEMICAL 
EQUIVALENCE  AND  THE  PREPARATION  OF  NORMAL 
TITRATING  SOLUTIONS. 

WHEN  analysis  by  measure  first  came  into  use,  the  test  solutions 
were  generally  prepared  so  that  each  substance  to  be  tested  had  its 
own  special  reagent ;  and  the  strength  of  the  standard  solution  was 
so  calculated  as  to  give  the  result  in  percentages.  Consequently,  in 
alkalimetry,  a  distinct  standard  acid  was  used  for  soda,  another  for 
potash,  a  third  for  ammonia,  and  so  on,  necessitating  a  great  variety 
of  standard  solutions. 

Griffin  and  Ure  appear  to  have  been  the  first  to  suggest  the  use 
of  standard  test  solutions  based  on  the  atomic  system  ;  and  following 
in  their  steps  Mohr  has  worked  out  and  verified  many  methods  of 
analysis  which  are  of  great  value  to  all  who  concern  themselves  with 
scientific  and  especially  with  technical  chemistry.  Not  only  has 
Mohr  done  this,  but  he  has  enriched  his  processes  with  so  many 
original  investigations,  and  improved  the  necessary  apparatus  to 
such  an  extent,  that  he  may  with  justice  be  called  the  father  of  the 
volumetric  system. 

Normal  Solutions. — It  is  of  great  importance  that  no  misconception 
should  exist  as  to  what  is  meant  by  a  normal  solution  ;  but  it  does 
unfortunately  occur,  as  may  be  seen  by  reference  to  the  chemical 
journals,  also  to  Muir's  translations  of  Fleischer's  book.* 
/•      Normal  solutions  may  be  defined  as  follows  : — 

A  normal  solution  of  a  reagent  is  one  that  contains  in  a  litre  that 
proportion  of  its  molecular  weight  in  grams  which  corresponds  to 
one  gram  of  available  hydrogen  or  its  equivalent. 

Seminormal,  quintinormal,  decinormal  and  centinormal  solutions 
are  also  required  and  are  shortly  designated  as  N/2  N/5  N/,  and  N/i0o 
solutions.f 

*See  Allen,  C.  N.  40,  239,  also  Analyst ,'13/181. 

t  It  is  much  to  be  regretted  that  the  word  "  normal,"  originally  based  on  the 
equivalent  system,  should  now  be  appropriated  by  those  who  advocate  the  use  of 
solutions  based  on  molecular  weights,  because  it  not  only  leads  to  confusion  between 
the  two  systems,  but  to  utter  confusion  between  the  advocates  of  the  change  them- 
selves. In  Fleischer's  German  edition  of  his  Maasanalyse  the  molecular  system 
is  advocated,  but,  as  the  old  atomic  weights  are  used,  the  solutions  are  really,  in  the 
main,  of  the  same  strength  as  those  based  on  the  equivalent  system.  Pattinson 
Muir,  however,  in  his  translation,  has  thought  proper  to  use  modern  atomic  weights, 
and  the  cxirious  result  is  that  one  is  directed  to  prepare  a  normal  solution  of  caustic 
potash,  with  39'1  grams  K  to  the  litre,  while  a  normal  potassium  carbonate  is  to 
contain  138'2  grams  K2CO3,  or  78'2  grams  K,  in  the  same  volume  of  solution.  Again, 
Muter,  in  his  Manual  of  Analytical  Chemistry,  defines  a  normal  solution  as  having 
one  molecular  weight  of  the  reagent  in  grams  per  litre  ;  then  follows  the  glaring  incon- 
sistency, among  others,  of  directing  that  a  decinormal  solution  of  iodine  should  contain 
12'7  grams  of  I  per  litre,  whereas,  if  it  was  made  strictly  according  to  the  original 
definition,  it  should  contain  25'4  grams  in  the  litre.  Menschu  tkin  's  Analytical 


NORMAL   SOLUTIONS.  29 

Thus,  a  normal  solution  of  hydrochloric  acid  contains  36 -47 
grams  in  a  litre,  because  this  weight  of  the  acid  contains  one  gram 
of  hydrogen  ;  similarly  normal  sodium  hydroxide  contains  40*01 
grams  per  litre.  Oxalic  acid  contains  in  the  molecule  two  atoms 
of  hydrogen,  both  available,  and  consequently  normal  oxalic  acid 
contains  ±*$°*  =  63*03  grams  of  the  crystallized  acid  (H2C2O42H2O) 
in  the  litre.  Similarly,  a  normal  solution  of  sodium  phosphate 
would  be  made  by  dissolving  one- third  of  the  molecular  weight  of 
the  crystallized  salt,  in  grams,  in  water  and  diluting  the  solution 
to  1  litre,  because  orthophosphoric  acid  contains  3  atoms  of 
available  hydrogen  in  its  molecule. 

In  preparing  a  normal  solution  of  potassium  permanganate  for 
use  as  an  oxidizing  agent  we  know  that  K2Mn2O8  gives  up  5  atoms 
of  available  oxygen,  equivalent  to  10  atoms  of  available  hydrogen. 
Hence,  a  normal  solution  of  potassium  permanganate  contains 
3_i_G_Q_o  _  31  .(jog  grams  of  the  crystallized  salt  per  litre. 

Normal  alkali  solutions  are  always  such  that  a  given  volume 
requires  for  neutralization  an  equal  volume  of  a  normal  acid 
solution. 

Other  instances  will  be  given  later  on  and  explained  in  detail  in 
their  proper  place. 

A  further  illustration  may  be  given  in  order  to  show  the  method 
of  calculating  the  results  of  this  kind  of  analysis. 

Each  c.c.  of  N/10  silver  nitrate  solution  will  contain  Toihre  °f  ^ne 
atomic  weight  of  silver =0*010788  gm.,  and  will  exactly  precipitate 
T^inn  of  the  atomic  weight  of  chlorine  =0*003546  gm.  from  any 
solution  of  a  chloride. 

In  the  case  of  normal  oxalic  acid  each  c.c.  will  contain  ^oVs  of 
the  molecular  weight  of  the  crystallized  acid =0*06303  gm.,  and 
will  neutralize  ^(foo  °^  the  molecular  weight  of  sodium  carbonate 
=0*053  gm.,  or  will  combine  with  ^oo  °f  tne  atomic  weight  of 
a  dyad  metal  such  as  lead  =0*10355  gm.,  or  will  exactly  saturate 
Tifoo  °f  the  molecular  weight  of  sodium  hydrate  =0*040  gm.,  and 
so  on. 

Where  the  1000  grain  measure  is  used  as  the  standard  in  place 
of  the  litre,  63*03  grains  of  oxalic  acid  would  be  used  for  the  normal 
solution  ;  but  as  1000  grains  is  too  small  a  quantity  to  make,  it  is 
better  to  weigh  630  grains,  and  make  up  the  solution  to  10,000  grain 

Chemistry,  translated  by  Locke,  recently  published  by  Macmillaii  and  Co.,  unfortu- 
nately adopts  the  molecular  system. 

If  the  unit  H  be  adopted  as  the  basis  or  standard,  everything  is  simplified,  and 
actual  normal  solutions  may  be  made  and  used  ;  but,  on  the  molecular  system,  this  is, 
in  many  cases,  not  only  unadvisable  but  impossible,  besides  leading  to  ridiculous 
inconsistencies.  As  Allen  points  out  in  the  reference  above,  it  is,  to  say  the  least  of 
it,  highly  inconvenient  that  the  nomenclature  of  a  standard  solution  should  be  capable 
of  two  interpretations.  I  have  given  the  term  systematic  to  this  handbook,  and 
I  maintain  that  the  equivalent  system  used  is  the  only  systematic  and  consistent  one  ; 
it  was  adopted  originally  by  M  o h r,  followed  by  Fresenius,  and  continued  by 
Classen  in  the  new  edition  of  M  o  h  r '  s  Titrirmethode.  Allen  himself  has  un- 
hesitatingly preferred  to  use  it  in  his  Organic  Analysis,  and  these,  together  with  this 
treatise,  being  all  text-books  having  a  wide  circulation,  ought  to  settle  definitely  the 
meaning  of  the  term  normal  as  applied  to  systematic  standard  solutions.  Anyhow,  it 
is  to  be  hoped  that  those  who  communicate  processes  to  the  chemical  journals,  or 
abstractors  of  foreign  articles  for  publication,  will  take  care  to  distinguish  between 
the  conflicting  systems. 


30  CONVENIENCE    OF   THE   NORMAL   SYSTEM. 

measure  (  =  1000  dm.).  The  solution  Mould  then  have  exactly  the 
same  strength  as  if  prepared  on  the  litre  system,  as  it  is 
proportionately  the  same  in  chemical  value  ;  and  either  solution 
may  be  used  indiscriminately  for  instruments  graduated  on  either 
scale,  bearing  in  mind  that  the  substance  to  be  tested  with  a  burette 
graduated  in  c.c.  must  be  weighed  on  the  gram  system,  and  vice 
versa,  unless  it  be  desired  to  calculate  one  system  of  weights  into 
the  other. 

The  great  convenience  of  this  equivalent  system  is  that  the 
numbers  used  as  coefficients  for  calculation  in  any  analysis  are 
familiar,  and  the  solutions  agree  with  each  other,  volume  for 
volume.  We  have,  hitherto,  however,  looked  only  at  one  side  of 
its  advantages.  For  technical  purposes  the  plan  allows  the  use  of 
all  solutions  of  systematic  strength,  and  simply  varies  the  amount 
of  substance  tested  according  to  its  equivalent  weight. 

Thus,  the  normal  solutions  say,  are — 

Crystallized  oxalic  acid               =  63*03  gm.  per  litre 

Sulphuric  acid  =49*043  gm.  ., 

Hydrochloric  acid  =36*47  gm.  ,, 

Nitric  acid  =63*02  gm. 

Anhydrous  sodium  carbonate     =53  gm.  „ 

Sodium  hydroxide  =40*01  gm.  „ 

Ammonia  =17*034  gm.  „ 

100  c.c.  of  any  one  of  these  normal  acids  should  exactly  neutralize 
100  c.c.  of  any  of  the  normal  alkalies,  or  the  corresponding  amount 
of  pure  substance  which  the  100  c.c.  contain.  In  commerce  we 
continually  meet  with  substances  used  in  manufactures  which  are 
not  pure,  and  it  is  necessary  to  know  how  much  pure  substance  they 
contain. 

Take,  for  instance,  refined  soda  ash  (sodium  carbonate).  If  it 
were  absolutely  pure,  5*3  gm.  of  it  should  require  exactly  100  c.c. 
of  any  normal  acid  to  saturate  it.  If  we  therefore  weigh  tha,t 
quantity,  dissolve  it  in  wrater,  and  deliver  into  the  mixture  the 
normal  a.cid  from  a  burette,  the  number  of  c.c.  required  to  saturate 
it  will  show  the  percentage  of  pure  sodium  carbonate  in  the  sample. 
Suppose  90  c.c.  are  required  =  90  %. 

Again — a  manufacturer  buys  common  oil  of  vitriol,  and  requires 
to  know  the  exact  percentage  of  pure  hydrated  acid  in  it ;  4'9  grams 
are  weighed,  diluted  with  water;  and  normal  alkali  delivered  in 
from  a  burette  till  saturated  ;  the  number  of  c.c.  used  will  be  the 
percentage  of  real  acid.  Suppose  58'5  c.c.  are  required  =  58'5  %. 

On  the  grain  system,  in  the  same  way,  53  grains  of  the  sample  of 
soda  ash  would  require  90  dm.  of  normal  acid,  also  equal  to  90  %. 

Or,  suppose  the  analyst  desires  to  know  the  equivalent  percentage 
of  sodium  oxide,  free  and  combined,  contained  in  the  above  sample 
of  soda  ash,  without  calculating  it  from  the  carbonate  found  as 
above,  3'1  gm.  is  treated  as  before,  and  the  number  of  c.c.  required 


THE    NORMAL   SYSTEM   APPLIED    TO    ANALYSIS.  31 

is  the  percentage  of  sodium  oxide.  In  the  same  sample  52*6  c.c. 
would  be  required  =  52'6  per  cent,  of  sodium  oxide,  or  90  per  cent. 
of  sodium  carbonate. 

Method  for  percentage  of  Purity  in  Commercial  Substances.  — 

The  rules,  therefore,  for  obtaining  the  percentage  of  pure 
substances  in  any  commercial  article,  such  as  alkalies,  acids,  and 
various  salts,  by  means  of  systematic  normal  solutions  such  as 
have  been  described  are  these  — 

1.  With  normal  solutions  fa  or  -fa  (according  to  its  atomicity)  of 
the  molecular  weight  in  grams  of  the  substance  to  be  analysed  is  to 
be  weighed  for  titration,  and  the  number  of  c.c.  required  to  produce 
the  desired  reaction  is  the  percentage  of  the  substance  whose 
atomic  weight  has  been  used. 

With  decinormal  solutions  T^n  or  ^J^  of  the  molecular  weight  in 
grams  is  taken,  and  the  number  of  c.c.  required  will,  in  like  manner, 
give  the  percentage. 

Where  the  grain  system  is  used  it  will  be  necessary,  in  the  case 
of  titrating  with  a  normal  solution,  to  weigh  the  whole  or  half  the 
molecular  weight  of  the  substance  in  grains,  and  the  number  of 
decems  required  will  be  the  percentage. 

With  decinormal  solutions,  fa  or  -fa  of  the  molecular  weight  in 
grains  is  taken,  and  the  number  of  decems  will  be  the  percentage. 

It  now  only  remains  to  say,  with  respect  to  the  system  of  weights 
and  measures  to  be  used,  that  the  analyst  is  at  liberty  to  choose  his 
own  plan.  Both  systems  are  susceptible  of  equal  accuracy,  and  he 
must  study  his  own  convenience  as  to  which  he  will  adopt.  The 
normal  solutions  prepared  on  the  gram  system  are  equally  applicable 
for  that  of  the  grain,  and  vice  versa,  so  that  there  is  no  necessity 
for  having  distinct  solutions  for  each  system 

Factors,    or    Coefficients,   for   the   Calculation   of   Analyses.—  It 

frequently  occurs  that  from  the  nature  of  the  substance,  or  from  its 
being  in  solution,  this  percentage  method  cannot  be  conveniently 
followed.  For  instance,  suppose  the  operator  has  a  solution  con- 
taining an  unknown  quantity  of  caustic  potash,  the  strength  of  which 
he  desires  to  know  ;  a  weighed  or  measured  quantity  of  it  is  brought 
under  the  normal  acid  burette  and  exactly  saturated,  32  c.c.  being 
required.  The  calculation  is  as  follows  :  — 

The  molecular  weight  of  potassium  hydroxide  being  56'  11  : 
100  c.c.  of  normal  acid  will  saturate  5*611  -gm.  ;  therefore,  as  100  c.c. 


are  to  5-611  gm.,  so  are  32  c.c.  to  x,  ~  =  l-796  gm.  KH(X 

100 

The  simplest  way,  therefore,  to  proceed,  is  to  multiply  the  number 
of  c.c.  of  test  solution  required  in  any  analysis  by  the  y^nu  (or  TO%U 
if  bivalent)  of  the  molecular  weight  of  the  substance  sought,  which 
gives  at  once  the  amount  of  substance  present. 

An  example  may  be  given  —  1  gm.  of  marble  or  limestone  is  taken 
for  the  determination  of  pure  calcium  carbonate,  and  exactly 


32  VOLUMETRIC   PROCESSES. 

saturated  with  normal  nitric  or  hydrochloric  acid — (sulphuric  or 
oxalic  acid  is,  of  course,  not  admissible)  17*5  c.c.  are  required, 
therefore  17'5  xO'050  (the  -2000  °f  the  molecular  weight  of  CaCO3) 
gives  0*875  gm.  and  as  1  gm.  of  substance  only  was  taken  =  87*5  % 
of  calcium  carbonate 


ON    THE    DIRECT    AND    INDIRECT    PROCESSES    OF 
ANALYSIS    AND    THEIR    TERMINATION. 

THE  direct  method  includes  all  those  analyses  where  the  substance 
under  examination  is  decomposed  by  simple  contact  with  a  known 
quantity  or  equivalent  proportion  of  some  other  body  capable  of 
combining  with  it,  and  where  the  end  of  the  decomposition  is 
manifest  in  the  solution  itself. 

It  also  properly  includes  those  analyses  in  which  the  substance 
reacts  upon  another  body  to  the  expulsion  of  a  representative 
equivalent  of  the  latter,  which  is  then  determined  as  a  substitute 
for  the  thing  required. 

Examples  of  this  method  are  readily  found  in  the  process  for  the 
determination  of  iron  by  permanganate,  where  the  beautiful  rose 
colour  of  the  permanganate  asserts  itself  as  the  end  of  the  reaction. 

The  testing  of  acids  and  alkalies  comes,  also,  under  this  class,  the 
great  sensitiveness  of  litmus,  or  other  indicators,  causing  the  most 
trifling  excess  of  acid  or  alkali  to  alter  their  colour. 

The  indirect  method  is  exemplified  in  the  analysis  of  manganese 
ores,  and  also  other  peroxides  and  oxygen  acids,  by  boiling  with 
hydrochloric  acid.  The  chlorine  evolved  is  determined  as  the 
equivalent  of  the  quantity  of  oxygen  which  has  displaced  it.  We 
are  indebted  to  Bunsen  for  a  most  accurate  and  valuable  series  of 
processes  based  on  this  principle. 

The  residual  method  is  such  that  the  substance  to  be  analysed  is 
not  itself  determined,  but  the  excess  of  some  other  body  added  for 
the  purpose  of  combining  with  it  or  of  decomposing  it ;  and  the 
quantity  or  chemical  value  of  the  body  added  being  known,  and  the 
conditions  under  which  it  enters  into  combination  being  also  known, 
by  deducting  the  remainder  or  excess  (which  exists  free)  from  the 
original  quantity,  it  gives  at  once  the  proportional  quantity  of  the 
substance  sought. 

An  example  will  make  the  principle  obvious  : — Suppose  that 
a  sample  of  native  calcium  or  barium  carbonate  is  to  be  titrated. 
It  is  not  possible  to  determine  it  with  standard  nitric  or  hydrochloric 
acid  in  the  exact  quantity  it  requires  for  solution.  There  must  be 
an  excess  of  acid  and  heat  applied  also  to  get  it  dissolved  ;  if, 
therefore,  a  known  excessive  quantity  of  standard  acid  be  first 
added  and  solution  obtained,  and  the  liquid  then  titrated  back 
with  standard  alkali  and  an  indicator,  the  quantity  of  free  acid 
can  be  exactly  determined,  and  consequently  that  which  is  combined 
also. 


USE    OF   INDICATORS.  33 

In  some  analyses  it  is  necessary  to  add  a  substance  which  shall  be 
an  indicator  of  the  end  of  the  process  ;  such,  for  instance,  is  litmus 
or  the  azo  colours  in  alkalimetry,  potassium  chromate  in  silver  and 
chlorine,  and  starch  in  iodine  determinations. 

There  are  other  processes,  the  end  of  which  can  only  be  determined 
by  an  indicator  separate  from  the  solution  ;  such  is  the  case  in  the 
determination  of  iron  by  potassium  bichromate,  where  a  drop  of 
the  liquid  is  brought  into  contact  with  another  drop  of  solution  of 
potassium  ferricyanide  on  a  white  slab  or  plate  ;  when  a  blue  colour 
ceases  to  form  by  contact  of  the  two  liquids,  the  end  of  the  process 
is  reached. 


34 


INDICATORS. 


PART    II. 
ALKALIMETRY. 

GAY  LUSSAC  based  his  system  of  alkalimetry  upon  a  standard 
solution  of  sodium  carbonate,  with  a  corresponding  solution  of 
sulphuric  acid.  It  possesses  the  recommendation  that  a  pure 
standard  solution  of  sodium  carbonate  can  be  more  readily  obtained 
than  any  other  form  of  alkali.  Mohr  introduced  the  use  of  caustic 
alkali  instead  of  a  carbonate,  the  strength  of  which  is  established  by 
a  standard  solution  of  oxalic  or  sulphuric  acid.  The  advantage  in 
the  latter  system  is  that  in  titrating  acids  with  a  caustic  alkali  the 
well-known  interference  produced  in  litmus  by  carbonic  acid  is 
avoided  ;  this  difficulty  is  now  overcome  wherever  it  is  desired  by 
the  new  indicators  to  be  described. 


INDICATORS    USED    IN    ALKALIMETRY. 

1.  Litmus  Solution. — It  has  been  the 
custom  since  the  introduction  of  the  azo 
and  other  modern  indicators  to  regard 
litmus  as  old  fashioned  and  of  very 
doubtful  sensitiveness.  This  is  a  mistake, 
for  if  properly  prepared  it  is,  in  the 
absence  of  carbonic  acid,  one  of  the  most 
sensitive  of  the  indicators  used  for  alkalies. 
The  litmus  of  commerce  differs  consider- 
ably in  purity  and  colour,  but  a  careful 
examination  will  at  once  detect  a  good 
specimen  by  the  absence  of  a  greyish 
muddy  colour,  due  to  inert  matters,  both 
of  vegetable  and  mineral  nature.  The 
sensitive  colouring  matter  in  litmus  is 
azolitmin,  and  by  purifying  ordinary 
litmus,  as  described  below,  some  inter- 
fering bodies  are  removed,  with  the  result 
that  the  indicator  is  far  more  sensitive. 

A  simple  solution  may  be  made  by 
treating  the  cubes  repeatedly  with  small 

quantities  of  hot  water,  mixing  all  the  extracts,  and  allowing  the 
liquid  to  stand  in  a  covered  beaker  for  a  day  or  night.  The  clear 
blue  liquid  is  then  poured  off  and  placed  in  the  stock  bottle,  together 
with  two  or  three  drops  of  chloroform.  This  latter  agent  prevents 
the  development  of  bacteria,  and  if  the  bottle  is  simply  closed  with 
a  loose  cork,  through  which  the  delivery  pipette  is  passed,  the 
solution  will  keep  for  a  long  period.  If  the  colour  is  a  deep  blue 
it  must  be  modified  by  a  few  drops  of  weak  acid,  until  it  is  a  faint 


Fig.  27. 


LITMUS   SOLUTION.  35 

purple.  In  course  of  time  it  may  lose  its  colour,  but  this  may  be 
restored  by  simple  exposure  to  the  air  in  a  basin.  Another  method 
of  preparing  an  extract  of  litmus  in  a  concentrated  form  for 
dilution  whenever  required  is  as  follows.  Extract  all  soluble 
matters  from  the  solid  litmus  by  repeated  treatment  with  hot 
water  ;  evaporate  the  mixed  extracts  to  a  moderate  bulk,  and  add 
acetic  acid  in  slight  excess  to  decompose  carbonates  ;  evaporate  to 
a  thick  extract,  transfer  this  to  a  beaker,  and  add  a  large  proportion 
of  hot  85  per  cent,  alcohol  or  methylated  spirit.  By  this  treatment 
the  blue  colour  is  precipitated,  and  the  alkaline  acetates,  together 
with  some  red  colouring  matter,  remain  dissolved  ;  the  fluid  with 
precipita-te  is  thrown  on  a  filter,  washed  with  hot  spirit,  and  the 
purified  extract  finally  evaporated  to  a  paste,  which  is  placed  in 
a  wide-mouthed  bottle.  This  extract  will  keep  for  years  unchanged. 

Another  method  also  gives  good  results.  The  crushed  litmus  is 
extracted  with  warm  distilled  water,  as  before  described,  and  the 
several  extracts  mixed,  then  allowed  to  stand  in  a  beaker  till  quite 
clear.  This  clear  extract  is  poured  off,  strongly  acidified  with 
hydrochloric  acid,  and  put  in  a  dialyser,  which  is  surrounded  by 
running  water  and  kept  so  for  about  a  week.  The  colouring  matter 
of  litmus  being  a  colloid,  all  the  calcium  and  other  salts  are  removed, 
and  a  pure  colour  soluble  in  hot  distilled  water  remains,  which  may 
be  preserved  in  the  manner  previously  described,  or  evaporated  to 
a  soft  extract. 

A  sensitive  and  stable  solution  of  litmus  is  said  to  be  prepared 
as  follows  : — 

100  gm.  of  commercial  litmus  are  extracted  with  successive  quantities  of  hot 
water  until  the  united  extracts  measure  600  c.c.  The  extract  is  then  allowed 
to  settle,  preferably  at  a  low  temperature,  and  the  clear  solution  is  decanted 
and  evaporated  to  about  200  c.c.  After  filtration,  the  solution  is  diluted  with 
water  to  300  c.c.,  100  c.c.  of  16  per  cent,  sulphuric  acid  are  added,  and  the 
mixture  is  heated  on  a  water-bath  for  4  hours.  The  flocculent  precipitate  formed 
is  separated  by  filtration  and  washed  with  cold  water  until  all  sulphuric  acid 
has  been  removed  and  the  faintly  red  wash-water  becomes  bright  blue  when 
rendered  alkaline.  The  precipitate  on  the  filter  is  then  dissolved  in  about 
100  c.c.  of  hot  alcohol,  which  is  used  in  small  quantities  at  a  time  ;  a  few  drops 
of  ammonia  may  be  added  to  the  later  quantities  of  alcohol.  The  alcoholic 
solution  is  now  evaporated  to  dryness,  the  residue  dissolved  in  600  c.c.  of^hot 
water,  and  the  solution  neutralized  with  potash. 

Free  carbonic  acid  interferes  considerably  with  the  production  of 
the  blue  colour,  and  its  interference  in  titrating  acid  solutions  with 
alkali  carbonates  can  only  be  got  rid  of  by  boiling  the  liquid  during 
the  operation,  in  order  to  dispel  the  gas.  If  this  is  not  done,  it  is 
easy  to  overstep  the  exact  point  of  neutrality  in  endeavouring  to 
produce  the  blue  colour.  The  same  difficulty  is  also  found  in  obtain- 
ing the  pink-red  when  acids  are  used  for  titrating  alkali  carbonates, 
hence  the  value  of  the  caustic  alkali  solutions  free  from  carbonic 
acid  when  this  indicator  is  used. 

It  sometimes  occurs  that  titration  by  litmus  is  required  at  night. 

•A.  Puschel,  Oesterr.  Chem.-Zeit.,  1910,  13,  185. 

D    2 


36  LITMUS.       COCHINEAL. 

Ordinary  gas  or  lamp  light  is  not  adapted  for  showing  the  reaction  in 
a  satisfactory  manner  ;  but  a  very  sharp  line  of  demarcation  between 
red  and  blue  may  be  found  by  using  a  monochromatic  light.  With 
the  yellow  sodium  flame  the  red  colour  appears  perfectly  colourless, 
while  the  blue  or  violet  appears  like  a  mixture  of  black  ink  and 
water.  The  transition  is  very  sudden,  and  even  sharper  than  the 
change  by  daylight. 

The  operation  should  be  conducted  in  a  perfectly  dark  room  ; 
and  the  flame  may  be  obtained  by  heating  a  coiled  platinum  wire 
sprinkled  with  salt,  or  a  piece  of  pumice  saturated  with  a  concentrated 
solution  of  salt,  in  the  Bunsen  flame,  or  by  means  of  one  of  the 
sodium  lamps  now  on  the  market. 

According  to  R.  R  e  i  n  i  t  z  e  r*  litmus  solution  is  the  most  serviceable  indicator, 
excelling  methyl  orange  in  sharpness  of  change  of  colour  and  sensitiveness  (about 
8  times  as  great)  while  it  possesses  an  advantage  over  phenolphthalein  in  being 
capable  of  being  used  in  the  presence  of  ammonium  salts.  The  final  change  of 
colour  is  sharpest  when  the  liquid  to  be  titrated  is  boiled  for  seven  or  eight  minutes 
and  then  well  cooled.  In  order  to  avoid  the  influence  of  atmospheric  carbonic 
acid  it  should  not  be  allowed  to  stand  exposed  for  long,  and  dilution  with  unboiled 
distilled  water  should  be  avoided.  It  is  important  to  note  that  the  liquid  must 
be  cold  when  titrated.  Lungef  admits  that  litmus,  when  prepared  and  used 
according  to  Reinitzer's  directions,  is  more  sensitive  than  methyl  orange,  but 
found  it  to  be  only  twice  as  sensitive.  With  normal  acid  practically  identical 
results  are  obtained,  but  methyl  orange  is  preferable  on  account  of  its  speed  and 
the  precautions  to  be  observed  in  the  use  of  litmus.  With  semi-normal  acid  the 
change  of  colour  is  more  difficult  to  observe  in  the  case  of  methyl  orange,  but  a 
practised  observer  can  be  sure  to  a  drop.  It  is  only  with  N/iO  acid  that  litmus  is 
undoubtedly  superior,  and  Reinitzer's  method  of  titration  must  be  observed. 

2.  Litmus   Paper. — Is   made   by   dipping   strips   of  calendered 
unsized  paper  in  the  solution  and  drying  them  ;  the  solution  used 
being  rendered  blue,  red,  or  violet  as  may  be  required. 

3.  Cochineal  Solution. — This  indicator  .possesses  the  advantage 
over  litmus  that  it  is  not  so  much  modified  in  colour  by  the  presence 
of  carbonic  acid  and  may  be  used  by  gas-light.     It  may  also  be  used 
with  the  best  effect  with  solutions  of  the  alkaline  earths,  such  as 
lime  and  baryta  water  ;  the  colour  with  pure  alkalies  and  earths  is 
especially  sharp  and  brilliant.     The  solution  is  made  by  digesting 
1  part  of  crushed  cochineal  with  10  parts  of  25  per  cent,  alcohol. 
Its  natural  colour  is  yellowish-red,  which  is  turned  to  violet  by 
alkalies  ;  mineral  acids  restore  the  original  colour.     It  is  not  so 
easily  affected  by  weak  organic  acids  as  litmus,  and  therefore  for 
these  acids  the  latter  is  preferable.     It  cannot  be  used  in  the 
presence  of  even  traces  of  iron  or  alumina  compounds  or  acetates, 
which  facet  limits  its  use.     Carminic   acid  is   the  true  sensitive 
element  in  cochineal,  and  it  is  sometimes  preferable  to  cochineal  in 
very  delicate  operations. 

4.  Turmeric   Paper. — Pettenkofer,   in   his  determination   of 
carbonic  acid  by  baryta  water,  prefers  turmeric  paper  as  an  in- 
dicator.    For  this  purpose  it  is  best  prepared  by  digesting  pieces 

*  See  The  Analyst,  1894,  p.  255.          t  Ibid,  1895,'p.  65. 


TURMERIC   PAPER.  37 

of  the  root,  first  in  repeated  small  quantities  of  water  to  remove 
a  portion  of  objectionable  colouring  matter,  then  in  alcohol,  and 
dipping  strips  of  calendered  unsized  paper  into  the  alcoholic  solution, 
drying  and  preserving  them  in  the  dark.  Or  one  part  of  powdered 
turmeric  may  be  digested  for  about  an  hour  with  six  parts  of  weak 
alcohol  (3  volumes  of  alcohol  to  1  of  water)  in  a  covered  flask,  with 
occasional  shaking.  The  filtered  liquid  (tincture  of  turmeric)  is 
used  to  prepare  turmeric  paper.  Both  liquid  and  paper  should  be 
protected  from  light. 

Thomson,  in  continuance  of  his  valuable  experiments  on  various 
indicators,  found  that  turmeric  paper  is  of  very  little  use  for 
ammonia,  or  the  alkali  carbonates,  or  sulphides  or  sulphites,  but 
he  prepared  a  special  paper  of  a  light  red-brown  colour  by  dipping 
it  into  tincture  of  turmeric  rendered  slightly  alkaline  by  caustic  soda. 
If  this  paper  is  wetted  with  water  the  colour  is  intensified  to  a  dark 
red-brown  ;  when  partly  immersed  in  a  very  dilute  solution  of  an 
acid,  the  wetted  portion  becomes  bright  yellow,  while  immediately 
above  this  a  moistened  dark  red-brown  band  is  formed  and  the 
upper  dry  portion  retains  its  original  colour.  This  appearance  only 
occurs  in  the  titration  of  a  comparatively  large  proportion  of  an 
acid,  when  the  latter  is  nearly  all  neutralized,  and  thus  serves  to 
indicate  the  near  approach  to  the  end-reaction.  When  neutral  or 
alkaline,  the  colour  of  the  immersed  portion  of  paper  is  simply 
intensified  as  already  described.  This  intensification  is  quite  as 
decided  as  a  change  of  tint.  This  red-brown  paper  is  as  sensitive 
as  phenolphthalein  for  the  titration  of  citric,  acetic,  tartaric,  oxalic 
and  other  organic  acids  by  standard  soda  or  potash,  and  may  be 
used  for  highly  coloured  solutions.  It  is  also  available,  like 
phenolphthalein,  for  the  determination  of  small  quantities  of  acid 
in  strong  alcohol. 


Indicators  derived  from  the  Azo  Colours,  etc. 

A  great  stride  has  been  taken  in  the  application  of  these  modern 
indicators,  and  the  thanks  of  all  chemists  are  due  to  R.  T.  Thomson 
for  his  valuable  researches  on  them,  read  before  the  Chemical 
Section  of  the  Philosophical  Society  of  Glasgow,  and  published  in 
their  Transactions.*  The  experiments  recorded  in  these  papers  are 
carefully  carried  out,  and  the  truthfulness  of  their  results  has  been 
verified  by  Lunge  and  other  practical  men  as  well  as  by  myself. 

Space  will  only  permit  here  of  a  record  of  the  results,  fuller  details 
being  given  in  the  publications  to  which  reference  has  been  made. 

5.  Methyl  Orange,  or  sodium  dimethylamido-azo-benzene-sulpho- 
nate,  is  prepared  by  the  action  of  diazotized  sulphanilic  acid  upon 
dimethylaniline,  the  commercial  product  being  the  sodium  salt  of 
the  sulphonic  acid  thus  produced.  If  carefully  prepared  from  the 

*  Reprinted  C.  N.  47,  123,  185  ;  49,  32,  119 ;  J.  S.  C.  I.  6,  195. 


38  METHYL  ORANGE.   PHENACETOLIN. 

purest  materials  it  possesses  a  bright  orange-red  colour,  and  is 
perfectly  soluble  in  water  ;  but  the  commercial  product  is  often  of 
a  dull  colour,  due  to  slight  impurities  in  the  substances  from  which 
it  is  produced,  and  not  completely  soluble  in  water.  These  im- 
purities may  generally  be  removed  by  recrystallization  from  hot 
alcoholic  solution.  Complaints  have  been  made  by  some  operators 
that  the  commercial  article  is  sometimes  unreliable  as  an  indicator  ; 
it  may  be  so,  but,  although  I  have  examined  many  specimens,  I  have 
not  yet  found  any  in  which  the  impurities  sensibly  affected  its 
delicate  action  when  used  in  the  proper  manner.  The  common 
error  is  the  use  of  too  much  of  it ;  again,  there  is  the  personal  error 
of  observation,  some  eyes  being  much  more  sensitive  to  the  change 
of  tint  than  others.  The  great  value  of  this  indicator  is  that,  unlike 
litmus  and  some  other  agents,  it  is  comparatively  unaffected  by 
carbonic  acid,  sulphuretted  hydrogen,  hydrocyanic,  silicic,  boric, 
arsenious,  oleic,  stearic,  palmitic,  carbolic  acids,  etc.  It  must 
not  be  used  for  the  organic  acids,  such  as  oxalic,  acetic,  citric, 
tartaric,  etc.,  since  the  end-reaction  is  indefinite  ;  nor  can  it  be  used 
in  the  presence  of  nitrous  acid  or  nitrites,  which  decompose  it.* 
It  may  safely  be  used  for  the  determination  of  free  mineral  acids  in 
alum,  ferrous  sulphate  or  chloride,  zinc  sulphate,  cupric  sulphate 
or  chloride.  The  acid  radical  (and  consequently  its  equivalent 
metal)  in  cupric  sulphate  and  similar  salts  may  be  determined  with 
accuracy  by  precipitating  the  solution  with  sulphuretted  hydrogen, 
filtering,  and  titrating  the  filtrate  at  once  with  normal  alkali  and 
methyl  orange. 

Methyl  orange  is  especially  useful  for  the  accurate  standardizing 
of  any  of  the  mineral  acids  by  means  of  pure  sodium  carbonate  in  the 
cold,  the  liberated  carbonic  acid  having  practically  no  effect,  as  is 
the  case  with  many  indicators.  Its  effect  is  also  excellent  with 
ammonia  or  its  salts.  A  convenient  strength  for  the  indicator  is 
I  gram  of  the  powder  in  a  litre  of  distilled  water  ;  a  single  drop  of 
the  liquid  is  sufficient  for  100  c.c.  of  any  colourless  solution — the 
colour  being  faint  yellow  if  alkaline,  and  pink  if  acid  ;  if  too  much  is 
used  the  end-reaction  is  slower  and  much  less  definite.  All  titrations 
with  methyl  orange  should  be  carried  on  at  ordinary  temperatures 
if  the  utmost  accuracy  is  desired,  and  the  liquid  titrated  should  not 
be  too  dilute  Ethyl  orange  is  similar  to  the  methyl  orange,  but 
is  not  quite  so  sensitive. 

6.  Phenacetolin. — This  indicator  is  slightly  soluble  in  water  but 
readily  in  50  per  cent,  alcohol,  and  a  convenient  strength  is  2  gm. 
per  litre.  The  solution  is  greenish  brown,  giving  a  scarcely 
.perceptible  yellow  with,  caustic  soda  or  potash  when  a  few  drops 
are  used  with  ordinary  volumes  of  liquid.  With  ammonia  and 

*  Some  operators  have  used  methyl  orange  in  the  titration  of  alkaloids,  but  in 
a  series  of  very  careful  experiments,  carried  out  by  L.  F.  Kebler,  it  was  found  in 
some  cases  very  defective  (Jour.  Am.  Chem.  Soc.  Oct.,  1895).  Probably  the  most 
useful  indicator  for  alkaloids  is  Brazilin,  or  a  decoction  of  Brazil-wood;  inapplicable 
in  the  presence  of  sulphuretted  hydrogen  or  sulphurous  acid. 


PHENOLPHTHALEIN.  39 

the  normal  alkali  carbonates  it  gives  a  dark  pink,  with  bicarbonate 
a  much  more  intense  pink,  and  with  mineral  acids  a  golden  yellow. 
This  indicator  may  be  used  to  determine  the  amount  of  caustic 
potash  or  soda  in  the  presence  of  their  normal  carbonates  if  the 
proportion  of  the  former  is  not  very  small,  or  of  caustic  lime  in  the 
presence  of  carbonate,  but  no  ammonia  must  be  present. 

Practice,  however,  is  required  with  solutions  of  known  composition, 
so  as  to  acquire  knowledge  of  the  exact  shades  of  colour. 

7.  Phenolphthalein  (C20H1404). — This  indicator  is  of  a  resinous 
nature,  but  quite  soluble  in  50  per  cent,  alcohol.  A  convenient 
strength  is  5  gm.  per  litre. 

A  few  drops  of  the  indicator  show  no  colour  in  ordinary 
volumes  of  neutral  or  acid  liquids  ;  the  faintest  excess  of  caustic 
alkalies,  on  the  other  hand,  gives  a  sudden  change  to  purple-red. 

This  indicator  is  useless  for  the  titration  of  free  ammonia  or  its 
compounds,  or  for  the  fixed  alkalies  when  salts  of  ammonia  are 
present ;  except  with  alcoholic  solutions,  in  which  case  caustic  soda 
and  potash  displace  the  ammonia  in  equivalent  quantities  at 
ordinary  temperatures,  and  the  indicator  forms  no  compound  with 
the  ammonia. 

It  may,  however,  be  used  like  phenacetolin  for  determining  the 
proportions  of  hydrate  and  carbonate  of  soda  or  potash  in  the  same 
sample  where  the  proportion  of  hydrate  is  not  too  small.  Unlike 
methyl  orange,  this  indicator  is  especially  useful  in  titrating  all 
varieties  of  organic  acids;  viz.,  oxalic,  acetic,  citric,  tartaric,  etc. 

One  great  advantage  possessed  by  phenolphthalein  is  that  it  may 
be  used  in  alcoholic  solutions,  or  mixtures  of  alcohol  and  ether,* 
and  therefore  many  organic  acids  insoluble  in  water  may  be 
accurately  titrated  by  its  help  ;  in  addition  to  this  it  may  be  used 
to  determine  the  acid  combined  with  many  organic  bases,  such  as 
morphine,  quinine,  brucine,  etc.,  the  base  having  no  effect  on  the 
indicator. 

Reinitze»f  found  that  phenolphthalein  was  three  times  more 
sensitive  in  a  cold  solution  than  in  a  hot  one. 

The  following  substances  can  be  determined  by  standard  alcoholic 
potash,  with  phenolphthalein  as  indicator.     One  c.c.  normal  caustic 
potash  (1  c.c.  =  '056  gm.  KHO)  is  equal  to — (Hehner  and  Allen) 
•088  gm.  butyric  acid.  '1007  gm.  tributyrin. 

•282    „     oleic  acid.  -2947    „    triolein. 

•256    ,,    palmitic  acid.  -2687    ,,    tripalmitin, 

•284    „     stearic  acid.  -2967    ,,    tristearin. 

•410    ,,     cerotic  acid.  *6760    ,,    myricin. 

•329    ,,     resin  acids  (ordinary  colophony,  chiefly  sylvic  acid). 

*  H.  N.  and  C.  Draper  (C.  N.  55,  143)  have  shown  that  this  indicator  is  rapidly 
decomposed  by  atmospheric  carbonic  acid,  which  is  more  readily  absorbed  by  alcohol 
than  by  water.  Fortunately  this  is  less  the  case  with  hot  solutions  than  with  cold ; 
titration  of  this  kind  should  therefore  be  quickly  done,  and  with  not  too  small 
a  quantity  of  the  indicator. 

tSee  The  Analyst,  1894,  p.  256. 


40  OTHER   INDICATORS. 

Mixture  of  Phenolphthalein  and  Methyl  Orange. — This  is 
a  neutrality  indicator  made  by  dissolving  5  gm.  of  the  first  and  1  gm. 
of  the  latter  in  a  litre  of  50  per  cent,  alcohol.  The  neutral  point  is 
shown  by  a  lemon-yellow  colour.  The  slightest  excess  of  either 
acid  or  alkali  shows  in  turn  pink  and  bright  red. 

8.  Rosolic  Acid,  Corallin,  or  Aurine  (C20H16O3)  is  soluble  in  60  per 
cent,  alcohol,  and  a  convenient  strength  is  2  gm.  per  litre.     Its 
colour  is  pale  yeUow,  unaffected  by  acids,  but  turning  to  violet-red 
with  alkalies.     It  possesses  the  advantage  over  litmus  and  the  other 
indicators  that  it  can  be  relied   upon  for  the  neutralization  of 
sulphurous  acid  with  ammonia  to  normal  sulphite  (Thomson). 
Its  delicacy  is  sensibly  affected  by  salts  of  ammonia  and  by  carbonic 
acid.     It  is  excellent  for  all  the  mineral,  but  useless  for  the  organic 
acids,  excepting  oxalic. 

9.  Lacmoid. — This  indicator  is  a  product  of  resorcin,   and  is 
therefore  somewhat  allied  to  litmus  ;  nevertheless,  it  differs  from  it 
in  many  respects,  and  has  a  pronounced  and  valuable  character  of 
its  own,  especially  when  used  in  the  form  of  paper.     Lacmoid  is 
soluble  in  dilute  alcohol,  and  the  indicator  is  made  by  dissolving 
3  gm.  to  the  litre.* 

10.  Lacmoid    Paper. — This   is   prepared    by    dipping    slips    of 
calendered  unsized  paper  into  the  blue  or  red  solution,  and  drying 
them. 

Thomson  states  that,  in  nearly  every  particular,  lacmoid  paper, 
either  blue  or  red,  is  an  excellent  substitute  for  methyl  orange,  and 
may  be  employed  in  titrating  coloured  solutions  where  the  latter 
would  be  useless.  Solution  of  lacmoid,  on  the  other  hand,  is  not 
so  valuable  as  the  paper,  inasmuch  as  it  is  more  easily  affected  by 
weak  acids  such  as  carbonic,  boric,  etc.  See  p.  42. 

There  is  a  large  number  of  other  indicators,  either  in  solution  or 
as  test  papers,  and  where  necessary  these  will  be  mentioned. 

11.  Congo  Red. — This  is  specially  useful  in  determining  free 
mineral  acids  in  the  presence  of  most  organic  acids.     A  solution 
of  1  part  of  the  indicator  in  100  parts  of  10  per  cent,  alcohol  is  used. 
It  is  red  in  alkaline  solution,  turning  blue  with  excess  of  acid. 

12.  Extra  Sensitive  Indicators. — Mylius  and  For  sterf  describe 
a  series  of  experiments  on  the  determination  of  minute  traces  of 
alkali  and  the  recognition  of  the  neutrality  of  water  by  means  of 
an    ethereal    solution    of   iodeosin    or    erythrosin.     This    body    is 
a  derivative  of  fluorescin,  and  occurs  plentifully  in  commerce  as 
a  dye  for  fabrics  and  paper.     The  commercial  material  is  purified 

*  This  solution  is  rendered  much  more  effective  as  an  indicator  if  Forster's  sug- 
gestion is  adopted,  namely,  the  addition  of  about  5  gm.  of  napthol  green  to  a  litre  of 
the  solution*  The  effect  is  to  produce  a  more  decided  blue  colour  with  alkalies  than 
is  given  by  lacmoid  alone. 

t  Berichte,  24,  1482 ;  also  C.  N.  64,  228,  ef  seq. 


METHYL   BED.  41 

by  solution  in  aqueous  ether,  and  the  filtered  solution  is  shaken 
with  dilute  caustic  soda,  which  removes  the  colour  ;  the  latter  is 
then  precipitated  with  stronger  alkali.  The  salt  is  then  filtered  off, 
washed  with  spirit  and  finally  recrystallized  from  hot  alcohol. 
The  indicator  used  by  the  operators  was  made  by  dissolving  1  deci- 
gram of  the  dry  powder  in  a  litre  of  aqueous  pure  ether.  The  ether 
of  commerce  is  purified  and  rendered  neutral  by  washing  with  weak 
alkali,  afterwards  with  pure  water,  and  keeping  the  ether  over  pure 
water.  The  indicator  so  prepared  is  quite  useless  for  the  ordinary 
titration  of  acids  and  alkalies  ;  its  chief  use  is  for  the  detection  and 
measurement  of  very  minute  proportions  of  alkali  such  as  would 
occur  in  water  kept  in  glass  vessels,  or  the  solubility  of  calcium  or 
other  earthy  carbonates  in  water  free  from  carbonic  acid,  or  in  the 
use  of  millinormal  solutions  of  alkalies  and  acids,  also  the  neutrality 
of  so-called  pure  salts  or  water.  The  method  of  using  the  indicator 
is  to  shake  up,  say,  20  c.c.  with  100  c.c.  of  the  liquid  to  be  examined, 
when,  if  alkali  is  present,  a  rose  colour  will  be  communicated  to  the 
layer  of  ether  which  rises  to  the  top.  The  indicator  may  be  used 
in  conjunction  with  millinormal  standard  solutions,  or  colori- 
metrically,  like  the  well-known  Nessler  test.  Further  details  of 
its  use  are  described  in  the  contributions  mentioned.  Another 
similar  indicator  is  mentioned  by  Ruhemann,  viz.,  the  imide  of 
dicinnamylphenylazimide.*  This  material  gives  a  violet  rose  colour 
with  the  most  minute  traces  of  alkali,  such,  for  instance,  as  would 
appear  from  merely  heating  alcohol  in  a  test  tube, — the  faint  trace  of 
alkali  thus  derived  from  the  glass  being  sufficient  to  cause  a  rapid 
development  of  colour. 

Another  indicator  highly  sensitive  to  alkalies  has  been  introduced 
by  Rupp  and  Loose,|  viz.,  p-dimethylamino-azobenzene-o-car- 
boxylic  acid,  which  they  term  "  methyl  red."  It  is  faintly  yellow 
in  alkaline  and  neutral  solutions,  and  violet-red  in  acid  solutions. 
A  0*2  per  cent  solution  in  alcohol  forms  the  indicator.  It  can  be 
used  to  titrate  ammonia,  even  at  centinormal  strength,  as  well  as 
some  alkaloids,  e.g.,  quinine. 

SHORT  SUMMARY  OF  THOMSON'S  RESULTS  WITH  INDI- 
CATORS AND  PURE  SALTS  OF  THE  ALKALIES  AND 
ALKALINE  EARTHS. 

The  whole  of  the  base  or  acid  in  the  following  list  of  substances 
may  be  determined  with  delicacy  and  precision  unless  otherwise 
mentioned. 

Litmus  Cold. — Hydrates  of  soda,  potash,  ammonia,  lime,  baryta, 
etc.  ;  arsenites  of  soda  and  potash,  and  silicates  of  the  same  bases  ; 
nitric,  sulphuric,  hydrochloric,  and  oxalic  acids. 

*J.  C.  S.  Trans.  61,  285. 
t  Ber  1908,  41,  3905,  and  Analyst,  34,  29. 


42  APPLICABILITY    OF   INDICATORS. 

Litmus  Boiling. — The  neutral  and  acid  carbonates  of  potash,  soda, 
b'me,  baryta,  and  magnesia,  the  sulphides  of  sodium  and  potassium, 
and  silicates  of  the  same  bases. 

Methyl  Orange  Cold. — The  hydrates,  carbonates,  bicarbonates, 
sulphides,  arsenites,  silicates,  and  borates  of  soda,  potash,  ammonia, 
lime,  magnesia,  baryta,  etc.  ;  all  the  mineral  acids,  sulphites,  half  the 
base  in-  the  alkaline  and  alkaline  earthy  phosphates  and  arseniates. 

Rosolic  Acid  Cold. — The  whole  of  the  base  or  acid  may  be 
determined  in  the  hydrates  of  potash,  soda,  ammonia,  and  arsenites 
of  the  same  ;  the  mineral  acids  and  oxalic  acid. 

Rosolic  Acid  Boiling. — The  alkaline  and  earthy  hydrates  and 
carbonates,  bicarbonates,  sulphides,  arsenites,  and  silicates. 

Phenacetolin  Cold. — The  hydrates,  arsenites,  and  silicates  of  the 
alkalies  ;  the  mineral  acids. 

Phenacetolin  Boiling. — The  alkaline  and  earthy  hydrates,  car- 
bonates, bicarbonates,  sulphides,  arsenites,  and  silicates 

Phenolphthalein  Cold. — The  alkaline  hydrates,  except  ammonia  ; 
the  mineral  acids,  oxalic,  citric,  tartaric,  acetic,  and  other  organic 
acids. 

Phenolphthalein  Boiling. — The  alkaline  and  earthy  hydrates, 
carbonates,  bicarbonates,  and  sulphides,  always  excepting  ammonia 
and  its  salts. 

Lacmoid  Cold. — The  alkaline  and  earthy  hydrates,  arsenites  and 
borates,  and  the  mineral  acids.  Many  salts  of  the  metals  which 
are  more  or  less  acid  to  litmus  are  neutral  to  lacmoid,  such  as  the 
sulphates  and  chlorides  of  iron,  copper,  and  zinc  ;  therefore  this 
indicator  serves  for  determining  free  acids  in  such  solutions. 

Lacmoid  Boiling. — The  hydrates,  carbonates,  and  bicarbonates  of 
potash,  soda,  and  alkaline  earths. 

Lacmoid  Paper. — The  alkaline  and  earthy  hydrates,  carbonates, 
bicarbonates,  sulphides,  arsenites,  silicates,  and  borates  ;  the  mineral 
acids  ;  half  of  the  base  in  sulphites,  phosphates,  arseniates. 

This  indicator  reacts  alkaline  with  the  chromates  of  potash  and 
soda,  but  neutral  with  the  bicarbonates,  so  that  a  mixture  of  the 
two,  or  of  bichromates  with  free  chromic  acid,  may  be  titrated  by 
its  aid,  which  could  also  be  done  with  methyl  orange  were  it  not  for 
the  colour  of  the  solutions. 

General  Characteristics  of  the  Foregoing  Indicators. 

It  is  interesting  to  notice  the  different  degrees  of  sensitiveness 
shown  by  indicators  used  in  testing  acids  and  alkalies.  This  is  well 


SENSITIVENESS   OF   INDICATORS.  43 

illustrated  by  Thomson's  experiments,  where  he  used  solutions  of 
the  indicator  containing  a  known  weight  of  the  solid  material,  and 
so  adjusted  as  to  give,  as  near  as  could  be  judged,  the  same  intensity 
of  colour  in  the  reaction. 

It  was  found  that  lacmoid,  rosolic  acid,  phenacetolin,  and 
phenolphthalein  were  capable  of  showing  the  change  of  colour  with 
one-fifth  of  the  quantity  of  acid  or  alkali  which  was  required  in  the 
case  of  methyl  orange  or  litmus  ;  that  is  to  say,  in  100  c.c.  of  liquid, 
where  the  latter  took  0'5  c.c.,  the  same  effect  with  the  former  was 
observed  with  O'l  c.c. 

Another  important  distinction  is  shown  in  their  respective 
behaviour  with  mineral  and  organic  acids. 

It  is  true  the  whole  of  them  are  alike  serviceable  for  the  mineral 
acids  and  fixed  alkalies  ;  but  they  differ  considerably  in  the  case  of 
the  organic  acids  and  ammonia.  Methyl  orange  and  lacmoid  appear 
to  be  most  sensitive  to  alkalies,  while  phenolphthalein  is  most 
sensitive  to  acids  ;  the  others  appear  to  occupy  a  position  between 
these  extremes,  each  showing,  however,  special  peculiarities.  The 
distinction,  however,  is  so  marked,  that,  as  Thomson  says,  it  is 
possible  to  have  a  liquid  which  may  be  acid  to  phenolphthalein  and 
alkaline  to  lacmoid. 

The  presence  of  certain  neutral  salts  has,  too,  a  definite  effect  on 
the  sensitiveness  of  certain  indicators.  Sulphates,  nitrates, 
chlorides,  etc.,  retard  the  action  of  methyl  orange  slightly,  while  in 
the  case  of  phenacetolin  and  phenolphthalein  they  have  no  effect. 
On  the  other  hand,  neutral  salts  of  ammonia  have  such  a  disturbing 
influence  on  the  last  named  indicator  as  to  render  it  useless,  unless 
special  precautions  be  taken. 

Nitrous  acid  alters  the  composition  of  methyl  orange  ;  so  also  do 
nitrites  when  existing  in  any  quantity.  Forbes  Carpenter  has 
noted  this  effect  in  testing  the  exit  gases  of  vitriol  chambers.* 

Sulphites  of  the  fixed  alkalies  and  alkaline  earths  are  practically 
neutral  to  phenolphthalein,  but  alkaline  to  litmus,  methyl  orange, 
and  phenacetolin. 

Sulphides,  again,  can  be  accurately  titrated  with  methyl  orange 
in  the  cold,  and  on  boiling  off  the  H2S  a  tolerably  accurate  result 
can  be  obtained  with  litmus  and  phenacetolin,  but  with 
phenolphthalein  the  neutral  point  occurs  when  half  the  alkali  is 
saturated.  The  phosphates  of  the  alkalies,  arseniates,  and  arsenites 
also  vary  in  their  effects  on  the  various  indicators. 

Thomson  classifies  the  usual  neutrality  indicators  into  three 
groups.  The  methyl  orange  group,  comprising  that  substance, 
together  with  lacmoid,  dimethylamidobenzene,  cochineal  and  Congo 
red  ;  the  phenolphthalein  group,  consisting  of  itself  and  turmeric  ; 
and  the  litmus  group, including  litmus,  rosohc  acid,  and  phenacetolin. 
The  methyl  orange  group  are  most  susceptible  to  alkalies^  the 
phenolphthalein  to  acids,  and  the  litmus  intermediate  between  the 
two.  This  classification  has  nothing  to  do  with  delicacy  of  reaction. 

*  J.  S.  C.  I.  5,  287. 


44 


THOMSON'S  OBSERVATIONS. 


but  with  the  special  behaviour  of  the  indicator  under  the  same 
circumstances  ;  for  instance,  saliva,  which  is  generally  neutral  to 
litmus  paper,  is  always  strongly  alkaline  to  lacmoid  or  Congo  red, 
and  acid  to  turmeric  paper.  Fresh  milk  reacts  in  very  much  the 
same  way.  Such  substances  are  termed  amphoteric. 

Thomson  gives  the  following  table  as  an  epitome  of  the 
results  obtained  with  indicators,  on  which  several  processes  have 
been  based.  The  figures  refer  to  the  number  of  atoms  of  hydrogen 
displaced  by  the  monatomic  metals,  sodium  or  potassium,  in  the 
form  of  hydrates.  Where  a  blank  is  left  it  is  meant  that  the  end- 
reaction  is  obscure.  The  figures  apply  also  to  ammonia,  except 
where  phenolphthalein  is  concerned  and  when  boiling  solutions  are 
used.  Calcium  and  barium  hydrates  give  similar  results,  except 
where  insoluble  compounds  are  produced.  Lacmoid  paper  acts  in 
every  respect  like  methyl  orange,  except  that  it  is  not  affected  by 
nitrous  acid  or  its  compounds.  Turmeric  paper  behaves  exactly 
like  phenolphthalein  with  the  mineral  acids  and  also  with  thio- 
sulphuric  and  organic  acids. 


Acids. 

Methyl  Orange. 

Phenolphthalein  . 

Litmus. 

Name. 

Formula. 

Cold. 

Cold. 

Boilintr 

Cold. 

Boiling. 

Sulphuric     . 

H2S04 

2 

2 

2 

2 

2 

Hydrochloric 

HC1 

1 

1 

1 

1 

1 

Nitric     .      . 

HNC-3 

1 

1 

1 

1 

1 

Thiosulphuric 

H2S203 

2 

2 

2 

2 

2 

Carbonic 

H2C03 

0 

1  dilute 

0 

— 

0 

Sulphurous 

H2S03 

1 

2 

— 

j     

Hydrosulphuric 

H2S 

0 

1  dilute 

0 

0 

Phosphoric 

H3P04 

1 

2 

— 

—           — 

Arsenic  .         •    * 

H3As04 

1 

2 

— 

—         !      — 

Arsenious 

H3As03 

0 

— 

— 

0             0 

Nitrous 

HN02 

indicator  destroyed 

1 



1 

Silicic 

H4Si04 

0 

— 

0             0 

Boric 

H3B03 

0 

— 

— 

—       •     — 

Chromic           • 

H2Cr04 

1 

2 

2 

—           — 

Oxalic 
Acetic             •  .  , 

H2C204 
HC2H302 

z 

2 

1 

2 

2 
1  nearly 

L 

Butyric 
Succinic 
Lactic    . 

HC4H702 
H2C4H404 
HC3H503 

— 

1 
2 
1 

— 

1  nearly 
2  nearly 

Tartaric 

H2C4H406 

— 

2 

2 

Citric      . 

H3C6H507 

— 

3 

—     •       —       |     — 

A.  H.  Allen  clearly  points  out  that  the  acid  which  enters  into 
the  composition  of  an  indicator  must  be  weaker  than  the  acid  which 
it  is  required  to  determine  by  its  means.  The  acid  of  which  methyl 
orange  is  a  salt  is  a  tolerably  strong  one,  since  it  is  only  completely 
displaced  by  the  mineral  acids  ;  the  organic  acids  are  not  strong 
enough  to  overpower  it  completely,  hence  the  uncertainty  of  the 
end-reaction.  The  still  weaker  acids,  such  as  carbonic,  hydrocyanic, 
boric,  oleic,  etc.,  do  not  decompose  the  indicator  at  all,  hence  their 
salts  may  be  titrated  by  it  just  as  if  the  bases  only  were  present. 


CLASSIFICATION    OF   INDICATORS.  45 

On  the  other  hand  the  acid  of  phenolphthalein  is  extremely  weak, 
hence  its  salts  are  easily  decomposed  by  the  organic  and  carbonic 
acids.  A  combination  of  the  two  indicators  is  frequently  of  service  ; 
say  for  instance,  in  a  mixture  of  normal  and  acid  sodium  carbonates. 
If  first  titrated  with  phenolphthalein  and  standard  mineral  acid, 
the  rose  colour  disappears  exactly  at  the  point  when  the  normal 
carbonate  is  saturated  ;  the  bicarbonate  can  then  be  found  by 
continuing  the  operation  with  methyl  orange.  The  study  of  these 
new  indicators  is  still  somewhat  imperfect  and  requires  further 
elucidation ;  more  especially  if  we  take  into  consideration  some  new- 
aspects  of  the  question  mentioned  in  a  paper  by  R.  T.  Thomson*. 
The  experiments  there  recorded,  which  are  too  voluminous  to 
reproduce  here,  are  of  a  very  interesting  character  and  point  to  the 
conclusion  that  molecular  condition,  viscosity  of  the  liquid,  or 
some  such  influence  was  at  work,  so  as  to  modify  very  considerably 
the  action  of  the  indicator.  The  irregularities  occurring  in  the  cases 
mentioned  are  no  doubt  exceptional,  and  need  not  disturb  the 
faith  hitherto  reposed  in  well-known  and  much-used  methods  of 
titration. 

The  particular  indicator  whose  erratic  action  was  under  discussion 
was  phenolphthalein,  and  it  was  demonstrated  that  in  using  this 
indicator  in  the  titration  of  boric  acid  with  soda  no  satisfactory  end- 
reaction  could  be  got  in  a  merely  aqueous  solution,  but  that  by  the 
addition  of  not  less  than  30  per  cent,  of  glycerin  to  the  mixture 
a  perfectly  correct  determination  could  be  made.  Other  substances 
such  as  starch,  glucose,  and  cane  sugar  had  a  similar  effect,  but 
not  to  the  same  extent  as  glycerin.  Mannitol  acts  just  as  well. 

The  result  of  these  investigations  is  to  give  a  fairly  satisfactory 
method  of  determining  volumetrically  boric  acid  existing  in  its 
natural  compounds  and  as  a  preservative  in  various  kinds  of  food. 

An  excellent  classification  of  the  more  modern  indicators  as  well 
as  those  previously  described  is  given  by  F.  Glaser.f 

GROUP  I.  (sensitive  to  alkalies). 
Tropaeolin  00. 

Methyl-  and  Ethylorange,  Dimethylamido-azobenzene. 
Congo  Red,  Benzopurpurin,  lodo-esoin,  Cochineal. 
Lacmoid. 

GROUP  II. 

Fluorescein,  Phenacetolin. 
Alizarin,  Orseille,  Haematoxylin,  Gallein. 
Litmus. 

p-Nitrophenol,  Guaiacum  tincture. 
Rosblio  Acid. 

GROUP  III.  (sensitive  to  acids). 

Tropaeolin  000. 

Phenolphthalein,  Turmeric,  Curcumin  W.  Flavescin. 

a-Naphtholbenzein. 

Poirrier's  Blue  C4B. 

*  J.  S.  C.  1. 12,  432. 
\Z.  a\C.,  1899,  273-8;  J.  S.  C.  /.,  1899,  708. 


46  THEORY    OF   INDICATORS. 

The  above  indicators  are  all  either  of  acid  or  saline  nature,  and  the  classification 
is  based  upon  the  strength  of  the  acid  radicle  contained  in  each.  Members  of 
Group  I.  are  of  strong  acid  nature,  consequently  they  react  readily  with  bases 
forming  stable  salts  ;  they  are  not  sensitive  to  weak  acids.  The  acid  character  of 
the  indicators  in  Group  III.  is  only  weakly  marked,  consequently  they  are  but 
slightly  sensitive  to  bases ;  their  salts  are  unstable  and  easily  decomposed  by 
acids.  Members  of  Group  II.  are  intermediate  in  character  between  those  of 
Groups  I.  and  III.  The  table  ib  so  drawn  up  that  the  sensitiveness  of  the 
successive  indicators  to  alkalies  decreases  as  the  sensitiveness  to  acids  increases. 

The  knowledge  of  the  position  of  an  indicator  is  of  importance  when  bodies  are 
titrated  whose  basic  or  acid  character  is  not  well  marked,  e.g.,  the  salts  of  the 
mineral  acids  with  alumina,  carbonates,  silicates,  etc.  Further,  the  table  enables 
us  to  determine  to  some  extent  the  nature  and  strength  of  an  acid  or  base  by 
titrating  it  with  the  help  of  different  indicators,  e.g.,  if  one  acid  can  be  readily 
titrated  with  the  help  of  either  lacmoid  or  litmus,  and  another  only  with  the 
latter,  then  the  two  acids  must  be  of  different  strengths. 

When  titrating  formic  acid,  lacrnoid  is  a  fairly  good  indicator,  but  litmus  is 
better ;  with  acetic  acid  a  member  of  Group  III.  must  be  used.  Here  we  have 
a  confirmation  of  the  fact  that  among  homologous  organic  acids  with  the  same 
number  of  carboxyl  group?  the  acid  character  diminishes  with  increasing  molecular 
weight. 

In  titrating  the  alkalies  the  rule  holds  good  that  an  indicator  only  shows  the  end 
of  a  reaction  sharply  when  the  product  of  the  change  is  neutral.  The  change  of 
colour  is  only  sharp  when  strong  fixed  bases  are  used  ;  ammonium  salts  being 
readily  hydrolysed  by  the  water  present.  When  very  dilute  solutions  of  the 
fixed  bases  are  used,  the  colour  change  is  often  not  sharp ;  this  is  due  more  to 
the  hydrolytic  action  of  the  water  on  the  indicator  than  on  the  salt  formed. 

Hydrolytic  changes  in  presence  of  indicators  of  Group  III.  are  frequently 
ascribed  to  the  influence  of  C02  in  the  air.  The  author  shows  experimentally 
that  the  fading  of  the  colour  of  a  weak  alkaline  solution  containing  phenolph- 
thalein  is  due  more  to  the  hydrolytic  action  of  the  water  present  than  to 
atmospheric  C02 

For  Ostwald's  lonization  theory  of  Indicators  see  bis 
"  Scientific  Foundations  of  Analytical  Chemistry,"  translated  by 
Me  Go  wan.  A.  A.  Noyes*  has  published  a  mathematical  treat- 
ment of  the  theory  of  indicators  as  applied  to  volumetric 
analysis.  See  also  J.  T.  Hewittf  on  the  constitution  of 
indicators. 


PREPARATION    OF    THE    NORMAL    ACID    AND 
ALKALI    SOLUTIONS. 

IT  is  quite  possible  to  carry  out  the  titration  of  acids  and  alkalies 
with  only  one  standard  liquid  of  each  kind  ;  but  it  frequently 
happens  that  standard  acids  or  alkalies  are  required  in  other 
processes  of  titration  besides  mere  saturation,  and  it  is  therefore 
advisable  to  have  a  variety. 

Above  all  things  it  is  absolutely  necessary  to  have  at  least  one 
standard  acid  and  one  standard  alkali  prepared  with  the  most 
scrupulous  accuracy  to  use  as  foundations  for  all  others. 

•  J.  A.  C.  S.  1910,  32,  815-861,  and  J.  S.  C.  I.,  29,  1039  (abstr.). 
1  Analyst,  1908,  W,  85. 


STANDARDIZATION    OF   NORMAL   SOLUTIONS.  47 

It  is  preferable  to  use  sulphuric  acid  for  the  normal  acid  solution, 
inasmuch  as  there  is  no  difficulty  commercially  in  obtaining  the 
purest  acid.  The  normal  acid  made  with  it  is  totally  unaffected 
by  boiling,  even  when  of  full  strength,  which  cannot  be  said  of 
either  nitric  or  hydrochloric  acid.  Hydrochloric  acid  is,  however, 
generally  preferred  by  alkali  makers,  owing  to  its  giving  soluble 
compounds  with  lime  and  similar  bases.  Nitric  and  oxalic  acids  are 
also  sometimes  convenient. 

Sodium  carbonate,  on  the  other  hand,  is  to  be  preferred  for  the 
standard  alkali,  because  it  may  readily  be  prepared  in  a  pure  state, 
or  may  easily  be  made  from  pure  sodium  bicarbonate  as  described 
further  on.  Differences  of  opinion  exist  among  chemists  as  to  the 
best  substances  to  be  used  as  standards  in  preparing  the  various 
solutions  .used  in  alkalimetry  and  acidimetry.  My  experience 
satisfies  me  that,  although  many  of  these  modifications  may  serve 
very  well  as  controls,  there  is  no  more  reliable  standard  than  pure 
sodium  carbonate. 

The  chief  difficulty  with  sodium  carbonate  is  that,  with  litmus  as 
indicator,  the  titration  must  be  carried  on  at  a  boiling  heat  in  order 
to  get  rid  of  CO2,  which  obscures  the  pure  blue  colour  of  the 
indicator,  notwithstanding  the  alkali  may  be  in  great  excess.  This 
difficulty  is  now  set  aside  by  the  use  of  methyl  orange.  In  case  the 
operator  has  not  this  indicator  at  hand,  litmus  or  phenolphthalein 
give  perfectly  accurate  results  if  the  saturation  is  first  conducted 
by  rapid lv  boiling  the  liquid  for  a  minute  after  each  addition  of 
acid,  until  the  point  is  reached  when  one  drop  of  acid  in  excess  gives 
a  change  of  colour  which  is  not  altered  by  further  boiling.  This  is 
used  as  a  preliminary  test,  but  as  titrations  are  usually  conducted 
at  ordinary  temperatures  the  final  adjustment  should  be  made  by 
adding  in  the  second  trial  a  moderate  excess  of  the  acid,  then 
boiling  to  get  thoroughly  rid  of  C02,  rapidly  cooling  the  liquid,  and 
titrating  back  with  an  accurate  standard  alkali.  A  slight  calculation 
will  then  give  the  figures  for  adjustment.  If  great  accuracy  is 
required  this  boiling  must  not  take  place  in  glass  flasks,  but  in 
vessels  of  porcelain,  platinum,  or  silver,  as  even  Jena  glass  is 
affected  by  boiling  alkaline  solutions. 

As  has  previously  been  said,  these  two  standard  acid  and  alkali 
solutions  must  be  prepared  with  the  utmost  care,  since  upon  their 
correct  preparation  and  preservation  depends  the  verification  of 
other  standard  solutions. 

The  best  method  of  preparing  sodium  carbonate  for  standardizing 
sulphuric  or  hydrochloric  acid  is  to  half  fill  a  platinum  basin  with 
pure  sodium  bicarbonate  in  powder  (NaHCO3).  Place  it  in  an  air 
bath  already  heated  to  about  200°  C.,  and  raise  the  temperature  to 
270-80°,  but  not  more  than  300°  C.  Let  it  remain  at  this  temperature 
for  half  an  hour,  then  cool  it  in  an  exsiccator,  and  before  it  is  quite 
cold  transfer  it  to  a  warm,  dry,  stoppered  tube  or  bottle,  out  of 
which,  when  cold,  it  may  be  weighed  rapidly  as  wanted.  The 
carbonate  so  produced  will  be  free  from  lumps  and  easily  soluble  in 


48  STANDARDS    FOR   ACIDIMETRY. 

cold  distilled  water.  To  standardize  the  diluted  acid,  about  2  or,,  3 
grams  of  carbonate  should  be  quickly  weighed,  dissolved  in  about 
80  or  100  c.c.  of  water,  2  drops  of  methyl  orange  solution  added, 
and  the  operation  completed  by  running  the  acid  of  unknown 
strength,  in  small  quantities  at  a  time,  from  a  burette  divided  into 
i^  c.c.  into  the  soda  solution  until  exact  saturation  is  effected. 

A  second  trial  should  now  be  made,  but  preferably  with  a  different 
weight  of  the  salt.  The  saturation  is  carried  out  precisely  as  at  first. 
The  data  for  ascertaining  the  exact  strength  of  the  acid  solution  by 
calculation  are  now  in  hand. 

A  strictly  normal  acid  should  at  15°  C.  exactly  saturate  sodium 
carbonate  in  the  proportion  of  100  c.c.  to  5*3  gm. 

Suppose  that  2-46  gm.  carbonate  required  41 -5  c.c.  of  the  acid  in 
the  first  experiment,  then 

V 
2-46  :  5-3  :  :  41-5  :  z  =  89'4  c.c. 

Again  :  2*153  gm.  carbonate  required  36'3  c.c.  of  acid,  then 
2-153  :  5-3  :  :  36'3  :  a;  =  89-4  c.c. 

The  acid  may  now  be  adjusted  by  measuring  890  c.c.  into  the 
graduated  litre  cylinder,  adding  4  c.c.  from  the  burette,  or  with 
a  small  pipette,  and  filling  to  the  litre  mark  with  distilled  water. 

Finally,  the  strength  of  the  acid  so  prepared  must  be  proved  by 
taking  a  fresh  quantity  of  sodium  carbonate,  or  by  titration  with 
a  strictly  normal  sodium  carbonate  solution  previously  made,  and 
using  not  less  than  50  c.c.  for  the  titration,  so  as  to  avoid  as  much  as 
possible  the  personal  errors  of  measurement  in  small  quantities.  If 
the  measuring  instruments  all  agree,  and  the  operations  are  all 
conducted  with  due  care,  a  drop  or  two  in  excess  of  either  acid  or 
alkali  in  50  c.c.  should  suffice  to  reverse  the  colour  of  the  indicator. 

The  adoption  of  sodium  carbonate  as  a  standard  for  preparing 
normal  acid  solutions  is  strongly  recommended  in  Lunge's  Report 
to  the  International  Congress  of  Applied  Chemistry.  *  From  the  acid 
so  standardized  a  corresponding  normal  caustic  alkali  may  be  made, 
and  from  that  a  normal  oxalic  acid,  and  from  that  a  decinormal 
permanganate  solution. 

Potassium  bi-iodate,  KH  (I03)2,  an  acid  salt,  which  can  easily  be  obtained 
in  a  state  of  great  purity,  has  been  recommended  for  standardizing  normal 
potassium  hydroxide,  using  phenolphthalein  as  indicator.  3 '8995  grams 
dissolved  in  water  require  exactly  10  c.c.  of  the  alkali  solution. 

Sodium  oxalate,  prepared  as  recommended  by  S6rensen,f  is  a  reliable 
and  accurate  standard  for  acidimetry.  It  is  converted  into  sodium  carbonate 
by  moderate  ignition.  .^ 

C.  L.  Higgins  J  has  discussed  the  preparation  of  an  exact  standard  acid  in 
a  practical  manner. 

*  ZeUs.  1.  angew.  Chem.,  1903,  24,,  560,  also  Analyst,  28,  307. 

t  Z.  a.  C.,  1 903,  42,  333,  and  Analyst  1903,  306. 

U.S.C.  I.,  1900,19,958. 


NORMAL   ALKALIES    AND    ACIDS.  49 

^ 

1.     Normal  Sodium  Carbonate. 
53  gm.  Na2CO3  per  litre. 

This  solution  is  made  by  dissolving  crystals  of  pure  sodium 
carbonate,  then  ascertaining  its  strength  by  a  correct  normal  acid 
at  15°,  and  adjusting  its  volume  so  that  it  corresponds  exactly  with 
the  normal  acid.  Absolutely  pure  anhydrous  sodium  carbonate  is 
difficult  to  find  in  commerce,  and  even  if  otherwise  pure  is  generally 
contaminated  with  insoluble  dust  contracted  in  the  process  of 
drying. 

2.     Normal  Potassium  Carbonate. 
69-1  gm.  K2CO3  per  litre. 

This  solution  is  sometimes,  though  rarely,  preferable  to  the  soda 
salt,  and  is  of  service  for  the  determination  of  qombined  acids  in 
certain  cases  where,  by  boiling  with  this  reagent,  an  interchange 
of  acid  and  base  takes  place. 

It  cannot  be  prepared  by  direct  weighing  of  the  potassium 
carbonate,  and  is  therefore  best  made  by  titrating  a  solution  of 
unknown  strength  with  strictly  normal  acid  and  adjusting  as 
described  above. 

3.     Normal  Sulphuric  Acid. 
49-043  gm.  H2SO4  per  litre,    t 

About  30  c.c.  of  pure  sulphuric  acid  of  sp.  gr.  l'S40,  or  thereabouts, 
are  mixed  with  three  or  four  times  the  volume  of  distilled  water  and 
allowed  to  cool,  then  put  into  the  graduated  cylinder  and  diluted  up 
to  about  a  litre  at  the  proper  temperature.  The  solution  may  now 
be  titrated  with  sodium  carbonate,  as  previously  described,  and 
accurately  adjusted. 

Or  thus :  Suppose  that  two  litres  of  normal  sulphuric  acid  are  required. 
Counterbalance  a  beaker  with  shot  on  a  rough  balance  and  weigh  into  it  104  grams 
of  concentrated  sulphuric  acid.  This  is  easily  done  by  putting  a  5  gm.  weight 
on  the  pan  with  the  beaker  and  pouring  the  acid  in  a  thin  stream  till  it  is  in  excess 
of  the  weight,  then  removing  the  5  gm.  weight  and  adding  the  acid  drop  by  drop 
till  it  is  the  correct  weight  or  a  little  more.  Then  pour  it  into  a  litre  flask  containing 
about  500  c.e.  of  distilled  water,  rinse  the  beaker  several  times  with  water,  adding 
the  rinsings  to  the  flask,  and  after  shaking  the  flask  allow  it  to  stand  several  hours 
till  it  has  acquired  the  temperature  of  the  laboratory.  Now  make  up  to  the  mark 
with  distilled  water,  mix,  and  transfer  to  the  stock  bottle.  Fill  up  the  litre  flask 
with  distilled  water  and  pour  this  also  into  the  stock  bottle.  Mix  thoroughly 
and  we  have  a  solution  rather  stronger  than  normal.  Next  weigh  out  TOG  grams  " 
of  sodium  carbonate  and  dissolve  in  about  30  c.c.  of  water  in  a  Jena  beaker.  Fill 
a  burette  with  the  acid  and  run  carefully  into  the  soda  solution,  to  which  a  dropv 
or  two  of  methyl  orange  has  been  added,  until  a  pink  colour  remains  after  well 
stirring.  Let  18'9  c.c.  of  the  acid  be  required  (instead  of  20  for  normal).  Then 
the  solution  requires  the  addition  of  Tl  c.c.  of  water  for  every  18'9  c.c.  It 
measures  2000  -18'9  =  (say)  1981  c.c. 


50  NORMAL   SULPHURIC   ACID. 

.'.  18-9  :   1981=1-1  :  x. 
x  =115-3 

Now,  as  an  approximation  to  correct  strength  with  a  certainty  of  being  on  the 
right  side,  pour  carefully  110  c.c.  of  water  into  the  stock  bottle,  run  the  liquid  from 
the  burette  into  it,  mix  thoroughly,  fill  up  the  burette  twice  and  then  titrate  again, 
using  2' 12  grams  of  the  alkali  and  running  the  acid  into  it  slowly.  Suppose  that 
exactly  39'9  c.c.  are  required  (instead  of  40).  The  solution  now  measures  1981  + 
110—40=2051  c.c.  and  the  amount  of  water  to  be  added  is 

39-9  :  2051  =0'1  :  x. 
x=5'l  c.c. 

Add  this  quantity  of  water  to  the  bulk,  mix  thoroughly  as  before,  and  the 
solution  should  be  of  exactly  normal  strength,  and  should  be  proved  to  be  so  by 
a  final  titration  with  2*12  grams  of  sodium  carbonate  as  before. 

In  the  foregoing  directions  for  the  preparation  of  standard  acid 
and  alkali  it  is  evident  that,  with  the  exception  of  control  by  the 
rather  doubtful  method  of  precipitation  of  the  sulphuric  acid  by 
barium,  the  responsibility  for  an  accurate  acid  solution  is  thrown 
upon  the  sodium  carbonate,  and  though  my  experience  has  been 
that  with  proper  care  this  is  quite  reliable,  it  is  plain  that  any  other 
means  of  getting  at  the  accurate  strength  of  the  sulphuric  acid  will 
be  acceptable.  This  is  now,  owing  to  the  elaborate  and  careful 
experiments  of  Pickering  on  the  specific  gravities  of  solutions  of 
sulphuric  acid  of  various  strengths,  rendered  quite  possible.* 

It  is  true  that  the  conditions  under  which  the  working  strength 
of  the  acid  is  obtained  are  very  stringent,  and  need  the  utmost 
care  in  performance,  but  of  the  extreme  accuracy  of  the  result 
there  is  no  shadow  of  doubt. 

We  are  indebted  to  A.  Marshall,  who  has  made  use  of 
Pickering's  figures  to  calculate  a  formula  and  tables,  which  may 
be  used  for  making  up  standard  solutions  of  sulphuric  acid  with  great 
accuracy  and  ease.  Pickering's  percentages  are  based  upon  the 
freezing  points  of  concentrated  sulphuric  acid,  and  they  are  accurate 
within  0*01  per  cent.  As  practically  no  volumetric  method  can  be 
relied  upon  within  less  than  O'l  per  cent,  this  leaves  an  ample 
margin.  Consideration  of  the  figures  shows  that  the  strength  of  the 
acid  can  be  determined  with  the  necessary  accuracy  with  least 
difficulty  when  the  acid  contains  from  60  to  85  per  cent,  of  H2S04. 
Between  these  limits  an  error  of  O'OOl  in  the  specific  gravity  or  of  1°  C. 
in  the  temperature  will  introduce  an  error  of  about  0-14  per  cent, 
in  the  amount  of  acid,  whereas  outside  the  above  limits  the  error 
introduced  may  be  many  times  as  great. 

METHOD  OF  PROCEDURE  :  Highly  pure  sulphuric  acid  should  be  taken  and 
diluted  with  water  (preferably  by  adding  the  acid  to  the  water).  Cool  the 
mixture  to  a  convenient  temperature  and  then  determine  its  specific  gravity. 
The  temperature  must  be  known  within  0*5°  C.  and  the  specific  gravity  within 
0-0005.  If  a  Sprengel  tube  of  25  c.c.  capacity  be  used,  the  weighings  must  be 
correct  within  O'Ol  gm.  The  percentage  of  H2S04  in  the  acid  is  then  given  by 
the_forinula 

*  J.  C.  S.  Trans.,  1890.  64-184. 


STANDARDIZATION    BY    SPECIFIC   GRAVITY. 


51 


P  =D  (85-87  +'05  T  -'0004  t-)  -69'80 

where  P  =per  cent,  of  H2S04  in  the  acid 

and  D  =  density  of  the  acid  at  T°  C.  referred  to  water  at  t°  C. 
The  above  formula  may  be  used  for  any  temperatures  from  0°  to  40°  C.  'and 
for  acid  containing  62  to  82  per  cent,  of  H2S04.     The  percentages  given  by  it 
are  correct  within  +  *1  per  cent.  ^ 

The  weight  of  acid  required  for  the  preparation  of  the  standard  solution  can 
now  be  calculated. 

Let  A  =grams  of  H2S04  per  litre  in  the  required  solution 

and  n  —  number  of  litres  required 

and  W=  weight  of  the  acid  which  must  be  weighed  out 

Then 

™r         A      10° 
W  =n  A  x-p 

Weigh  out  W  grams  of  the  acid  and  make  it  up  to  n  litres. 

The  percentages  given  by  the  above  empirical  formula  are  quite  accurate 
enough  for  all  ordinary  purposes ;  the  maximum  error  which  could  be  introduced 
by  employing  them  is  about  1  in  1,500.  More  accurate  values  may,  however, 
be  obtained  from  Tables  I.  and  II.  ;  if  great  care  be  exercised,  the  error  in  the 
percentage  need  not  then  exceed  1  in  7,000.  The  weights,  on  which  these  tables 
were  based,  were  fully  corrected  for  air  displacement.  The  weights  of  acid  and 
water  contained  by  the  pyknometer,  or  Sprengel  tube,  must  therefore  be  similarly 
corrected.  Unless  a  very  high  degree  of  accuracy  be  aimed  at,  this  correction 
may  be  made  by  subtracting  O'OOl  from  the  uncorrected  specific  gravity  found. 

The  table  at  18°  C.  (Table  II.)  is  slightly  more  reliable  than  that  at  15°  C. 
(Table  I.),  as  18°  was  one  of  the  temperatures  at  which  Pickering  actually 
determined  the  densities.* 


TABLE  I. 

For  ascertaining  the  Percentage  Strength  of  Sulphuric  Acid  Solutions 
from  the  Specific  Gravities  at  15°  C.  (Water  at  15°  C.=l). 


!  Specific 
Gravity 

0. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

•60 

68-72 

68-80 

68-89 

68-97 

69-06 

69-15 

69-23 

69-32 

69-40 

69-49 

•61 

69-58 

69-66 

69-75 

69-84 

69-92 

70-01 

70-09 

70-18 

70-26 

70-35 

•62 

70-43 

70-52 

70-60 

70-69 

70-77 

70-86 

70-94 

71-03 

71-11 

71-20 

•63 

71-28 

71-37 

71-45 

71-54 

71-62 

71-71 

71-80 

71-88 

71-97 

72-05 

•64 

72-13 

72-22 

72-30 

72-39 

72-47 

72-56 

72-64 

72-73 

72-81 

72-90 

•65 

72-98 

73-07 

73-15 

73-24 

73-32 

73-41 

73-49 

73-57 

73-66 

73-74 

•66 

73-83 

73-91 

74-00 

74-08 

74-17 

74-25 

74-34 

74-42 

74-51 

74-59 

•67 

74-68 

74-76 

74-85 

74-93 

75-02 

75-10 

75-19 

75-27 

75-36 

75-44 

•68 

75-52 

75-61 

75-69 

75-78 

75-86 

75-95 

76-03 

76-12 

76-21 

76-29 

•69 

76-38 

76-47 

76-55 

76-64 

76-72 

76-81 

76-90 

76-98 

77-07 

77-15 

•70 

77-24 

77-33 

77-41 

77-50 

77-59 

77-67 

77-76 

77-84 

77-93 

78-02 

•71 

78-10 

78-19 

78-28 

78-36 

78-45 

78-53 

78-62 

78-71 

78-79 

78-88 

1-72 

78-97 

79-05 

79-14 

79-22 

79-31 

79-40 

79-48 

79-57 

79-65 

79-74 

1-73 

79-83 

79-91 

80-00 

80-09 

"80-18 

80-27 

80-37 

80-46 

80-55 

80-64 

1-74 

80-73 

80-82 

80-91 

81-00 

81-10 

81-19 

81-28 

81-37 

81-46 

81-55 

1-75 

81-64 

81-73 

81-83 

81-98 

82-01 

82-11 

82-21 

82-30 

82-40 

82-50 

1-76 

82-60 

82-70 

82-79 

82-89 

82-99 

83-09 

83-19 

83-28 

83-38 

83-48 

1-77 

83-58 

83-68 

83-77 

83-87 

83-97 

84-07 

84-16 

84-26 

84-36 

84-45 

*  The  figures  given  in  these  tables  are  only  applicable  to  pure  sulphuric  acid. 

E    '2 


52 


STANDARDIZATION    BY    SPECIFIC    GRAVITY. 


TABLE  II. 

For  ascertaining  the  Percentage  Strength  of  Sulphuric  Acid  Solutions 
from  the  Specific  Gravities  at  18°  C.  (Water  at  18°  C.=l). 


Specific 
Gravity 

0. 

1. 

2. 

3. 

4. 

5. 

6. 

7. 

8. 

9. 

1-60 

68-89 

68-97 

69-06 

69-15 

69-23 

69-32 

69-40 

69-49 

69-58 

69-66 

1-61 

69-75 

69-83 

69-92 

70-01 

70-09 

70-18 

70-26 

70-35 

70-43 

70-52 

1-62 

70-61 

70-69 

70-78 

70-86 

70-95 

71-03 

71-12 

71-20 

71-29 

71-37 

1-63 

71-46 

71-55 

71-63 

71-72 

71-80 

71-89 

71-97 

72-06 

72-14 

72-23 

•64 

72-31 

72-40 

72-48 

72-57 

72-65 

72-74 

72-82 

72-91 

72-99 

73-08 

•65 

73-16 

73-25 

73-33 

73-42 

73-50 

73-59 

73-67 

73-76 

73-84 

73-93 

•66 

74-01 

74-10 

74-18 

74-27 

74-35 

74-44 

74-52 

74-61 

74-69 

74-78 

•67 

74-86 

74-95 

75-03 

75-12 

75-20 

75-29 

75-37 

75-46 

75-54 

75-63 

•68 

75-71 

75-80 

75-88 

75-97 

76-05 

76-14 

76-22 

76-31 

76-40 

76-48 

•69 

76-57 

75-65 

76-74 

76-82 

76-91 

76-99 

77-08 

77-17 

77-25 

77-34 

•70 

77-42 

77-51 

77-59 

77-68 

77-77 

77-85 

77-94 

78-03 

78-11 

78-20 

•71 

78-29 

78-37 

78-46 

78-55 

78-63 

78-72 

78-81 

78-90 

78-98 

79-07 

•72 

79-16 

79-24 

79-33 

79-42 

79-51 

79-59 

79-68 

79-77 

79-85 

79-94 

•73 

80-03 

80-12 

80-21 

80-30 

80-39 

80-48 

80-57 

80-66 

80-75 

80-84 

•74 

80-93 

81-02 

81-12 

81-21 

81-30 

81-40 

81-49 

81-58 

81-67 

81-76 

•75 

81-86 

81-95 

82-04 

82-14 

82-24 

82-34 

82-44 

82-53 

82-63 

82-72 

1-76 

82-82 

82-92 

83-02 

83-13 

83-23 

83-32 

83-42 

83-52 

83-62 

83-72 

1-77 

83-82 

84-92 

84-02 

84-12 

84-22 

84-33 

84-43 

84-54 

84-65 

84-77 

Using  these  tables  I  have  found  that  a  normal  acid  of  great 
accuracy  may  readily  be  prepared,  and  the  strong  solution  may  be 
kept  intact  in  strength  if  placed  in  a  well-stoppered  bottle  so  as  to 
preserve  it  from  damp  air.  The  fact  that  the  concentrated  acid  is 
weighed,  and  not  measured,  is  an  additional  security,  and  the 
weighing  may  take  place  within  a  large  range  of  temperatures 
without  any  practical  loss  of  accuracy. 

As  a  check  to  the  normal  solution  thus  made  I  have  used  pure 
sodium  carbonate  prepared  and  weighed  with  the  utmost  care,  and 
titrated  by  the  help  of  methyl  orange,  with  the  most  satisfactory 
results.* 


4.    Normal  Oxalic  Acid. 
63-024  gm.  C204H2,2H20,  or  45-01  gm.  C204H2  per  litre. 

This  solution  cannot  very  well  be  correctly  prepared  by  direct 
weighing,  owing  to  uncertain  hydration  of  the  crystallized  acid  ; 
hence  it  must  be  titrated  by  normal  alkali  of  known  accuracy,  using 
phenolphthalein  as  indicator. 

The  solution  is  apt  to  deposit  some  of  the  acid  at  low  temperatures, 
but  keeps  fairly  well  if  preserved  from  direct  sunlight,  and  will  bear 
heating  without  volatilizing  the  acid.  Very  dilute  solutions  of 

*  For  valuable  tables  giving  the  relation  of  specific  gravity  to  percentage  for 
solutions  of  sulphuric,  hydrochloric,  and  oxalic  acids,  see  a  paper  by  Wordon  and 
Motion,  J.  S.  C.  I.  1905,  24,  178. 


NORMAL   ACID   SOLUTIONS.  53 

oxalic  acid  are  unstable  ;  therefore,  if  a  decinormal  or  centinormal 
solution  is  at  any  time  required  it  should  be  made  when  wanted. 

5.     Normal  Hydrochloric  Acid. 
36-47  gm.  HC1  per  litre. 

It  has  been  shown  by  Roscoe  and  Dittmar*  that  a  solution  of 
hydrochloric  acid  containing  20 '2  per  cent,  of  the  gas  when  boiled 
at  about  760  mm.  pressure  loses  acid  and  water  in  the  same 
proportion,  and  the  residue  will  therefore  have  the  constant  compo- 
sition of  20-2  per  cent.,  or  a  specific  gravity  of  TIO.  About  181  gm. 
of  acid  of  this  density,  diluted  to  one  litre,  serves  very  well  to  form 
an  approximately  normal  acid.  Or  120  grams,  or  105  c.c.,  of  the 
ordinary  strong  hydrochloric  acid,  which  contains  about  one-third 
of  its  weight  of  the  gas,  may  be  diluted  with  water  to  one  litre, 
which  will  then  form  a  solution  somewhat  stronger  than  normal. 
This  is  now  titrated  with  sodium  carbonate  exactly  as  described  in 
section  3,  p.  49,  and  diluted  to  exact  strength. 

The  actual  strength  may  be  determined  by  precipitation  with  silver 
nitrate,  or  by  titration  with  an  exactly  weighed  quantity  of  pure 
sodium  carbonate  or  pure  anhydrous  calcium  carbonate  (Iceland 
Spar).  Hydrochloric  acid  is  useful  on  account  of  its  forming 
soluble  compounds  with  the  alkaline  earths,  but  it  has  the  dis- 
advantage of  volatilizing  at  a  boiling  heat.  Dittmar  says  that 
this  may  be  prevented  by  adding  a  few  grains  of  sodium  sulphate. 
In  many  cases  this  would  be  inadmissible,  for  the  same  reason  that 
sulphuric  acid  cannot  be  used.  The  hydrochloric  acid  from  which 
standard  solutions  are  made  must  be  free  from  chlorine  gas  or 
metallic  chlorides,  and  should  leave  no  residue  when  evaporated  in 
a  platinum  vessel. 

G.  T.  Moodyf  describes  a  method  of  preparing  an  accurate 
standard  acid  which  consists  in  passing  .gaseous  HC1  into  water  and 
weighing  the  amount  absorbed.  This  requires  a  rather  delicate 
arrangement  of  apparatus,  but  it  is  undoubted^  capable  of  great 
accuracy  when  properly  carried  out. 

6.     Normal  Nitric  Acid. 

63-02  gm.  HN03  per  litre. 

A  rigidly  exact  normal  acid  should  be  prepared  by  means  of 
sodium  or  calcium  carbonate,  as  in  the  case  of  normal  hydrochloric 
acid. 

The  nitric  acid  used  should  be  colourless,  free  from  chlorine  and 
nitrous  acid,  sp.  gr.  about  1*3.  If  coloured  from  the  presence  of 
nitrous  or  hyponitric  acid  (nitrogen  peroxide),  it  should  be  mixed 
with  two  volumes  of  water  and  boiled  until  colourless.  When  cold 

*  J.  C.  S.  12, 128,  1860.  t  J.  C.  8   Trans.  1898,  658. 


54  NORMAL  ALKALI  SOLUTIONS. 

it  may  be  diluted  and  titrated  as  previously  described  for  sulphuric 
acid. 

Also,  93^grams  or  65  c.c.  of  the  ordinary  strong  nitric  acid  (sp.  gr. 
1*42)  diluted  to  one  litre  gives  a  solution  rather  stronger  than 
normal.  This  is  then  titrated  with  sodium  carbonate  exactly  as 
described  in  section  3,  p.  49,  and  diluted  to  exact  strength. 

7.    Normal" Alkali  Hydroxides. 
Caustic  Soda,  or  Potash. 

40-01  gm.  NaHO  or  56- 11  gm.  KHO  per  litre. 

Pure  sodium  hydroxide  made  from  metallic  sodium  may  now  be 
readily  obtained  in  commerce,  and  also  powdered  caustic  soda  of 
98/99  %  purity,  and  a  standard  solution  may  be  prepared  from 
either  of  these.  Weigh  out  rapidly  about  42  grams,  dissolve  in 
water,  make  up  to  a  litre,  and  pour  into  the  stock  bottle.  When 
quite  cold  fill  a  burette  with  the  solution  and  titrate  with  20  c.c. 
of  normal  sulphuric  acid  to  which  a  drop  or  two  of  methyl  orange 
has  been  added.  The  solution  will  be  slightly  strong  and  should 
be  carefully  adjusted  by  adding  water  till  of  correct  strength. 
The  final  adjustment  should  be  made  by  running  it  into  50  c.c.  of 
normal  acid.  To  make  normal  potassium  hydroxide  weigh  out 
58/grams  for  a  litre. 

However  pure  caustic  soda  or  potash  may  otherwise  be,  they  are 
both  in  danger  of  absorbing  carbonic  acid,  and  hence  in  using  litmus 
or  phenolphthalein  the  titration  must  be  conducted  with  boiling. 
Methyl  orange  permits  the  use  of  these  solutions  at  ordinary 
temperature  notwithstanding  the  presence  of  C02. 

Sodium  and  potassium  hydroxides  may  now  both  be  obtained  in 
commerce  sufficiently  pure  for  all  ordinary  titration  purposes,  and 
their  solutions  may  be  freed  from  traces  of  chlorine,  sulphuric, 
silicic,  and  carbonic  acids,  by  shaking  with  Mill  on '  s  base,  trimercur- 
ammonium.*  Carbonic  acid  may  also  be  removed  by  the  cautious 
addition  of  barium  hydrate  in  solution,  shaking  well,  and  then 
after  settling  clear  ascertaining  the  exact  strength  with  correct 
standard  acid. 

In  preparing  these  alkali  solutions,  they  should  be  exposed  as 
little  as  possible  to  the  air.  and  when  the  strength  is  finally 
determined,  should  be  preserved  in  a  bottle  similar  to  that  shown 
in  fig.  24,  or  in  full  bottles  having  their  glass  stoppers  slightly 
greased  with  vaseline. 

v 

8.    Semi-normal  Ammonia. 
8-517  gm.  NH3  per  litre. 

This  strength  of  standard  ammonia  is  useful  for  saturation 
analyses  in  some  cases  ;  it  is  cleanly,  does  not  readily  absorb  carbonic 

*  C.  N.  42,  8. 


NORMAL  ALKALI  SOLUTIONS.  55 

acid,  holds  its  strength  well  when  kept  in  a  cool  place  and  well 
stoppered,  but  is  liable  to  develop  flocculent  growths.  It  may, 
however,  be  prepared  in  a  few  minutes  by  simply  diluting  strong 
liquid  ammonia  with  freshly  distilled  water.  An  approximate 
solution  may  be  made  with  about  28  c.c.  of  *880  ammonia  to 
the  litre. 

A  normal  solution  cannot  be  used  with  safety,  owing  to 
evaporation  of  the  gas  at  ordinary  temperatures.* 

9.     Decinormal  Barium  Hydroxide. 

This  solution  is  best  made  from  the  crystallized  hydroxide  approxi- 
mately of  N/10  strength.  This  is  done  by  shaking  up  in  a  stoppered 
bottle  powdered  crystals  of,  barium  hydroxide  with  distilled  water, 
and  allowing  it  to  stand  a  day  or  two  until  quite  clear.  There  should 
be  an  excess  of  the  hydrate,  in  which  case  the  clear  solution,  when 
poured  off  into  a  stock  bottle  fitted  with  a  tube  to  prevent  the 
entrance  of  CO2  (see  fig.  24)  will  be  nearly  twice  the  required  strength. 
It  is  better  to  dilute  still  further  (after  taking  its  approximate 
strength  with  N/10  HC1  and  phenolphthalein)  with  freshly  boiled  and 
cooled  distilled  water  ;  the  actual  working  strength  may  be  checked 
by  evaporating  20  or  25  c.c.  to  dryness  with  a  slight  excess  of 
sulphuric  acid,  then  igniting  over  a  Bun  sen  flame  and  weighing 
the  BaS04.  The  corresponding  acid  may  be  either  N/10  oxalic,  nitric, 
or  hydrochloric,  and  the  proper  indicator  is  phenolpthalein.  Oxalic 
acid  is  recommended  by  Pettenkofer  for  carbonic  acid  deter- 
mination because  it  has  no  effect  upon  the  barium  carbonate 
suspended  in  weak  solutions  ;  but  there  is  the  serious  drawback 
with  oxalic  acid  that  in  dilute  solution  it  is  liable  to  lose  its  strength; 
therefore,  if  N/i0  oxalic  is  used  it  should  be  freshly  prepared  from 
a  normal  solution. 

The  bartya  solution  is  subject  to  constant  change  by  absorption 
of  carbonic  acid,  but  this  may  be  prevented  to  a  great  extent  by 
preserving  it  in  the  bottle  shown  in  fig.  24.  A  thin  layer  of  light 
petroleum  oil  on  the  surface  of  the  liquid  preserves  the  baryta  at 
one  strength  for  a  long  period  in  the  bottle  shown  in  fig.  25. 

The  reaction  between  baryta  and  yellow  turmeric  paper  is  very 
delicate,  so  that  the  merest  trace  of  baryta  in  excess  gives  a  decided 
brown  tinge  to  the  edge  of  the  spot  made  by  a  glass  rod  on  the 
turmeric  paper.  If  the  substance  to  be  titrated  is  not  too  highly 
coloured,  phenolphthalein  should  invariably  be  used. 

10.    Normal  Ammonium-Copper  Solution  for 

Acetic  Arid  and  free  Acids  and  Bases  in  Earthy  and 

Metallic  Solutions. 

This  acidimetric  solution  is  prepared  by  dissolving  pure  copper 

*Carulla  (J.  S.  C.  I.,  1907,  26,  186)  gives  a  series  of  experiments  proving  that  N/2 
ammonia  retains  its  strength  with  great  constancy  under  varying  laboratory  con- 
ditions. 


56  NORMAiTAMMONiUM-coppER  SOLUTION. 

sulphate  in  warm  water,  and  adding  to  the  clear  solution  liquid 
ammonia  until  the  bluish-green  precipitate  which  first  appears  is 
nearly  dissolved.  The  solution  is  then  filtered  into  the  graduated 
cylinder,  and  titrated  by  allowing  it  to  flow  from  a  pipette  graduated 
in  J  or  y1^  c.c.  into  10  or  20  c.c.  of  normal  sulphuric  or  nitric  acid 
(not  oxalic).  While  the  acid  remains  in  excess,  the  bluish-green 
precipitate  which  is  produced  as  each  drop  falls  into  the  acid 
rapidly  disappears  ;  but  so  soon  as  the  exact  point  of  saturation 
is  reached,  the  previously  clear  solution  is  rendered  turbid  by  the 
precipitate  remaining  insoluble  in  the  neutral  liquid. 

The  process  is  especially  serviceable  for  the  determination  of  the 
free  acid  existing  in  certain  metallic  solutions,  i.e.,  mother-liquors, 
etc.,  where  the  neutral  compounds  of  such  metals  have  an  acid  re- 
action on  litmus — such  as  the  oxides  of  zinc,  copper,  and  magnesia, 
and  the  protoxides  of  iron,  manganese,  cobalt,  and  nickel ;  it  is 
also  applicable  to  acetic  and  the  mineral  acids. 

If  cupric  nitrate  be  used  for  preparing  the  solution  instead  of 
sulphate,  the  presence  of  barium,  or  strontium,  or  metals  precipitable 
by  sulphuric  acid  is  of  no  consequence.  The  solution  is  standardized 
by  normal  nitric  or  sulphuric  acid;  and  as  it  slightly  alters  by 
keeping  ,a  coefficient  must  be  found  from  time  to  time  by  titrating 
with  normal  acid,  by  which  to  calculate  the  results  systematically. 
Oxides  or  carbonates  of  magnesium,  zinc,  or  other  admissible  metals, 
are  dissolved  in  excess  of  normal  nitric  acid,  and  titrated  residually 
with  the  copper  solution. 

EXAMPLE  :  1  gm.  of  pure  zinc  oxide  was  dissolved  in  27  c.c.  of  normal  acid, 
and  2'3  c.c.  of  normal  copper  solution  required  to  produce  the  precipitate  =24'7 
c.c.  of  acid  ;  this  multiplied  by  0-0405,  the  coefficient  for  zinc  oxide,  =1  gm 

DETERMINATION  OF  THE  ACTUAL  STRENGTH  OF  STANDARD 
SOLUTIONS  NOT  STRICTLY  NORMAL  OR  SYSTEMATIC. 

IN  discussing  the  preparation  of  the  foregoing  standard  solutions 
it  has  been  assumed  that  they  shall  be  strictly  and  absolutely 
correct ;  that  is  to  say,  if  the  same  measure  be  filled  first  with  any 
alkaline  solution,  then  with  an  acid  solution,  and  the  two  mixed 
together,  a  perfectly  neutral  solution  shall  result,  so  that  a  drop  or 
two  either  way  will  upset  the  equilibrium. 

Where  it  is  possible  to  weigh  directly  a  pure  dry  substance,  this 
approximation  may  be  very  closely  reached.  Sodium  carbonate, 
for  instance,  admits  of  being  thus  accurately  weighed.  On  the 
other  hand,  the  caustic  alkalies  cannot  be  so  weighed,  nor  can  the 
liquid  acids.  An  approximate  quantity,  therefore,  of  these 
substances  must  be  taken,  and  the  exact  strength  of  the  solution 
found  by  experiment. 

In  titrating  such  solutions  it  is  exceedingly  difficult  to  make  them 
so  exact  in  strength  that  equal  volumes,  to  a  drop  or  two,  shall 
neutralize  each  other.  In  technical  matters  a  near  approximation 
may  be  sufficient,  but  in  scientific  investigations  it  is  of  the  greatest 


ADJUSTMENT    OF    STANDARD    SOLUTIONS.  57 

importance  that  the  utmost  accuracy  should  be  obtained  ;  it  is 
therefore  advisable  to  ascertain  the  actual  difference,  and  to  mark 
it  upon  the  vessels  in  which  the  solutions  are  kept,  so  that  a  slight 
calculation  will  give  the  exact  result. 

Suppose,  for  instance,  that  a  standard  sulphuric  acid  is  prepared 
which  does  not  rigidly  agree  with  the  normal  sodium  carbonate  (not 
at  all  an  uncommon  occurrence,  as  it  is  exceedingly  difficult  to  hit 
the  precise  point)  ;  in  order  to  find  out  the  exact  difference  it  must 
be  carefully  titrated  as  on  page  49.  Suppose  the  weight  of  sodium 
carbonate  to  be  1*9  gm.,  it  is  then  dissolved  and  titrated  with  the 
standard  acid,  of  which  36'  1  c.c.'  are  required  to  reach  the  neutral 
point  exactly. 

If  the  acid  were  rigidly  exact  it  should  require  35'85  c.c.  ;  in  order, 
therefore,  to  find  the  factor  necessary  to  bring  the  quantity  of  acid 
used  in  the  analysis  to  an  equivalent  quantity  of  normal  strength, 
the  number  of  c.c.  actually  used  must  be  taken  as  the  denominator, 
and  the  number  which  should  have  been  used,  had  the  acid  been 
strictly  normal,  as  the  numerator,  thus  — 


0*993  is  therefore  the  factor  by  which  it  is  necessary  to  multiply  the 
number  of  c.c.  of  that  particular  acid  used  in  any  analysis  in  order 
to  reduce  it  to  normal  strength,  and  should  be  marked  upon  the 
bottle  in  which  it  is  kept. 

On  the  other  hand,  suppose  that  the  acid  is  too  strong,  and  that 
35-2  c.c.  were  required  instead  of  35'85, 


T0184  is  therefore  the  factor  by  which  it  is  necessary  to  multiply 
the  number  of  c.c.  of  that  particular  acid  in  order  to  bring  it  to  the 
normal  strength.  This  plan  is  much  better  than  dodging  about 
with  additions  of  water  or  acid. 

In  all  circumstances  it  is  safer  to  prove  J  the  strength  of  any 
standard  solution  by  experiment^  even  though  its  constituent  has 
been  accurately  weighed  in  the  dry  and  pure  state. 

Further,  let  us  suppose  that  a  solution  of  caustic  soda  is  to  be 
made  from  carbonate  by  means  of  fresh  lime.  After  pouring  off  the 
clear  liquid,  water  is  added  to  the  sediment  to  extract  more  alkali 
solution  ;  by  this  means  we  may  obtain  two  solutions,  one  of  which 
is  stronger  than  necessary,  and  the  other  weaker.  Instead  of 
mixing  them  in  various  proportions  and  repeatedly  trying  the 
strength,  we  may  find,  by  two  experiments  and  a  calculation,  the 
proportions  of  each  necessary  to  give  a  normal  solution,  thus  :  — 

The  exact  actual  strength  of  each  solution  is  first  found  by 
separately  running  into  10  c.c.  of  normal  acid  as  much  of.  each  alkali 
solution  as  will  exactly  neutralize  it.  We  have,  then,  in  the  case 
of  the  stronger  solution,  a  number  of  c.c.  required  less  than  10. 
Let  us  call  this  number  V. 


58  ADJUSTMENT    OF   STANDARD    SOLUTIONS. 

In  the  weaker  solution  the  number  of  c.c.  is  greater  than  10, 
represented  by  v.  A  volume  of  the  stronger  solution  —x  will  saturate 
10  c.c.  of  normal  acid  as  often  as  V  is  contained  in  x. 

A  volume  of  the  weaker  solution  =y  will,  in  like  manner,  saturate 

—  -    c.c.    of   normal    acid;    both    together   saturate    -y^~H  —  — 

and  the  volume  of  the  saturated  acid  is  precisely  that  of  the  two 
liquids,  thus- 


Whence  10  v  x  +  w  y  ^y  v  x+y  v  y 

v  x  (10-V)=V  y  (v-10). 

And  lastly, 

x_V  (v  +  10) 

y»-v(16-V) 

An  example  will  render  this  clear.  A  solution  of  caustic  soda 
was  taken,  of  which  5-  8  c.c.  were  required  to  saturate  10  c.c.  of 
normal  acid  ;  of  another  solution,  12*7  c.c.  were  required.  The 
volumes  of  each  necessary  to  form  a  normal  solution  were  found 
as  follows  :  — 

5-8  (12-7-10  ')  =  15-66 
12-7  (10    -5-8)  -53-34 

Therefore,  if  the  solutions  are  mixed  in  the  proportion  of  15*66  c.c. 
of  the  stronger  with  53-34  c.c.  of  the  weaker,  a  correct  solution  ought 
to  result.  The  same  principle  of  adjustment  is,  of  course,  applicable 
to  standard  solutions  of  every  class. 

EXAMPLE  :  Suppose  that  1  litre  of  normal  soda  were  required. 
Since  15  '66  +  53*34  =  69  it  is  clear  that  the  fraction  of  the  stronger 
solution  to  be  taken  is,  in  all  cases, 

15-66       1566         522 

AQ     or  or  ;  and  to  get  1000  c.c.  the  volume  required  is 

oy         oyuu      .ZoUU 

522  x  1000    5220     .._ 

-23007     =^=227C'C" 

which  has  to  be  mixed  with  1000  —  227  =  773  c.c.  of  the  weaker 
solution. 

Again  :  suppose  that  a  standard  solution  of  sulphuric  acid  has 
been  made  approximating  as  nearly  as  possible  to  the  normal 
strength,  and  its  exact  value  found  by  titration  with  sodium 
carbonate  (or  a  standard  hydrochloric  acid  with  silver  nitrate),  and 
that  such  a  solution  has  been  calculated  to  require  the  coefficient 
0-995  to  convert  it  to  normal  strength.  By  the  help  of  this  solution, 
though  not  strictly  normal,  we  may  titrate  an  approximately 
normal  alkali  solution  thus  :  —  Two  trials  of  the  acid  and  alkaline 
solution  show  that  50  c.c.  alkali  =  48*5  c.c.  acid  having  a  coefficient 
of  0*995  =  48*25  c.c.  normal;  then,  according  to  the  equation, 
50  x  =  48-25  is  the  required  coefficient  for  the  alkali,  i.e. 


FACTORS    FOR   NORMAL    SOLUTIONS. 


59 


And  here,  in  the  case  of  the  alkali  solution  being  sodium  carbonate, 
we  can  bring  it  to  exact  normal  strength  by  a  calculation  based  on 
the  equivalent  weight  of  the  salt,  thus — 

1    :  0-965  :   :  53  :  51*145. 

The  difference  between  the  two  latter  numbers  is  1/855  gm.,  and 
this  weight  of  pure  sodium  carbonate,  added  to  one  litre  of  the 
solution,  will  bring  it  to  normal  strength. 

TABLE  FOR  THE  SYSTEMATIC  ANALYSIS  OF  ALKALIES, 
ALKALINE    EARTHS    AND    ACIDS. 


Substance. 

Formula. 

Molecular 
Weight. 

Quantity  to  be 
weighed  so  that  1 
c.c.  Normal  Solu- 
tion =1  percent, 
of  substance. 

Normal 
Factor.* 

Sodium  Oxide    . 

Na20 

62 

grams. 

3-1 

0-031 

Sodium  Hydroxide 

NaOH 

40-01 

4-00 

0-040 

Sodium  Carbonate  . 

Na2C03 

106 

5-3 

0-053 

Sodium  Bicarbonate     . 

NaHC03 

84-01 

8-40 

0-084 

Potassium  Oxide     .      . 

K2O 

94-2 

4-71 

0-0471 

Potassium  Hydroxide  . 

KOH 

56-11 

5-61 

0-0561 

Potassium  Carbonate    . 

K2C03 

138-2 

6-91 

0-0691 

Potassium  Bicarbonate 

KHC03 

100-11 

10-01 

0-1001 

Ammonia      .... 

NH3 

17-03 

1-703 

0-0170 

Ammonium  Carbonate 

(NH4)2  C03 

96-08 

4-804 

0-0480 

Calcium  Oxide  (Lime)  . 

CaO 

56-09 

2-805 

0-0280 

Calcium  Hydroxide 
Calcium  Carbonate  '  .  .  -. 

CaH202 
CaCOs 

74-11 
100-09 

3-705 

:         5-0 

0-0370 
0-050 

Barium  Hydroxide  . 

BaH202 

171-39 

8-57 

0-0857 

Do.  (cryst.)     .     .    .j.; 
Barium  Carbonate  . 

BaH-Pa,  8H20 
BaC03 

315-51 
197-37 

15-775 

9-868 

0-1578 
0-0987 

Strontium  Oxide 

SrO 

103-63 

5-18 

0-0518 

Strontium  Carbonate    . 

SrC03 

147-63 

7-38 

0-0738 

Magnesium  Oxide    . 

MgO 

40-32 

2-016 

0-0202 

Magnesium  Carbonate  . 

MgC03 

84-32 

4-216      . 

0-0422 

Nitric  Acid   .... 

HNOS 

63-02 

6-30 

0-063 

Hydrochloric  Acid  .      . 

HC1 

36-47 

3-647 

0-03647 

Sulphuric  Acid  .      ."•'.-. 

H2S04 

98-09 

4-905 

0-04905 

Oxalic  Acid  .      .    •  .     •. 

Acetic  Acid  ...     »  " 

H2C204,  2H20 
C2H402 

126-06 
60-03 

6-30 
6-00 

0-063 
0-060 

Tartaric  Acid 
Citric  Acid    .... 

C4H606 
C6H807H20 

150-05 
210-08 

7-50 
7-00 

0-075 
0-070 

Carbonic  acid     .      .      . 

C02 

44 

0-022 

*  This  is  the  coefficient  by  which  the  number  of  c.c.  of  normal  solution  used  in  any 
analysis  is  to  he  multiplied  in  order  to  obtain  the  amount  of  pure  substance  present 
in  the  material  examined. 

If  grain  weights  arc  used  instead  of  grams,  the  decimal  point  must  be  moved  one 
place  to  the  right  to  give  the  necessary  weight  for  examination ;  thus  sodium  carbon- 
ate, instead  of  5'3  gm.,  would  be  53  grains,  the  normal  factor  in  this  case  would  also 
be  altered  to  0*53. 


60  ALKALIMETRY. 

THE    TITRATION    OF    ALKALI    SALTS. 
1.    Total  Alkali  in  Caustic  Soda  or  Potash,  or  their  Carbonates. 

THE  necessary  quantity  of  substance  being  weighed  or  measured, 
as  the  case  may  be,  and  mixed  with  distilled  water  to  a  proper 
state  of  dilution  (say  about  one  per  cent,  of  solid  material),  an 
appropriate  indicator  is  added,  and  the  solution  is  ready  for  the 
burette.  Normal  acid  is  then  cautiously  added  till  the  change  of 
colour  occurs.  In  the  case  of  caustic  alkalies  free  from  CO2  the 
end-reaction  is  very  sharp  with  any  of  the  indicators;  but  if  C02  is 
present,  the  only  available  indicators  in  the  cold  are  methyl  orange 
or  lacmoid  paper.  If  the  other  indicators  are  used,  the  C02  must 
be  boiled  off  after  each  addition  of  acid. 

In  examining  carbonates  of  potash  or  soda,  or  mixtures  of  caustic 
and  carbonate,  where  it  is  only  necessary  to  ascertain  the  total 
alkalinity,  the  same  method  applies. 

In  the  examinations  of  samples  of  commercial  refined  soda  or 
potash  salts,  it  is  advisable  to  proceed  as  follows  : — 

Powder  and  mix  the  sample  thoroughly,  weigh  10  gm.  in  a  platinum  or 
porcelain  crucible,  and  ignite  gently  over  a  spirit  or  gas  lamp,  and  allow  the 
crucible  to  cool  under  the  exsiccator.  Weigh  again,  the  loss  of  weight  gives  the 
moisture  ;  wash  the  contents  of  the  crucible  into  a  beaker,  dissolve  and  filter  if 
necessary,  and  dilute  to  the  exact  measure  of  500  c.c.  with  distilled  water ;  after 
mixing  thoroughly,  take  out  50  c.c.  (=1  gm.)  of  alkali  with  a  pipette,  transfer 
to  a  small  flask,  bring  the  flask  under  a  burette  containing  normal  acid  and 
graduated  to  £  or  ^  c.c.,  and  allow  the  acid  to  flow  cautiously  as  before  directed 
until  the  neutral  point  fs  reached.  The  process  may  then  be  repeated,  in  order 
to  make  certain  of  the  correctness  of  the  result. 

RESIDUAL  TITRATION  :  As  the  presence  of  carbonic  acid  with  litmus  and  the 
other  indicators,  except  methyl  orange,  always  tends  to  confuse  the  exact  end  of 
the  process,  the  difficulty  is  best  overcome,  in  the  case  of  not  using  methyl  orange, 
by  allowing  a  known  excess  of  acid  to  flow  into  the  alkali,  boiling  to  expel  the 
CO g,  cooling,  and  then  cautiously  adding  normal  caustic  alkali,  drop  by  drop, 
until  the  liquid  suddenly  changes  colour ;  by  deducting  the  quantity  of  caustic 
alkali  from  the  quantity  of  acid  originally  used,  the  exact  volume  of  acid  necessary 
to  saturate  the  1  gm.  of  alkali  is  ascertained. 

This  method  of  re-titration  gives  a  very  sharp  end-reaction,  as  there 
is  no  C02  present  to  interfere  with  the  delicacy  of  the  indicator.  It 
is  a  procedure  sometimes  necessary  in  other  cases,  owing  to  the 
interference  of  impurities  dissipated  by  boiling,  e.g.,  H2S,  which 
would  otherwise  bleach  the  indicator — except  in  the  case  of  methyl 
orange  and  lacmoid  paper,  either  of  which  is  indifferent  to  H2S  in 
the  cold.  An  example  will  make  the  plan  clear  : 

EXAMPLE  :  50  c.c.  of  the  solution  of  alkali  prepared  as  directed,  equal  to  1  gm. 
of  the  sample,  is  put  into  a  flask,  and  20  c.c.  of  normal  acid  added ;  it  is  then 
boiled  and  shaken  till  all  C02  is  expelled,  and  normal  caustic  alkali  added  till  the 
neutral  point  is  reached ;  the  quantity  required  is  3'4  c.c.,  which  deducted 
from  20  c.c.  of  acid  leaves  16*6  c.c.  The  following  calculation,  therefore,  gives 
the  percentage  of  real  alkali,  supposing  it  to  be  soda : — 31  is  the  half  molecular 
weight  of  anhydrous  soda  (Na20)  and  1  c.c.  of  the  acid  is  equal  to  0'031  gm., 
therefore  16'6  c.c.  is  multiplied  by  0'031,  which  gives  0'5146;  and  as  1  gm.  was 


ALKALIMETRY.  61 

taken,  the  decimal  point  is  moved  two  places  to  the  right,  which  gives  51446  per 
cent,  of  real  alkali.  If  calculated  as  carbonate,  the  16'6  would  be  multiplied  by 
0-053,  which  gives  0'8798  gm.  =87'98  per  cent. 

The  following  methods  of  ascertaining  the  proportions  of  mixed 
alkaline  hydroxides,  monocarbonates,  and  bicarbonates  must  not 
be  taken  as  absolutely  correct,  but  are  correct  enough  for  technical 
purposes  : — 


2.     Mixed  Caustic  and  Carbonated  Alkali  Salts. 

(a)    Precipitation    of     the     carbonate    by     barium    chloride.— 

The  alkaline  salts  of  commerce  consist  often  of  a  mixture  of 
caustic  and  carbonated  alkali.  If  it  be  desired  to  ascertain  the 
proportions  in  these  mixtures,  the  total  alkalinity  of  a  weighed  or 
measured  quantity  of  substance  (not  exceeding  1  or  2  gm.)  is 
ascertained  by  normal  acid  and  noted  ;  a  like  quantity  is  then  dis- 
solved in  about  150  c.c.  of  water  in  a  200  c.c.  flask,  and  exactly 
enough  solution  of  barium  chloride  added  to  remove  all  CO2  from 
the  alkali. 

Watson  Smith  has  shown*  that  whenever  an  excess  of  barium 
chloride  is  used  in  this  precipitation  so  as  to  form  barium  hydrate, 
there  is  invariably  a  loss  of  soda  :  exact  precipitation  is  the  only 
way  to  secure  accuracy. 

The  flask  is  now  filled  up  to  the  200  c.c.  mark  with  distilled  water,  securely 
stoppered,  and  put  aside  to  settle.  When  the  supernatant  liquid  is  clear,  take 
out  50  c.c.  with  a  pipette,  and  slowly  run  in  normal  hydrochloric  acid  till  neutral. 
The  number  of  c.c.  multiplied  by  4  will  be  the  quantity  of  acid  required  for  the 
caustic  alkali  in  the  original  weight  of  substance,  because  only  one-fourth  was 
taken  for  analysis.  The  difference  is  calculated  as  carbonate.  Or  the  precipitated 
barium  carbonate  may  be  thrown  upon  a  dry  filter,  washed  well  and  quickly  with 
boiling  water,  and  titrated  with  normal  acid,  instead  of  the  original  determination 
of  the  totaKalkalinity  ;  or  both  plans  may  be  adopted  so  that  one  forms  a  check 
to  the  other. 

The  principle  of  this  method  is  that  when  barium  chloride  is  added  to  a  mixture 
of  caustic  and  carbonated  alkali  the  C02of  the  latter  is  precipitated  as  an  equivalent 
of  barium  carbonate,  while  the  equivalent  proportion  of  caustic  alkali  remains  in 
solution  as  barium  hydroxide.  By  multiplying  the  number  of  c.c.  of  acid  required 
to  saturate  this  free  alkali  by  the  TTnnr  molecular  weight  of  caustic  potash  or 
soda,  according  to  the  alkali  present,  the  quantity  of  substance  originally  present 
in  this  state  will  be  ascertained. 

Sorensen  and  Andersonj  have  shown  that  this  method  only 
gives  trustworthy  results  if  the  precipitation  with  barium  chloride 
be  carried  out  in  the  warmed  solution,  and  when  the  latter  contains 
only  normal  carbonates. 

If  the  solution  contains  hydroxide  or  bicarbonate  it  must  be  treated  with 
hydrochloric  acid  or  sodium  hydroxide  before  heating  and  precipitating.  The 
quantity  of  acid  or  hydroxide  required  is  ascertained  by  a  previous  determination. 
When  a  solution  of  pure  normal  carbonate  is  treated,  while  hot,  with  an  excess 
of  barium  chloride,  only  normal  barium  carbonate  is  precipitated  ;  if,  however, 
the  precipitation  be  performed  at  the  ordinary  temperature,  more  or  less  acid 

*  J.  S.  C.  I.  1,  85.  t  Zeit.  a.Chem.,  1908,  47,  279-294. 


62  ALKALIMETRY. 

carbonate  is  thrown  down,  and  the  supernatant  liquid  becomes  distinctly 
alkaline.  Further,  if  the  solution  of  the  alkali  carbonate  contains  hydroxide, 
a  greater  or  less  quantity  of  basic  barium  carbonate  is  precipitated  on  treating 
the  hot  solution  with  bariumjjhloride. 

As  barium  hydroxide  solution  absorbs  C02  very  readily  when 
exposed  to  the  atmosphere,  it  is  preferable  to  allow  the  precipitate 
of  barium  carbonate  to  settle  in  the  flask  as  here  described  rather 
than  to  filter  the  solution,  especially  also  as  the  filter  obstinately 
retains  some  barium  hydroxide. 

A  very  slight  error,  however,  occurs  in  this  method  in  consequence 
of  the  volume  of  the  precipitate  being  included  in  the  liquid  in  the 
graduated  flask.  This  error  may  be  obviated  by  precipitating  a 
small  volume  of  the  solution  with  barium  chloride  and  titrating 
the-  liquid  and  precipitate  in  the,same  vessel  with  normal  oxalic 
acid  and  phenolphthalein,  this  acid  having  no  action  on  the  barium 
carbonate. 

(6)  Use  of  two  indicators. — On  titrating  the  cold  solution  (as 
near  0°  C.  as  possible)  of  mixed  hydroxide  and  carbonate  with 
normal  acid  and  phenolphthalein,  keeping  tip  of  burette  immersed 
in  the  liquid,  the  colour  of  the  indicator  is  discharged  as  soon  as 
all  the  hydroxide  is  neutralized  and  half  the  carbonate  (by  con- 
version into  bicarbonate).  On  adding  methyl  orange  (at  most  two 
drops)  to  the  colourless  solution  and  continuing  the  titration,  the 
solution  becomes  pink  as  soon  as  the  remaining  half  of  the 
carbonate  is  neutralized.  If  the  first  addition  of  acid  =  N  c.c,,  and 
the  second  n  c.c.,  then  N~n  corresponds  to  hydroxide,  2  n  to 
carbonate,  and  N  +  n  to  total  alkali. 

3.     Determination  of  Sodium  or  Potassium  Hydroxide  in 
presence  of  small  proportions  of  Carbonate. 

This  may  be  accomplished  by  means  of  phenacetolin.* 

The  alkaline  solution  is  coloured  a  scarcely  perceptible  yellow  with  a  few 
drops  of  the  indicator.  The  standard  acid  is  then  run  in  until  the  yellow  gives 
place  to  a  pale  rose  tint ;  at  this  point  all  the  caustic  alkali  is  saturated,  and  the 
volume  of  acid  used  is  noted.  Further  addition  of  acid  now  intensifies  this  red 
colour  until  the  carbonate  is  decomposed,  when  a  clear  golden  yellow  solution 
results.  The  neutralization  of  the  NaHO  or  the  KHO  is  indicated  by  a  rose  tint 
permanent  on  standing ;  that  of  Na2C03  or  K.2C03  by  the  sudden  passage  from 
red  to  yellow. 

Practice  is  required  with  solutions  of  known  composition  to 
accustom  the  eye  to  the  changes  of  colour.  Phenolphthalein  may 
also  be  employed  for  the  same  purpose  as  follows  :— 

Add  normal  acid  to  the  cold  alkaline  solution  till  the  red  colour  is  discharged, 
taking  care  to  use  a  very  dilute  solution,  and  keeping  the  point  of  the  burette  in 
the  liquid  so  that  no  C02  escapes.  The  period  at  which  the  colour  is  discharged 
occurs  when  all  the  hydrate  is  neutralized  and  the  carbonate  converted  into 
bicarbonate ;  the  volume  of  acid  is  noted,  and  the  solution  heated  to  boiling, 

"Lunge,  .7.  8.  C.  J.  1,  5fi. 


ALKALIMETRY.  63 

with  small  additions  of  acid,  till  the  red  colour  produced  by  the  decomposition  of 
the  bicarbonate  is  finally  destroyed. 

In  both  these  methods  it  is  preferable,  after  the  first  stage,  to  add 
excess  of  acid,  boil  off  the  CO2,  and  titrate  back  with  normal  alkali. 
The  results  are  quite  as  accurate  as  the  method  of  precipitation 
with  barium. 

4.     Determination  of  small  quantities  of  Sodium 
or  Potassium  Hydroxide  in  presence  of  Carbonates. 

A  method,  by  Thomson,  consists  in  precipitating  the  carbonates 
by  neutral  solution  of  barium  chloride  in  the  cold  :  the  barium 
carbonate  being  neutral  to  phenolphthalein,  this  indicator  can  be 
used  for  the  process.  When  the  barium  solution  is  added,  a  double 
decomposition  takes  place,  resulting  in  the  formation  of  an 
equivalent  quantity  of  sodium  or  potassium  chloride,  while  the 
barium  carbonate  is  precipitated  and  the  alkaline  hydrate  remains 
in  solution. 

EXAMPLE  (Thomson):  2  gm.  of  pure  sodium  carbonate  were  mixed  in 
solution  with  '02  gm.  of  sodium  hydroxide ;  excess  of  barium  chloride  was  then 
added,  together  with  the  indicator,  and  the  solution  titrated  with  N/K>  acid,  of 
which  in  three  trials  an  average  of  5  c.c.  was  required  ;  therefore,  5  x  "004  ='02  gm. 
—  exactly  the  quantity  used. 

In  this  process  chlorides,  sulphates,  and  sulphites  do  not  interfere  ; 
neither  do  phosphates,  as  barium  phosphate  is  neutral  to  the 
indicator.  With  sulphides,  half  of  the  base  will  be  determined ;  but 
if  hydrogen  peroxide  be  added  and  the  mixture  allowed  to  rest 
for  a  time,  the  sulphides  are  oxidized  to  sulphates,  which  have  no 
effect.  If  silicates  or  aluminates  of  alkali  are  present,  the  base  will 
of  course  be  recorded  as  hydroxide. 

Thomson  further  states  : — 

"  The  foregoing  method  can  also  be  applied  to  the  determination 
of  the  hydrates  of  sodium  or  potassium  in  various  other  compounds, 
which  give  precipitates  with  barium  chloride  neutral  to  phenolph- 
thalein, such  as  the  normal  sulphites  and  phosphates  of  the  alkali 
metals.  An  illustration  of  the  use  to  which  the  facts  stated  in  this 
and  former  papers  may  be  put  will  be  found  in  the  analysis  of 
sodium  sulphite.  Of  course  sulphate,  thiosulphate,  and  chloride 
are  determined  as  usual,  but  to  determine  sulphite,  carbonate  and 
hydrate,  or  sodium  bicarbonate  by  methods  in  ordinary  use  is 
rather  a  tedious  operation  To  find  the  proportion  of  hydroxide, 
all  that  is  necessary  is  to  precipitate  with  barium  chloride  and 
titrate  with  standard  acid,  as  above  described.  Then,  by  simple 
titration  of  another  portion  of  the  sample  in  the  cold,  using  phenolph- 
thalein as  indicator,  the  hydroxide  and  half  of  the  carbonate  can 
be  found,  and  finally,  by  employment  of  methyl  orange  as  indicator, 
and  further  addition  of  acid,  the  other  half  of  the  carbonate  and  half 
of  the  sulphite  can  be  determined.  By  simple  calculations,  the 


64  ALKALIMETRY. 

respective  proportions  of  these  three  compounds  can  be  obtained, 
a  result  which  can  be  accomplished  in  a  few  minutes.  It  must  be 
borne  in  mind  that  if  a  large  quantity  of  sodium  carbonate  is  in  the 
sample  the  proportion  of  that  compound  found  will  only  be  an 
approximation  to  the  truth,  as  the  end-reaction  is  only  delicate  with 
small  proportions  of  sodium  carbonate. 

5.  Determination  of  Alkali  Carbonate  and  Bicarbonate  in  the 

presence  of  each  other. 

(a)  The  total  alkali  is  first  determined  in  one  portion  of  the 
solution  by  titration  with  standard  acid  and  methyl  orange.  To 
another  portion  a  measured  volume  of  standard  sodium  hydroxide 
(free  from  C02)  is  added,  to  convert  the  bicarbonate  to  carbonate. 
The  sodium  carbonate  is  then  precipitated  by  barium  chloride  and 
the  excess  of  hydroxide  determined  by  titration  with  standard 
acid  as  in  paragraph  2  (a).  If  N  c.c.  of  normal  acid  correspond 
to  the  total  alkali,  n  c.c.  of  normal  sodium  hydroxide  be  added, 
and  n'  c  c.  normal  acid  correspond  to  the  excess  of  the  latter; 
then,  n  -  n7  c.c.=  bicarbonate,  and  N-(n-n')  c.c.=  carbonate. 
If  n  =  N,  thus  ensuring  excess  of  hydroxide,  n'  =  carbonate. 
(b)  Use  of  two  indicators. — Titratiug  with  acid  and  phenophtha- 
lein  to  neutralization,  then  with  acid  and  methyl  orange,  as  in 
paragraph  2  (6),  the  first  addition  of  acid  (n  c.c.)  corresponds  to 
half  the  carbonate,  and  the  second  addition  (N  c.c.)  to  half  the 
carbonate  +  the  total  bicarbonate.  Thus  2  n  c.c.  =  carbonate,  and 
N  -  n= bicarbonate. 

This  method  of  titration  can  be  used  for  soda  ash,  etc.,  when 
carbonate  and  hydroxide  or  carbonate  and  bicarbonate  occur  in 
the  same  solution.  [Cf.  par.  2  (6)].  Should  the  amount  of  N/x 
acid  used  be  greater  with  phenolphthalein  than  with  methyl 
orange,  NaOH  is  shown  to  be  present,  but  if  greater  with  methyl 
orange,  then  bicarbonate  is  present,  and  calculations  for  carbonate 
and  hydrate,  or  carbonate  and  bicarbonate,  are  made  in  the  usual 
way. 

6.  Determination  of  Alkalies  in  the  presence  of  Sulphites. 

It  is  not  possible  to  determine  the  alkaline  compounds  in  the 
presence  of  sulphites  by  titration  with  acids,  as  a  certain  quantity 
of  acid  is  taken  up  by  the  sulphite,  SO2  being  evolved.  This 
difficulty  may  be  completely  overcome  by  the  aid  of  hydrogen 
peroxide,  which  speedily  converts  the  sulphites  into  sulphates.* 
These  operators  proved  that  neither  caustic  nor  carbonated  alkali 
was  affected  by  H2O2,  nor  had  the  latter  any  prejudicial  effect  on 
methyl  orange  in  the  cold.  The  quantity  of  H2O2  required  in 
any  given  analysis  must  depend  on  the  amount  of  sulphite  present ; 

*  Grant  and  Cohen,  J.  8.  C.  I.  9,  19. 


ALKALIES  IN  THE  PRESENCE  OF  SULPHITES          65 

for  instance,  the  caustic  salts  of  commerce  contain  about  50  %  of 
sulphite,  and  it  suffices  to  take  10  c.c.  of  ordinary  10  vol.  H202  for 
every  O'l  gm.  of  the  salts  in  solution.  In  the  case  of  mixtures 
containing  less  or  more  sulphite  the  quantity  may  be  varied. 

METHOD  OF  PROCEDURE  :  A  measured  volume  of  the  peroxide  is  tun  into 
a  beaker,  and  three  or  four  drops  of  methyl  orange  added.  As  the  H202  is 
invariably  faintly  acid,  the  acidity  is  carefully  corrected  by  adding  drop  by  drop 
from  a  pipette  N/ioq  caustic  soda.  The  required  quantity  of  salt  to  be  analyzed 
is  then  added  in  solution,  and  the  mixture  gently  boiled,  whereby  the  methyl  orange 
will  be  bleached.  The  liquid  is  then  cooled,  a  drop  or  two  more  of  methyl  orange 
added,  and  the  titration  for  the  proportion  of  alkali  carried  out  with  normal  acid. 
The  results  are  very  satisfactory. 

7.     Determination   of   Caustic    Soda    or   Potash   by   standard 
Potassium  Dichromate. 

This  process,  or  rather  the  inverse  of  it,  was'de vised  by  Riehter, 
for  determining  dichromate  with  caustic  alkali  by  the  aid  of 
phenolphthalein.  Exact  results  may  be  obtained  by  it  in  titrating 
soda  or  potash  as  hydrates,  but  not  ammonia  as  recommended  by 
Riehter. 

For  the  process  there  are  required  a  decinormal  solution  of  bicarbonate 
containing  14*72  gm.  per  litre,  and  N/io  soda  or  potash  solution  titrated  against 
sulphuric  acid.  A  comparison  liquid  containing  about  1  gm.  of  potassium 
chromate  in  150  —200  c.c.  water  is  advisable  for  ascertaining  the  exact  end  of 
the  reaction ;  50  c.c.  of  the  alkali,  having  been  diluted  with  the  same  volume  of 
water,  is  coloured  with  phenolphthalein,  and  the  dichromate  run  in  from  a  burette  ; 
the  fine  red  tint  changes  to  reddish  yellow,  which  remains  till  the  neutral  point  is 
nearly  reached,  when  the  yellow  colour  of  the  chromate  is  produced.  The  change 
is  not  instantaneous,  as  with  mineral  acids,  so  that  a  little  time  must  be  allowed 
for  the  true  colour  to  declare  itself. 

3.     Direct  Determination  of  Sodium  by  Potassium 
dihydroxytartrate  and  Permanganate. 

An  interesting  series  of  researches  on  the  oxidation  products  of 
tartaric  acid  has  been  published  by  H.  J.  Horstman  Fen  ton, 
M.A.*  The  results  of  these  researches  have  been  to  develop  the  only 
method  of  obtaining  sodium  in  such  a  form  of  combination  as  to  admit 
of  its  volumetric  determination.  The  author  has  kindly  furnished  me 
with  specimens  of  dihydroxy tartaric  acid,  and  also  of  the  potassium 
salt,  with  which  to  verify  the  results  obtained  by  him,  and  I  am 
able  to  state  that  when  the  method  is  carried  out  with  extreme  care 
and  strict  attention  to  details  it  is  capable  of  giving  satisfactory 
results. 

Dihydroxytartaric  acid,  so  far  as  present  knowledge  is  concerned, 
is  best  prepared  from  dihydroxymaleic  acid,  and  as  both  these  acids 
are  comparatively  unknown  their  preparation  will  now  be  described. 

Preparation  of  Dihydroxymaleic  Acid.^-Tartaric  acid  is  dissolved  in  the  least 
possible  quantity  of  hot  water  ;  finely-divided  iron  (ferrum  redactum)  is  added, 

*  J.  C.  S.  Trans.,  1894,  pp.  899-910,  1898,  pp.  71-81,  ibid,  472-482,  and  on  the 
volumetric  determination  of  sodium  1898,  pp.  167-174. 


66  DIBECT   DETERMINATION    OF   SODIUM. 

and  the  liquid  boiled  until  all  the  iron  has  disappeared.  The  quantity  of  iron 
must  be  insufficient  to  cause  a  separation  of  ferrous  tartrate  when  the  action  is 
finished  ;  about  -^  part  of  the  weight  of  tartaric  acid  employed  answers  well, 
but  the  final  result  does  not  appear  to  be  much  influenced  by  the  proportion  of 
iron  in  solution,  at  any  rate  within  considerable  limits.  The  solution,  filtered, 
if  necessary,  through  cotton  wool,  is  carefully  cooled,  surrounded  by  ice,  and 
hydrogen  peroxide  (20  volume)  added  in  small  quantities  at  a  time,  allowing 
a  few  minutes  to  elapse  between  each  addition.  The  first  portions  of  the  peroxide 
merely  produce  a  yellowish  colour,  but,  as  the  action  proceeds,  each  addition 
produces  a  dark  green  or  nearly  black  appearance,  transient  at  first,  but  becoming 
more  and  more  persistent.  When  this  dark  colour  remains  for  two  or  three 
minutes,  it  is  a  rough  indication  that  sufficient  peroxide  has  been  added.  Great 
care  must  be  taken  not  to  add  an  excess  of  the  peroxide,  or  the  whole  of  the  material 
will  be  wasted.  Nordhausen  sulphuric  acid  is  now  added  by  means  of  a  thistle 
funnel,  drawn  out  to  a  fine  point,  in  very  small  quantities  at  a  time,  cooling 
carefully  between  each  addition,  preferably  by  ice  and  salt.  The  quantity  added 
is  a  matter  of  importance,  too  much  or  too  little  giving  an  indifferent  yield  of 
the  substance  ;  the  best  proportion  is  found  by  experience  to  be  about  r\th  of 
the  total  volume  of  the  liquid  operated  on.  The  mixture,  still  surrounded  by 
ice,  is  put  aside  in  a  cold  place,  and  after  a  few  hours  crystals  begin  to  form.  The 
first  deposit  is  often  discoloured  and  the  crystals  small,  but  the  subsequent  crops 
are  beautifully  white  and  pure.  If  the  experiment  is  properly  conducted,  and  the 
liquid  kept  sufficiently  cool,  crystals  continue  to  form  for  several  days,  but 
the  greater  part  are  deposited  within  about  24  hours. 

The  crystals  are  collected  with  the  aid  of  a  pump,  carefully  drained,  and 
washed  repeatedly  with  small  quantities  of  cold  water.  After  again  thoroughly 
draining,  they  are  spread  on  filter-paper  and  air-dried.  They  appear  to  undergo 
no  change  in  the  air,  even  after  several  weeks'  exposure. 

Preparation  of  Dihydroxytartaric  Acid. — Crystallized  dihydroxymaleic  acid  as 
above  described  (C4H4O6,2H,0)  is  well  triturated  with  from  4  to  5  times  its  weight 
of  glacial  acetic  acid,  and  rather  more  than  the  calculated  quantity  of  bromine 
( 1  mol.  acid  to  1  mol.  bromine)  dissolved  in  a  little  glacial  acetic  acid,  is  added  to 
the  mixture  in  small  portions  at  a  time.  The  first  portions  are  almost  instantly 
bleached,  bnt  the  action  afterwards  becomes  more  sluggish  and  apparently  ceases. 
A  few  drops  of  water  are  then  added,  whereupon  the  colour  of  the  bromine  is  again 
immediately  discharged.  The  addition  of  bromine  is  continued  until  the  colour 
is  quite  permanent  on  standing,  even  when  a  drop  or  two  of  water  is  added.  This 
final  stage  is  reached  when  the  bromine  has  been  added  in  about  the  calculated 
proportion ;  fumes  of  hydrogen  bromide  are  freely  evolved  during  the  operation. 
The  dihydroxymaleic  acid  is  nearly  insoluble  in  cold  glacial  acetic  acid,  but  when  the 
oxidation  is  finished  complete  solution  takes  place.  The  solution  is  allowed  to 
stand  for  an  hour  or  two  and  then  vigorously  stirred,  when  the  dihydroxy tartaric 
acid  quickly,  sometimes  suddenly,  separates  as  a  heavy,  white,  crystalline  powder. 

The  product  is  now  collected  and  drained  with  the  aid  of  the  pump,  washed 
once  or  twice  with  small  quantities  of  anhydrous  ether,  and  kept  in  a  vacuum 
desiccator  over  solid  potash  and  sulphuric  acid  to  remove  the  last  traces  of 
hydrobromic  and  acetic  acids,  bromine  and  ether.  The  yield  of  purified  product 
thus  obtained  is  70  per  cent,  or  more  of  the  theoretical.  The  formula  for  this 
acid  is  C4H6Ofl. 

The  acid  just  described  was  first  studied  in  relation  to  potassium 
permanganate  by  Fen  ton,  and  the  reaction  found  to  be  quite 
definite  ;  and  bearing  in  mind  the  very  sparingly  soluble  character 
of  sodium  dihydroxy  tartrate  it  appeared  probable  that  a  simple 
volumetric  method  for  sodium  might  be  devised.  For  complete 
precipitation  it  is  necessary  that  the  free  acid  shall  be  exactly 
neutralized,  and  this  is  most  conveniently  effected  by  first  preparing 
the  normal  potassium  salt.  The  employment  of  this  salt  as  pre- 
cipitant has  also  the  advantage  that  risk  is  avoided  of  the  pre- 


DIRECT    DETERMINATION    OF   SODIUM.  67 

cipitation  of  potassium  with  the  sodium  salt  when  the  former  metal 
is  present  in  the  mixture  analyzed. 

Preparation  of  Potassium  Dihydroxytartrate. — Weigh  equivalent  proportions 
of  the  acid  182,  and  dry  potassium  carbonate  138  parts.  Dissolve  separately  in 
the  least  possible  quantity  of  ice  cold  water,  then  mix  in  a  vessel  surrounded  by 
ice,  and  stir.  Crystals  soon  separate,  which  may  then  be  collected  on  a  filter, 
and  finally  dried  on  changes  of  filter-paper  in  the  air  or  in  a  desiccator.  The  salt 
so  obtained  does  not,  however,  keep  in  proper  condition  for  more  than  a  few  days, 
and  therefore  it  is  better  to  prepare  it  specially  when  sodium  determinations  have  to 
be  made.  The  formula  of  the  salt  is  K2(C4H408)H2O. 

Standardizing  the  Permanganate  Solution. — In  this  method  of  titrating  soda 
it  is  preferable  to  standardize  the  permanganate  upon  pure  sodium  chloride  rather 
than  to  depend  on  the  relations  between  the  acid  and  permanganate.  The  strength 
of  the  latter  solution  is  best  about  N/5.  i.e.,  6'321  gm.  of  MnK04to  the  litre.  Its 
strength  as  regards  the  sodium  to  be  determined  is  ascertained  by  the  following 
procedure,  bearing  in  mind  that  exactly  the  same  process  in  every  respect  must 
be  carried  out  in  determining  sodium  in  any  given  salt. 

METHOD  OF  PROCEDURE  :  About  0'2  gm.  of  pure  sodium  chloride  is  accurately 
weighed  and  dissolved  in  a  small  beaker  with  the  least  possible  quantity  of  water, 
then  placed  in  a  basin  and  surrounded  by  ice.  Then  an  excess,  say  0'8  gm.  of 
the  potassium  salt  is  weighed  and  dissolved  in  another  small  beaker,  with  not  more 
than  25  c.c.  of  ice  cold  water,  placed  in  ice  and  stirred  till  dissolved  ;  this  occurs 
with  some  difficulty,  but  if  the  liquid  is  not  free  from  floating  particles  or  deposit, 
it  must  be  quickly  filtered  into  the  sodium  solution  still  standing  in  ice.  The 
mixture  is  then  allowed  to  stand  in  ice  for  half  an  hour  with  occasional  stirring. 
The  precipitated  sodium  salt  is  then  collected  by  means  of  a  small  filter  on  filter 
plate  and  quickly  drained  with  the  water  pump,  then  washed  with  3  or  4  c.c. 
of  ice  cold  water  three  or  four  times  in  succession,  and  rinsing  out  the  precipitating 
beaker.  Finally,  the  precipitate  is  dissolved  through  the  filter  in  a  large  excess 
of  dilute  H2S04,  rinsing  out  the  precipitating  beaker  with  dilute  acid  in  the  process, 
and  titrated  with  the  permanganate  at  ordinary  temperature.  The  action  on  the 
permanganate  is  at  first  very  slow,  but  when  once  commenced  increases  in  force 
like  the  action  of  oxalic  acid,  and  the  end  is  quite  distinct.  The  volume 
of  permanganate  having  been  noted,  its  working  strength  in  relation  to  sodium 
in  any  available  compound  is  ascertained,  and  marked  on  the  bottle. 

EXAMPLE  :  0'21  gm.  of  pure  NaCl  was  treated  strictly  according  to  the 
procedure  just  described,  and  required  48'3  c.c.  of  permanganate,  not  strictly 
N/5,  but  about  that  strength.  The  same  weight  of  the  same  NaCl  was  then 
taken  with  about  the  same  quantity  of  pure  KC1  in  the  same  manner.  The 
volume  of  permanganate  used  was  48 '9  c.c.  Taking  into  account  the  large  volume 
of  permanganate  required  for  so  small  a  quantity  of  sodium  the  difference  was 
infinitesimal  as  regards  the  amount  of  sodium  found.  Practice  undoubtedly 
renders  the  results  more  certain  if  exactly  the  same  conditions  are  observed,  more 
especially  in  keeping  the  temperature  down  to  as  near  0°  as  possible. 

The  process  seems  complicated,  but  when  once  the  cooling 
arrangements  are  satisfactorily  made  it  becomes  very  simple,  and 
if  there  are  a  series  of  sodium  determinations  to  be  made  such  as  the 
alkali  chlorides  in  mineral  water  residues,  etc.,  it  is  far  more  rapid, 
and  probably  more  exact  than  the  determination  of  the  potassium 
by  platinum,  and  calculating  the  sodium  by  difference. 

It  must  be  noted  that  the  method  is  not  available,  so  far  as  is 
known,  in  the  presence  of  metals  other  than  sodium,  potassium,  and 
magnesium.  Ammonium  should  be  removed,  and  borax  cannot 
accurately  be  examined  for  its  sodium.  The  metals  should  prefer- 
ably be  present  as  chlorides,  sulphates,  or  nitrates  ;  carbonates  and 
hydroxides  "of  sodium  must  be  neutralized  exactly  with  one  of  the 
mineral  acids. 

F  2 


68 


TECHNICAL   ANALYSIS 


9.     Determination  of  Potassium  by  means  of 
Sodium  Cobaltinitrite. 

Sodium  cobaltinitrite  [Na3Co  (N02)6]  under  the  name  of  de 
Koningh's  reagent  has  long  been  known  as  a  valuable  qualitative 
test  for  potassium,  with  solutions  of  which  it  gives  a  canary-yellow 
precipitate.  Its  use  for  the  quantitative  determination  of 
potassium,  both  gravimetrically  and  volumetrically,  has  been 
investigated  by  Adi e*  who.  on  adding  a  solution  of  sodium  cobalt- 
initrite to  a  potassium  salt,  obtained  a  compound  to  which  he 
gave  the  formula  K2NaCo  (NO2)6,H2O.  The  amount  of  K2O  in 
the  precipitate  may  be  found  by  means  of  a  decinormal  solution 
of  permanganate.  Cunningham  and  Perkinf  have  recently 
shown,  however,  that  the  precipitate  may  be  a  mixture  of  the  tri- 
and  di-potassium  salts,  K3Co  (NO2)6  and  K2NaCo  (N02)6,  and 
emphasize  the  difficulty,  also  referred  to  by  Adie,  of  washing 
the  precipitate  so  as  to  obtain  a  clear  filtrate.  The  constitution 
of  the  precipitate  is  shown  to  depend  on  whether  the  sodium 
cobaltinitrite  or  the  potassium  salt  is  in  excess.  Hence  the  authors 
consider  that  this  method  cannot  be  recommended  for  the  analysis 
of  potassium  or  cobalt  compounds. 

10.     Titration  of  Organic  Salts  of  the  Alkalies. 

The  following  organic  salts  yield  alkali  carbonate  on  ignition. 
When  the  latter,  in  aqueous  solution,  is  titrated  with  normal 
acid,  the  number  of  c.c.  used  multiplied  by  the  factor  tabulated 
below  gives  the  weight  of  the  corresponding  salt. 


Name  of  Salt. 

Formula. 

Normal 
Factor. 

Logarithm. 

Sodium  acetate 
,,          benzoate  . 

NaC2H3Oo.     3H,0 
Na  C7H562 

0-13608 
0-14404 

M3379    | 
1-15848 

,,          salicylate 

Na  CVH503 

0-16004 

1-20423 

Potassium  acetate 
„             bitartrate 

K  C2  H3  02 
KHC4H406 

0-09812 
0-18814 

2-99176 
1-27448 

,,             citrate 

K3C6H507.     H20 

0-10812 

1-03391 

,,             tartrate 

2K2C4H406.     H20 

0-11762 

1-07048 

,,             &  sodium  tartrate 

KNaC4H406. 

0-14110 

1-14953 

4H20 

TECHNICAL  EXAMINATION  OF  SOME  ALKALI  COMPOUNDS 
FOUND  IN  COMMERCE  OR  PRODUCED  IN  COURSE  OF 
MANUFACTURE. 

There  is  now  considerable  unanimity  among  English  and  foreign 
manufacturers  of  alkali  compounds  as  to  methods  of  analysis  to  be 
adopted  either  for  guidance  in  manufacture  or  for  commercial 

*J.  C.  S.  1900,77,  1076;  see  also  W.  A.  Dr ushel:  Zeit.  Anal.  Chem.  1907,  56,  223 
1908,  59,  97  ;  Analyst,  33.  35  and  378. 

t  J.  C.  S.,  1909,  95,  1562. 


OF   ALKALI   COMPOUNDS.  69 

valuation.  Lunge*  has  contributed  important  papers  on  the 
subject,  also  in  conjunction  with  Hurter  in  the  Alkali  Makers' 
Handbook,^  which  contains  valuable  tables  and  processes  of 
analysis.  So  far  as  volumetric  methods  are  concerned,  the  same 
processes  will  be  given  here,  together  with  others. 


11.     Soda  Ash,  Black  Ash,  Mother-liquors,  etc. 

Soda  Ash  or  Refined  Alkali. — 5  or  10  gm.  are  dissolved  in  about  150  c.c.  of 
warm  distilled  water,  and  any  insoluble  matter  filtered  off  (German  chemists  do 
not  filter),  and  the  volume  diluted  to  \  or  1  litre. 

The  total  alkali  is  determined  in  50  c.c.  by  normal  sulphuric,  nitric,  or 
hydrochloric  acid,  as  on  page  60.  \% 

The  caustic  alkali  present  in  any  sample  is  determined  as  on  page  61.  2. 

The  presence  of  sulphide  is  ascertained  by  the  smell  of  sulphuretted  hydrogen 
when  the  alkali  is  saturated  with  an  acid,  or  by  dipping  paper  steeped  in  sodium 
nitro-prusside  into  the  solution  :  if  the  paper  turns  blue  or  violet,  sulphide  is 
present. 

The  quantity  of  sulphide  and  sulphite  may  be  determined  by  saturating 
a  dilute  solution  of  the  alkali  with  a  slight  excess  of  acetic  acid,  adding  starch, 
and  titrating  with  N/iO  iodine  solution  till  the  blue  colour  appears.  The 
quantity  of  iodine  required  is  the  measure  of  the  sulphuretted  hydrogen  and 
sulphurous  acid  present. 

The  proportion  of  sulphide  is  determined  as  follows  :  13*820  gm.  of  pure  silver 
are  dissolved  in  dilute  nitric  acid,  and  the  solution  together  with  an  excess  of 
liquid  ammonia  made  up  to  a  litre.  Each  c.c.  =0'005  gm.  Na2S. 

METHOD  OF  PROCEDURE  :  100  c.c.  of  the  alkali  liquor  is  heated  to  boiling, 
some  ammonia  added,  and  the  silver  solution  dropped  in  from  a  burette  until  no 
further  precipitate  of  Ag2S  is  produced.  Towards  the  end,  filtration  will  be 
necessary  in  order  to  ascertain  the  exact  point,  to  which  end  the  Be  ales  filter  is 
serviceable  (fig.  23).  The  amount  of  Na2S  so  found  is  deducted  from  the  total 
sulphide  and  sulphite  found  by  iodine. 

Sodium  chloride  (common  salt)  may  be  determined  by  carefully  neutralizing 
1  gm.  of  the  alkali  with  nitric  acid,  and  titrating  with  decinormal  silver  solution 
and  potassium  chromate.  Each  c.c.  represents  0'005846  gm.  of  common  salt. 
Since  the  quantity  of  acid  necessary  to  neutralize  the  alkali  has  already  been 
found,  the  proper  measure  of  N/io  nitric  acid  may  at  once  be  added. 

Sodium  sulphate  is  determined  either  directly  or  indirectly,  as  under  Sulphuric 
Acid.  Each  c.cr  of  normal  barium  chloride  is  equal  to  0'071  gm.  of  dry  sodium 
sulphate. 

Examination  of  Grade  Soda  Lyes  and  Red  Liquors.— K  a  1  m  a  n  n  and  S  p  ii  1 1  e  r§ 

recommend  a  process  based  on  the  insolubility  of  barium  sulphite  and  the 
solubility  of  barium  thiosulphate  in  alkaline  solutions.  The  determination  is 
performed  in  the  following  manner : — 1. — The  total  alkalinity  is  determined  in 
a  measured  volume  of  the  liquor  under  examination  by  titration  with  normal  acid, 
methyl  orange  being  used  as  indicator.  The  acid  consumed  equals  sodium 
carbonate  +  sodium  sulphide  +  sodium  hydroxide,  +  one-half  the  sodium  sulphite 
present  (Na2S03  is  alkaline  and  NaHS03  neutral  to  methyl  orange).  2. — An 
equal  volume  of  the  liquor  is  titrated  with  N/IO  solution  of  iodine,  the  volume 
consumed  corresponding  with  the  sodium  sulphide  +  the  sodium  sulphite  +  the 
sodium  thiosulphate.  3.— Twice  the  volume  of  liquor  used  in  (1)  and  (2)  is 
precipitated  with  an  alkaline  zinc  solution,  and  the  mixture  made  up  to  a  certain 

*J.  S.  C.I- 1,  12,  16,  55,  92. 

t  This  Avork  now  appears  in  an  extended  form  under  the  title  of  Technical  Chemists' 
Handbook.    (Gurney  &  Jackson,  1908.) 

J  This  gives  a  slight  error,  owing  to  traces  of  aluininate  of  soda  and  lime,  which 
consume  acid. 

§  DingL  polyt.  J.,  264,  456-459. 


70  BLACK  ASH.      SALT   CAKE. 

measure,  one-half  of  which  is  filtered,  acidified,  and  titrated  with  N/1O  iodine. 
The  iodine  consumed  equals  sodium  sulphite  +  thiosulphate.  4. — Three  or  four 
times  the  volume  of  liquor  used  in  (1)  and  (2)  is  treated  with  an  excess  of  a  solution 
of  barium  chloride,  the  mixture  made  up  to  a  known  volume  with  water,  and 
filtered,  (a)  One-third  or  one-fourth  (as  the  case  may  be)  of  the  filtrate  is  titrated 
with  normal  acid  the  amount  used  corresponding  with  the  sodium  hydroxide  +  the 
sodium  sulphite.  (6)  A  further  third  or  fourth  of  the  filtrate  is  acidified  and 
titrated  with  N/iO  iodine,  the  iodine  consumed  being  equal  to  sodium  sulphite  + 
sodium  thiosulphate.  The  calculation  is  made  as  follows  : — 

2  —46  =A  c.c.  N/i0  iodine  corresponding  to     ....     Na2S03 

2  — 3     =B  c.c.  N/io  iodine  corresponding  to       ...     Na2S 


4& — (2 — 3)  . .  =C  c.c.  N/io  iodine  corresponding  to 
47 — rVB  ....  =D  c.c.  normal  acid  corresponding  to 
1  — (4a  +TiiA)  =E  c.c.  normal  acid  corresponding  to 


Na2S203 

NaOH 

NaoC03 


Black  Ash. — Digest  50  gm.  with  warm  water  in  a  half-1  tre  flask,  fill  up  to 
mark,  shake,  and  allow  to  stand  till  settled. 

(1)  Total  Alkali  existing  as  carbonate,  hydrate,  and  sulphide,   is  found  by 
titrating  10  c.c.  of  the  clear  liquid  (  =1  gm.  of  ash)  with  standard  acid  and  methyl 
orange  in  the  cold. 

(2)  Caustic  Soda. — 20  c.c.  of  the  liquid  are  put  into  a  100  c.c.  flask  with  10 
c.c.  of  solution  of  barium  chloride  of  10  per  cent,  strength,  filled  up  with  hot 
water,  well  shaken,  and  corked  after  settling  a  few  minutes.     The  clarified  liquid 
is  filtered,  and  50  c.c.  (=  1  gm.  ash),  titrated  with  standard  acid  and  methyl  orange  ; 
or  it  may  be  titrated  without  filtration  if   standard  oxalic  acid  and  phenolph- 
thalein  are  used,  this  acid  having  no  effect  on  the  barium  carbonate.     Each  c.c. 
normal  acid  =0'031  Na20.     This  includes  sulphides. 

(3)  Sodium  Sulphide. — Put  10  c.c.  of  liquor  into  a  flask,  acidulate  with  acetic 
acid,  dilute  to  about  200  c.c.  and  titrate  with  N/io  iodine  and  starch.     Each  c.c. 

=0-0039  Na2S,  or  0'0031  Na20. 

(4)  Sodium   Chloride. — 10   c.c.    are   neutralized   exactly   with   normal   nitric 
acid,  and  boiled  till  all  H2S  is  removed.      Any  sulphur  which  may  have  been 
precipitated  is  filtered  off,  and  the  filtrate  titrated  with  N/1O  silver  and  chromate. 
Each  c.c.  =0-005846  gm.  NaCl. 

(5)  Sodium  Sulphate. — This  is  best  determined  by  precipitation  as  barium 
sulphate,   and   weighing,   the   quantity   being   small.      If,    however,  volumetric 
determination  is  desired,  it  may  be  done  as  under  Sulphuric  Acid,  taking  50  c.c.  of 
liquor. 

For  other  methods  of  examining  the  various  solid  and  liquid 
alkali  wastes  used  for  soda  and  sulphur  recovery,  etc.,  the  reader  is 
referred  to  the  Technical  Chemists'  Handbook  already  mentioned. 

12.    Salt  Cake. 

This  is  the  somewhat  impure  sodium  sulphate  used  in  alkali 
manufacture  or  left  in  the  retorts  in  preparing  hydrochloric  acid 
from  sulphuric  acid  and  salt. 

Free  acid  is  determined  by  dissolving  20  gm.  of  the  sample, 
diluting  to  250  c.c.,  taking  out  50  c.c.  (  =  4  gm.  salt  cake)  with 
a  pipette,  adding  methyl  orange,  and  titrating  with  standard 
sodium  carbonate.  The  total  acidity  is  calculated  as  SO3,  in- 
cluding HC1  and  NaHSO4. 

The  common  salt  present  is  determined  by  decinormal  silver  solution  and 
potassium  chromate,  having  first  saturated  the  free  acid  with  pure  sodium 
carbonate ;  1  c.c.  silver  solution  is  equal  to  0-005846  gm.  of  salt. 

Sulphuric  acid,  combined  with  soda,  is  determined  either  directly  or  indirectly 
as  under  Sulphuric  Acid;  1  c.c.  of  normal  barium  solution  is  equal  to  0'071  gm. 
of  dry  sodium  sulphate. 


SOAP.  71 

Iron  is  precipitated  from  a  filtered  solution  of  the  salt  cake  with  ammonia  in 
excess,  the  precipitate  of  ferric  hydrate  re-dissolved  in  sulphuric  acid,  reduced  to 
the  ferrous  state  with  zinc  and  titrated  with  permanganate. 

13.    Soap. 

The  following  is  a  resume  of  the  methods  given  for  the  analysis 
of  soaps  by  Lewkowitsch.* 

(1)  Water. — This  is  usually  found  by  difference.     The  sum  of  the  percentages 
of  fatty  anhydrides  and  alkali  in  its  various  forms  subtracted  from  100  gives  the 
water.     For  direct  determination  in  exceptional  cases  about  5  grams, — carefully 
taken  from  the  centre  of  a  cake  by  cutting  away  all  the  outer  portions — in  thin 
shavings  are  put  into  a  porcelain  dish  and  weighed  with  a  glass  rod,  which  is  used 
from  time  to  time  to  break  the  skin  that  forms  on  drying  and  prevents  the  escape 
of  water  from  the  inner  portions.     Dry  at  100°  C.  till  of  constant  weight. 

(2)  Fatty  matter  and  total  alkali. — Weigh  accurately  5 — 10  gm.  of  the  sample 
and  dissolve,  with  constant  stirring,  in  hot  water  in  a  beaker  or  porcelain  dish, 
heating  gently.     Add  a  few  drops  of  methyl  orange  and  acidify  with  a  known 
volume  of  normal  HC1  or  H2S04.     Heat  with  constant  stirring  until  the  separated 
fatty  acids  have  melted  into  oily  drops.     Add  a  known  weight  (about  5  gm.)  of 
dry  beeswax  or  paraffin  wax,  and  heat  again  until  the  mixture  of  fatty  matter 
and  wax  appears  as  a  clear,  transparent  layer  on  the  surface  of  the  liquid.     Remove 
the  stirrer,  after  rinsing  with  boiling  water,  heat  till  the  fatty  matter  again  collects 
into  one  mass,  remove  the  vessel  from  the  source  of  heat  and  set  aside  in  a  cool 
place.     The  solid  cake  that  forms  on  the  surface  is  then  removed  by  means  of  a 
platinum  spatula,  rinsed  with  cold  water  and  placed  on  filter  paper.     The  sides 
of  the  vessel  are  carefully  scraped  and  the  scrapings  added  to  the  cake.     The  cake 
is  dried  by  touching  lightly  with  filter  paper  and  placed  bottom  upwards  on  a 
watch  glass,  allowed  to  dry  in  a  desiccator  and  weighed.     The  weight  thus  obtained, 
less  the  wax  added,  gives  the  fatty  matter.     If  neutral  fat,  wax  and  unsaponifiable 
matter  are  absent,  the  result  may  be  returned  as  fatty  acids  (rosin  acids  are 
regarded  as  so  much  fatty  acids).     In  a  complete  analysis  of  a  soap  the  fatty  acids 
are  multiplied  by  0'9675  and  the  value  so  obtained  returned  as  fatty  anhydrides. 
The  latter  form  a  direct  measure  of  the  actual  amount  of  soap  present. 

The  acid  liquid  is  filtered  to  remove  traces  of  fatty  acids  and  titrated  back  with 
N/2  soda  or  potash.  The  total  alkali  thus  obtained  is  calculated  to  Na20  in  the  case 
of  hard  soaps  and  to  K20  in  the  case  of  soft  soaps. 

(3)  Free  caustic  alkali. — Dissolve  a  weighed  portion  of  the  soap  in  absolute 
alcohol  and  filter.     The  alkaline  salts  remain  on  the  filter  and  the  titration  of  the 
alcoholic  filtrate  with  N/io  HC1,  using  phenolphthalein  as  indicator,  gives  the 
result  required. 

Should  the  alcoholic  solution  be  acid,  it  must  be  titrated  with  N/io  alkali  and  the 
latter  calculated  to  free  fatty  acids,  expressed  as  oleic. 

(4)  The  precipitate  on  the  filter  in  (3)  may  contain  carbonate,  silicate,  borate, 
etc.     It  is  washed  with  cold  water  and  the  filtrate  titrated  with  N/io  acid,  using 
methyl  orange  as  indicator.     The  number  of  c.c.  used  calculated  as  Na20  give 
the  alkali  contained  in  the  alkaline  salts. 

The  sum  of  (3)  and  (4)  subtracted  from  (2),  gives  the  combined  alkali. 
In  valuing  a  hard  soap  it  should  be  noted  that  the  presence  of  more  than  64  per 
cent,  of  fatty  acids  shows  that  the  soap  has  either  dried  spontaneously  on  keeping 
or  been  dried  artificially ;  a  less  percentage  indicates  the  presence  of  an  excess  of 
water  or  of  alkali,  or  some  adulterant. 

A  genuine  potash  soap  (soft  soap)  should  theoretically  have  the  following 
composition : — 

Fatty  anhydrides 38*70  % 

Combined  potassium  oxide  (K20)  . .          . .       6'84 
Glycerol,  water,  and  potassium  carbonate       54*46 

100-00 


Oils,  Fats  and  Waxes,  4th  edition,  vol.  iii.,  p.  287  et  seq. 


72  ALKALINE  EARTH  COMPOUNDS. 

TITRATION    OF    ALKALINE    EARTHS    AND    THEIR 
COMPOUNDS. 

STANDARD  hydrochloric  or  nitric  acid  must  in  all  cases  be  used 
for  the  titration  of  the  caustic  or  carbonated  alkaline  earths,  as 
these  are  the  only  acids  yielding  soluble  compounds,  except  in 
the  case  of  magnesia.  In  titrating  the  oxides,  such  as  caustic 
lime,  baryta,  strontia,  or  magnesia,  any  of  the  indicators  may  be 
used  and  the  residual  method  should  be  adopted,  viz.,  adding 
a  known  excess  of  standard  acid,  boiling  to  expel  any  trace  of 
carbon  dioxide,  and  then  titrating  the  residual  free  acid  by  means 
of  standard  alkali. 

The  carbonates  of  the  same  bases  may,  of  course,  also  be  determined 
in  the  same  way,  bearing  in  mind  that  when  methyl  orange  is  used 
the  liquid  is  best  cooled  before  the  final  titration.  All  heating  may 
be  avoided  when  using  methyl  orange  in  titrating  mixtures  of 
oxides  or  hydroxides  and  carbonates,  or  the  latter  only,  unless  it  is 
impossible  to  dissolve  the  substance  in  the  cold.  A  good  excess 
of  acid  is  generally  advisable. 

The  total  amount  of  base  in  mixtures  of  caustic  and  carbonated 
alkaline  earths  is  also  determined  in  the  same  way. 

(1)  Determination  of  Mixed  Hydroxides  and  Carbonates. — This 
may   be   done   using   either   phenacetolin   or   phenolphthalein   as 
indicator.     The  former  has  been  recommended  by  Degener  and 
Lunge  :  the  method,  however,  requires  practice  in  order  to  mark 
the  exact  change  of  colour. 

METHOD  OF  PROCEDURE  :  The  liquid  containing  the  compound  in  a  fine  state 
of  division  is  tinted  with  phenacetolin  so  as  to  be  of  a  faint  yellow ;  normal  acid 
is  then  cautiously  added  until  a  permanent  pink  occurs  (at  this  stage  all  the 
hydroxide  is  saturated),  more  acid  is  now  cautiously  added  until  the  colour  becomes 
deep  yellow, — the  volume  of  acid  so  used  represents  the  carbonate. 

The  method  is  especially  adapted  to  mixtures  of  calcium  hydroxide  and 
carbonate.  It  is  also  applicable  to  barium,  but  not  to  magnesium,  owing  to 
the  great  insolubility  of  magnesium  hydroxide  in  dilute  acid. 

If  phenolphthalein  is  used  as  indicator,  the  method  is  as  follows  : 

fHeat  the  liquid  to  boiling,  and  cautiously  add  normal  acid  until  the  red  colour 
is  just  discharged.  The  carbonates  of  calcium  and  barium,  rendered  dense  by 
boiling,  are  both  quite  neutral  to  the  indicator.  To  obtain  the  whole  of  the  base, 
excess  of  normal  acid  is  used,  and  the  mixture  re-titrated  with  normal  alkali. 

Magnesium  in  solution  as  bicarbonate  may  be  accurately 
determined  in  the  cold  with  methyl  orange  as  indicator. 

(2)  Determination  of  Calcium,  Barium,  Magnesium  and  Strontium 
in  Neutral  Soluble  Salts. — The  amount  of  base  in  the  chlorides  and 
nitrates   of   the    alkaline    earths   may   be   readily   determined    as 
follows  :— 

The  weighed  salt  is  dissolved  in  water,  cautiously  neutralized  if  acid  or  alkaline, 
phenolphthalein  added,  heated  to  boiling,  and  normal  sodium  carbonate  delivered 
in  from  time  to  time  with  boiling  until  the  red  colour  is  permanent. 

Magnesium  salts  cannot,  however,  be  determined  in  this  way,  or  even  mixtures 
of  lime  and  magnesia,  as  magnesium  carbonate  affects  the  indicator  in  a  different 
manner  from  the  other  carbonates. 


NITRATE    OF   LIME.  73 

Nitrate  of  Lime  (atmospheric).  The  nitrogen  in  this  fertiliser  may  be  indirectly 
determined  by  titrating  the  calcium  present  as  nitrate  with  standard  sodium 
carbonate. 

The  nitrate  of  lime  must  not  contain  the  following : — (a)  Nitrates  of  metals, 
the  carbonates  of  which  are  soluble  in  water  (alkali  nitrates) ;  (6)  soluble  oxides 
of  metals,  the  carbonates  of  which  are  soluble  in  water  (alkali  oxides) ;  (c)  soluble 
salts  of  metals,  the  carbonates  of  which  are  insoluble  in  water,  except  nitrites  and 
nitrates  (e.g.,  MgS04,  CaCl2).  I  have  found  that  these  conditions  are  fulfilled  by 
the  leading  brand  of  this  fertiliser  now  on  the  English  market,  and  this  indirect 
method  gives  quite  satisfactory  results  in  comparison  with  the  reduction  and 
distillation  process. 

METHOD  OF  PROCEDURE  :  10  grams  Nitrate  of  Lime  [containing  about  80  % 
Ca  (N03)2]  are  dissolved  in  water  and  made  up  to  500  c.c.  without  filtration.  50  c.c. 
are  transferred  to  a  beaker  or  conical  flask  and  50  c.c.  **/5  sodium  carbonate 
added,  boiled,  and  the  precipitated  calcium  carbonate  together  with  other 
insoluble  substances,  if  present,  filtered  off.  In  the  cooled  filtrate  the  excess  of 
sodium  carbonate  is  titrated  with  N/5  hydrochloric  acid  and  methyl  orange.  The 
Xa2C03  consumed  corresponds  to  the  Ca(N03)2  present. 

1  P.C.  N/5  Naa003  = -0028  gram  N. 

Should  the  nitrate  of  lime  contain  free  limet  this  does  not  affect  the  titration, 
as  an  amount  of  sodium  hydroxide  is  formed  in  the  filtrate  equivalent  to  the 
sodium  carbonate  consumed  by  the  calcium  hydroxide. 

EXAMPLE:  50  c.c.  of  solution  (  =  1  gm.  salt) +  50  c.c.  N/§  Na2C03  required 
2-3  c.c.  N/5  HC1.  50-2-3  =  47'7  c.c.  N/6  Na,C08  consumed.  47 '7  x  -0028  x  100 
-13-35  °/0  Nitrogen. 

(3)  Precipitation  of  the  Alkaline  Earths  from  their  Neutral  Salts  as  Carbonates. 

— Soluble  salts  of  calcium,  barium,  and  strontium,  such  as  chlorides,  nitrates,  etc., 
are  dissolved  in  water,  and  the  base  precipitated  as  carbonate,  with  excess  of 
ammonium  carbonate  and  some  free  ammonia.  The  mixture  is  heated  to  about 
60°  C.  for  a  few  minutes.  The  precipitated  carbonate  is  then  to  be  filtered,  well 
washed  with  hot  water  till  all  soluble  matters,  especially  ammonia,  are  removed, 
and  the  precipitate  with  filter  titrated  with  normal  acid  as  already  described. 
Magnesium  salts  cannot  be  determined  in  this  way. 

(4)  Calcium  and  Magnesium  Carbonates  in  Waters. — The  amount  of  calcium 
or  calcium  and  magnesium  carbonates  dissolved  in  ordinary  non-alkaline  waters 
may  be  very  readily,  and  with  accuracy,  found  by  taking  200  or  300  c.c.  of  the 
water,  heating  nearly  to  boiling,  adding  phenacetolin  or  lacmoid,  and  titrating 
cautiously  with  N/iq  acid.     An  equally  accurate  result  may  be  obtained  by  methyl 
orange  in  the  cold  liquid. 

(5)  Determination     of     Calcium     and     Magnesium     Sulphates, 
Chlorides,  Nitrates,  and  Carbonates  in  Waters  and  the  degrees  of 
hardness   obtained   without   the   use   of   Clark's  Standard  Soap 
Solution.     As  is  generally  known,   the  soap-destroying  power  of 
a  water  is  ascertained  in  Clark's  process  by  a  standard  solution  of 
soap  in  weak  alcohol,  titrated  against  a  standard  solution  of  calcium 
chloride.     The  valuation  is  in  so-called  degrees,  each  degree  being 
equal  to  1  grain  of  calcium  carbonate,  or  its  equivalent,  in  the 
Imperial  gallon.     The  process  is  an  old  and  familiar  one,  but  open 
to  many  objections  from  a  scientific  point  of  view.     The  scale  of 
degrees  is  arbitrary,  and  is  seriously  interfered  with  by  the  presence 
of  varying  proportions  of  magnesium  salts. 

We   are   indebted,    primarily    to   Mohr,    and    subsequently   to 


74  HARDNESS    IN    WATER. 

H  e  h  n  e  r,  f  or  an  ingenious  method  of  determining  both  the  temporary 
and  permanent  hardness  of  a  water  without  the  use  of  soap 
solution. 

The  standard  solutions  required  are.N/50  sodium  carbonate  and 
N/50  sulphuric  acid.  Each  c.c.  of  standard  acid  exactly  neutralizes 
1  mgm.  of  CaCO3  and  each  c.c.  of  the  alkali  precipitates  the  like 
amount  of  CaC03,  or  its  equivalent  in  magnesium  salts,  in  any 
given  water. 

METHOD  OF  PROCEDURE  :  100  c.c.  of  the  water  are  tinted  with  an  indicator 
of  suitable  character,  heated  nearly  to  boiling,  and  standard  acid  cautiously  added 
until  the  proper  change  of  colour  occurs.  Hehner  recommends  phenacetol in ; 
but  my  own  experiments  give  the  preference  to  lacmoid,  which  is  also  commended 
by  T  h  o  m  s  o  n.  Draper*  points  out  the  value  of  lacmoid  and  carminic  acid  for 
such  a  process,  and  I  fully  endorse  his  opinion  with  respect  to  both  indicators. 

Another  indicator,  erythrosin,  is  recommended  by  J.  W.  Ellmsf.  The 
advantage  this  indicator  possesses  is  that  it  is  less  affected  by  C02,  the  titration 
may  be  made  in  the  cold,  and  it  also  gives  more  accurate  results  with  fairly  turbid 
or  coloured  water  than  with  the  indicators  above  mentioned.  It  is  not,  however, 
the  preparation  described  on  page  40,  but  is  simply  a  sodium  salt  of  erythrosin 
in  ordinary  use,  dissolved  in  distilled  water  in  the  proportion  of  O'l  gm.  per  litre. 
The  titration  is  made  in  a  250  c.c.  stoppered  white  bottle  that  it  may  be  well  shaken 
without  loss.  100  c.c.  of  the  water  together  with  2'5  c.c.  of  the  indicator,  and 
5  c.c.  of  chloroform.  These  are  well  shaken  and  the  acid  added  in  small  quantities, 
and  well  shaken  after  each  addition.  The  rose  colour  of  the  water  toward  the  end 
of  the  titration  becomes  less  marked,  and  with  a  very  slight  excess  of  acid  quite 
colourless.  The  chloroform  produces  a  milky  appearance  from  frequent  shaking, 
but  this  is  no  hindrance  to  the  end-point ;  if  desired,  however,  it  will  settle  in  a  short 
time.  A  piece  of  white  paper  behind  the  bottle  will  facilitate  the  detection  of 
any  trace  of  colour  remaining  as  the  titration  approaches  the  end-point. 

If  the  most  accurate  results  are  desired,  any  of  the  indicators  should  be 
submitted  to  a  blank  experiment  by  taking  a  measured  volume  of  it  with  100 
c.c.  of  distilled  water,  and  finding  how  much  of  the  acid  is  required  to  remove  the 
colour ;  the  quantity  of  acid  so  found  should  then  be  deducted  from  all  readings 
before  converting  them  into  calcium  carbonate. 

The  number  of  c.c.  of  acid  used  represents  the  temporary  hardness  in  parts 
per  100,000.  To  obtain  "  degrees  of  hardness,"  multiply  the  number  of  c.c.  by 
0'7.  The  permanent  hardness  is  ascertained  by  taking  100  c.c.  of  the  water  and 
adding  to  it  a  rather  large  known  excess  of  the  standard  sodium  carbonate.  The 
quantity  must  of  course  be  regulated  by  the  amount  of  sulphates,  chlorides,  or 
nitrates  of  calcium  and  magnesium  present  in  the  water ;  as  a  rule,  a  volume 
equal  to  that  of  the  water  will  more  than  suffice.  Evaporate  in  a  platinum  dish 
to  dryness  (glass  or  porcelain  will  not  do,  as  they  affect  the  hardness),  then  extract 
the  soluble  portion  with  small  quantities  of  distilled  water,  pass  through  a  very 
small  filter,  and  titrate  the  filtrate  with  the  standard  acid  for  the  excess  of  sodium 
carbonate :  the  difference  represents  the  permanent  hardness. 

Some  waters  contain  alkali  carbonates,  in  which  case  there  is 
of  course  no  permanent  hardness,  because  the  salts  to  which  this  is 
due  are  decomposed  by  the  alkali  carbonate.  In  examining 
a  water  of  this  kind,  the  temporary  hardness  will  be  shown  to  be 
greater  than  it  really  is,  owing  to  the  alkali  carbonate  ;  and  the 
experiment  for  permanent  hardness  will  show  more  sodium  carbonate 
than  was  actually  added.  If  the  difference  so  found  is  deducted 
from  the  temporary  hardness,  at  first  noted,  the  remainder  will  be 
the  true  temporary  hardness. 

*  C.  N.  51,  206.  t  J.  Am.  C.  S.  1899,  p.  359. 


AMMONIUM    SALTS. 


75 


1. 


AMMONIA. 

NH3  =  17-034. 

Determination  of  Combined  Ammonia  by  distillation  with 
Alkalies  or  Alkaline  Earths. 


THIS  method  brings  about  the  expulsion  of  ammonia  from  all  its 
salts.  Caustic  soda,  potash,  or  lime  may  any  of  them  be  used. 
Where  an  organic  nitrogenous  compound  exists  in  the  substance 
it  is  in  most  cases  necessary  to  submit  it  to  preliminary  treatment 
by  Kjeldahl's  method  (p.  83). 

There  is  a  great  variety  of  distilling  vessels  convenient  for  this 
process. 

Any  of  the  ordinary  forms  of  apparatus  will  be  found  useful  for 
accurately  determining  the  ammonia  in  any  of  its  compounds 
which  can  be  decomposed  by  soda,  potash,  or  lime.  The  gas  so 
evolved  is  collected  in  a  known  volume  in  excess  of  standard  acid, 
the  excess  of  acid  being  afterwards  found  by  residual  titration 
with  standard  alkali.  A  compact  modern  form  of  apparatus  is 
shown  in  fig.  28. 


Fig.  28. 


. 

Fig.  29. 


An  ingenious  and  useful  distillation  tube  for  rapid  determinations 
of  ammonia  designed  by  Hopkins*  is  shown  in  fig.  29.  It  is 
made  from  tubing  of  7  to  8  mm.  bore.  The  side  openings,  A  and 
Av  should  be  nearly  as  large,  and  the  bulb  about  5  cm.  in 


*J.  Am.  C>S.  1896,  p.  227. 


76  AMMONIUM   SALTS. 

diameter.  The  length  of  the  tube  below  the  bulb  is  12  cm.,  and 
that  above  the  bulb  about  the  same.  The  jets  C  and  Cx  are  2  mm. 
inside  diameter.  In  use,  the  tube  is  pushed  through  the  cork  of  the 
distilling  flask  until  the  opening  At  is  below  the  cork  ;  the  vapour 
then  passes  through  the  side  openings,  and  whatever  condenses  in 
the  tube  below  the  bend  B,  runs  back  into  the  flask  through  the 
jets  C  and  C15  which  always  remain  filled  with  liquid. 

METHOD  OF  PROCEDURE  :  The  distilling  flask  a  (see  Fig.  28),  capacity  about 
400  c.c.,  is  placed  upon  the  wire  gauze,  and  contains  the  ammoniacal  substance. 
The  tube  d  is  filled  with  strong  caustic  potash  or  soda.  The  flask  b  holds  about 
300  c.c.  and  contains  a  measured  quantity  of  standard  acid,  part  being  contained 
in  the  tube  c  which  is  filled  with  glass  wool  or  broken  glass,  and  through  which 
the  standard  acid  has  been  poured.  The  stoppers  of  the  flasks  should  be  of 
caoutchouc,  failing  which,  good  corks  soaked  in  melted  paraffin  may  be  used. 

The  substance  to  be  examined  is  weighed  or  measured,  and  put  into  the  dis- 
tilling flask  a  with  a  little  water.  The  apparatus  then  being  made  tight  at  every 
point,  some  of  the  caustic  alkali  is  allowed  to  flow  into  a  by  opening  the  tap  of  d, 
and  the  gas  or  spirit  lamp  is  lighted  under  it. 

The  contents  are  brought  to  gentle  boiling,  taking  care  that  the  froth,  if  any, 
does  not  enter  the  distilling  tube.  It  is  as  well  to  use  a  movable  gas  burner  or 
common  spirit  lamp,  so  that,  if  there  is  any  tendency  to  boil  over,  the  heat  can 
be  removed  immediately  and  the  flask  blown  upon  by  the  breath,  which  reduces 
the  pressure  in  a  moment.  In  examining  guano  and  other  substances  containing 
ammoniacal  salts  and  organic  matter  the  tendency  to  frothing  is  considerable; 
and  unless  the  above  precautions  are  taken  the  accuracy  of  the  results  will  be  inter- 
fered with.  A  small  piece  of  beeswax,  solid  paraffin,  or  granulated  zinc  is  very 
serviceable  in  preventing  frothing  or  bumping. 

The  distilling  tube  has  both  ends  cut  obliquely,  and  the  lower  end  nearly, 
but  not  quite,  reaches  to  the  surface  of  the  acid,  to  which  a  little  methyl  orange 
may  be  added.  The  quantity  of  standard  acid  used  must,  of  course,  be  more  than 
sufficient  to  combine  with  the  ammonia  produced ;  the  excess  is  afterwards 
ascertained  by  titration  with  standard  alkali.  The  boiling  should  be  continued 
till  about  two-thirds  of  the  liquor  in  the  distilling  flask  have  distilled  over.  The 
tube  c  must  be  thoroughly  washed  out  into  the  flask  &  with  distilled  water,  so  as 
to  carry  down  the  acid  with  any  combined  gas  which  may  have  reached  it.  The 
titration  then  proceeds  as  usual.  Each  c.c.  of  N/i  acid  neutralized  by  the  ammonia 
=0-017  gram,  of  NH3. 


2.     Indirect  Method. 

In  the  case  of  tolerably  pure  ammoniacal  salts  or  liquids  free 
from  acid,  or  in  which  the  free  acid  has  previously  been  determined, 
a  simple  indirect  method  can  be  used,  as  follows  : — 

If  the  ammoniacal  salt  be  boiled  in  an  open  vessel  with  normal  eaustic  alkali 
the  ammonia  is  entirely  set  free,  leaving  its  acid  combined  with  the  fixed  alkali. 
If,  therefore,  the  quantity  of  alkaline  solution  is  known,  the  excess  beyond  that 
necessary  to  replace  the  ammonia  may  be  found  by  titration  with  normal  acid. 
The  boiling  of  the  mixture  must  be  continued  till  a  piece  of  red  litmus  paper, 
held  in  the  steam  from  the  flask,  is  no  longer  turned  blue. 

EXAMPLE  :  5  grams  of  a  dirty  sample  of  sulphate  of  ammonia  were  boiled  on 
a  sand-bath  in  a  large  flask  with  100  c.c.  of  N/i  NaOH  till  all  ammonia  was  expelled. 
On  then  titrating  back  with  Normal  H2SO4  29'4  c.c.  were  required.  The  free 
acid  in  5  grams  required  1-2  c.c.  of  N/i  NaHO.  Hence  100  -1-2  -29'4  =  69'4, 
and  -017  x  69 '4  x  20  =23 '59  %  NH3. 


GAS    LIQUOR.  77 

3.     Technical  Analysis  of  Ammoniacal  Gas  Liquor, 

Sulphate  of  Ammonia,  Sal  Ammoniac,  etc.  (arranged  for 

the  use  of  Manufacturers). 

This  process  depends  upon  the  fact  that  when  ammoniacal  salts 
are  boiled  with  caustic  soda,  potash,  or  lime,  the  whole  of  the 
ammonia  is  expelled  in  a  free  state  and  may  by  a  suitable  apparatus 
(fig.  28)  be  determined  with  extreme  accuracy  (see  p.  76). 

Technical  Analysis  of  Gas  Liquor. — This  liquid  consists  of 
a  solution  of  carbonates,  sulphates,  hyposulphites,  sulphides, 
sulphocyanides,  and  other  salts  of  ammonia.  The  object  of  the 
ammonia  manufacturer  is  to  get  all  these  out  of  his  liquor  into  the 
form  of  sulphate  or  chloride  as  economically  as  possible.  The 
whole  of  the  ammonia  existing  free  or  as  carbonate  in  the  liiquor 
can  be  distilled  off  at  a  steam  heat ;  the  fixed  salts,  however, 
require  to  be  heated  with  soda,  potash,  or  lime  (the  latter  is  generally 
used  on  a  large  scale  as  being  most  economical,  sometimes  with  an 
addition  of  caustic  soda  towards  the  end  of  the  distillation),  in 
order  to  liberate  the  ammonia  contained  in  them. 

The  apparatus  here  described  is  the  same  on  a  small  scale  as  is 
necessary  in  the  actual  manufacture  of  sulphate  of  ammonia  in 
quantities  ;  and  its  use  enables  any  manufacturer  to  tell  to  a  fraction 
how  much  sulphate  of  ammonia  he  ought  to  obtain  from  any  given 
quantity  of  gas  liquor.  It  also  enables  him  to  tell  exactly  how 
much  ammonia  can  be  distilled  off  with  heat  alone,  and  how  much 
exists  in  a  fixed  condition  requiring  lime  or  caustic  soda. 

The  measures  used  in  this  process  are  on  the  metric  system,  the 
use  of  which  may,  perhaps,  at  first  sight  appear  strange  to  English 
manufacturers  ;  but  as  the  only  object  of  the  process  is  to  obtain 
the  percentage  of  ammonia  in  any  given  substance,  it  is  a  matter  of 
no  importance  which  system  of  measures  or  weights  is  used,  as 
when  once  the  percentage  is  obtained  the  tables  will  show  the  result 
in  English  terms  of  weight  or  measure. 

METHOD  OF  PROCEDURE  :  Whatever  the  strength  of  the  liquor  10  c.c.,  measured 
by  a  pipette,  is  the  quantity  invariably  used  for  the  analysis.  This  quantity  is 
transferred  without  spilling  a  drop  to  the  distilling  flask — the  fittings  having  been 
previously  removed— and  the  tube  d  (Fig.  28)  is  then  filled  with  strong  caustic  soda 
solution.  The  cork  is  then  replaced,  and  the  flask  securely  imbedded  in  perfectly 
dry  sand  in  a  sand-bath  or  supported  on  wire  gauze.  20,  30,  40  or  50  c.c.  of  standard 
acid— according  to  the  estimated  strength  of  the  liquor — are  allowed  to  flow  into 
the  receiving  flask  through  the  tube  c  (Fig.  28),  which  is  filled  with  broken  glass 
placed  on  a  layer  of  glass  wool  or  fibrous  asbestos.  The  broken  glass  should  be 
completely  wetted  with  the  acid,  so  that  any  vapours  of  ammonia  which  may 
escape  the  acid  in  the  flask  shall  become  absorbed  by  the  acid.  The  quantity  of 
standard  acid  to  be  used  is  regulated  by  the  approximately  known  strength  of  the 
liquor,  which  of  course  can  be  told  by  Twaddle's  hydrometer :  thus  for  a  liquor 
of  3°  Twaddle  (  =6-oz.  liquor),  20  c.c. — 8-oz.,  25  c.c. — 10-oz.,  30  c.c.  of  acid  will 
be  sufficient — but  there  must  always  be  an  excess.  The  required  quantity  can 
always  be  approximately  known,  since  every  10  c.c.  of  acid  represents  1  per  cent, 
of  ammonia.  The  standard  acid  having  been  carefully  passed  through  c  (Fig.  28), 
the  apparatus  is  fitted  together  by^the  elastic  tube  and  the  india-rubber  stoppers 


78  GAS   LIQUOR. 

securely  inserted  in  both  flasks  ;  this  being  done,  the  lamp  is  lighted  under  the 
sand-bath,  and  at  the  same  time  the  tap  on  d  (Fig.  28)  is  opened,  so  as  to  allow 
caustic  soda  to  flow  into  the  distilling  flask.  The  heat  is  continued  to  boiling, 
and  allowed  to  go  on  till  the  greater  bulk  of  the  liquid  has  distilled  over  into  the 
receiving  flask.  A  quarter  of  an  hour  is  generally  sufficient  for  this  purpose, 
but  if  the  boiling  is  continued  till  the  liquid  just  covers  the  bottom  of  the  flask, 
all  the  ammonia  will  have  gone  over ;  during  the  whole  operation  the  distilling 
tube  must  never  dip  into  the  acid  in  the  receiving  flask.  The  apparatus  may  then 
be  detached  ;  distilled  or  good  boiled  drinking  water  is  then  poured  repeatedly 
through  the  tubes  in  small  quantities,  till  all  traces  of  acid  are  washed  down  into 
the  receiving  flask.  This  latter  now  contains  all  the  ammonia  out  of  the  sample  of 
liquor,  with  an  excess  of  acid,  and  it  is  necessary  now  to  find  out  the  quantity  of 
acid  in  excess.  This  is  done  by  means  of  a  standard  solution  of  caustic  soda, 
which  is  of  exactly  the  same  strength  as  the  standard  acid.  In  order  to  determine 
the  amount  of  the  standard  acid  which  has  been  neutralized  by  the  ammonia 
distilled  from  the  liquor  we  first  cool  the  receiving  flask  containing  the  distillate, 
add  to  it  one  drop  of  methyl  orange  or  a  sufficiency  of  some  other  suitable  indicator 
(but  not  phenolphthalein),  and  cautiously  run  into  it  from  a  burette  the  standard 
caustic  soda  solution,  with  constant  shaking,  until  the  indicator  changes  colour. 
The  number  of  c.c.  of  soda  so  used,  deducted  from  the  number  of  c.c.  of  standard 
acid  originally  taken,  gives  the  number  of  the  latter  neutralized  by  the  ammonia 
in  the  distillate  and  hence  in  the  10  c.c.  of  gas  liquor  distilled.  The  strengths 
of  the  standard  solutions  used  are  such  that  the  result  required  is  obtained  without 
any  calculation. 

EXAMPLE  :  Suppose  that  a  liquor  is  to  be  examined  which  marks  5°  T  w  a  d  d  1  e, 
equal  to  10-oz.  liquor ;  10  c.c.  of  it  are  distilled  into  30  c.c.  of  the  standard  acid, 
and  it  has  afterwards  required  6  c.c.  of  standard  soda  to  neutralize  it ;  this  leaves 
24  c.c.  as  the  volume  of  acid  saturated  by  the  distilled  ammonia,  and  this 
represents  2*4  per  cent.  ;  and  on  referring  to  the  table  it  is  found  that  this  number 
corresponds  to  a  trifle  more  than  11  ounces,  the  actual  figures  being  2*384  per 
cent,  for  11-ounce  strength. 

The  strength  of  the  standard  soda  and  acid  solutions  is  so  arranged 
that  when  10  c.c.  of  liquor  are  distilled  every  10  c.c.  of  acid  solution 
represents  1  per  cent,  of  ammonia  in  the  liquor.  Thus,  13  c.c.  of 
acid  will  represent  1-3  per  cent,  of  ammonia,  corresponding  to 
6-ounce  liquor. 

The  burette  is  divided  into  tenths  of  a  cubic-centimetre,  and  those 
who  are  familiar  with  decimal  calculations  can  work  out  the  results 
to  the  utmost  point  of  accuracy  ;  the  calculation  being  that  every 
1  per  cent,  of  ammonia  requires  4*61  ounces  of  concentrated  oil  of 
vitriol  (sp.  gr.  1'845)  per  gallon  of  liquor,  to  convert  it  into  sulphate. 
Thus,  suppose  that  10  c.c.  of  any  given  liquor  have  been  distilled, 
and  the  quantity  of  acid  required  amounts  to  18'6  c.c.,  this  is  1*86 
per  cent.,  and  the  ounce  strength  is  4'61  x  1*86  =  8'5746.  The 
liquor  is  therefore  a  trifle  over  8J-ounce  strength. 

Spent  Liquors. — It  is  frequently  necessary  to  ascertain  the  per- 
centage of  ammonia  in  spent  liquors  in  order  to  see  if  the  workmen 
have  extracted  all  the  available  ammonia.  In  this  case  the  same 
measure,  10  c.c.  of  the  spent  liquor,  is  taken,  and  the  operation 
conducted  precisely  as  in  the  case  of  a  gas  liquor. 

EXAMPLE  :  10  c.c.  of  a  spent  liquor  were  distilled,  and  found  to  neutralize 
3  c.c.  of  acid  :  this  represents  three-tenths  of  a  per-cent.  equal  to  1-oz.  and  four- 
tenths  of  an  ounce,  or  nearly  1£  oz.  Such  a  liquor  is  too  valuable  to  throw  away, 
and  should  be  worked  longer  to  extract  more  ammonia. 


GAS    LIQUOR. 


79 


METHOD  OF  PROCEDURE  FOR  SULPHATE  OF  AMMONIA  OR  SAL  AMMONIAC  : 
An  average  sample  of  the  salt  being  drawn,  ten  grams  are  weighed,  transferred 
without  loss  to  a  100  c.e.  flask,  distilled  or  boiled  drinking  water  poured  on  it, 
and  well  stirred  till  disolved,  and  finally  water  added  exactly  to  the  mark.  The 
10  c.c.  measure  is  then  filled  with  the  solution,  and  emptied  into  the  distilling 
flask  ;  30  c.c.  of  standard  acid  are  put  into  the  receiving  flask  and  the  distillation 
carried  on  precisely  as  in  the  case  of  the  gas  liquor.  The  number  of  c.c.  of 
standard  acid  required  shows  directly  the  percentage  of  ammonia ;  thus  if  24' 6 
c.c.  are  used  in  the  case  of  sulphate,  it  contains  24'6  per  cent,  of  ammonia. 

The  liquors  when  tested  must  be  measured  at  ordinary 
temperatures,  say  as  near  60°  F.  as  possible.  The  standard 
solutions  must  be  kept  closely  stoppered  and  in  a  cool  place. 

The  following  table  is  given  to  avoid  calculations  ;  of  course,  it 
will  be  understood  that  the  figures  given  are  on  the  assumption  that 
the  whole  of  the  ammonia  contained  in  the  liquor  is  extracted  in  the 
manufacture  as  closely  as  it  is  in  the  experiment.  With  the  most 
perfect  arrangement  of  plant,  however,  this  does  not  as  a  rule  take 
place  ;  but  it  ought  to  be  very  near  the  mark  with  proper  apparatus, 
and  care  on  the  part  of  workmen. 


Approxi- 
mate 
measure  of 
Standard 
Acid  in  c.c. 
and  tenths. 

Percentage 
of  Ammonia 
NH3. 

Ounce 
strength 
per 

gallon. 

Weight  of  Sulphuric  Acid  in  pounds 
and  decimal  parts  required  for  each 
gallon  of  liquor. 

Yield  of 
Sulphate 
per  gallon  in 
Ib.  and 
decimal 
parts. 

0.  O.  V. 
169°  Tw. 

B.  0.  V. 
144°  Tw. 

Chamber 
Acid 
120°  Tw. 

2-2 

•2168 

1 

•0625 

•0781 

•0893             -0841 

4-3 

•4336 

2 

•1250 

•1562 

•1786             -1682 

6'5 

•6504 

3 

•1875 

•2343 

•2679            -2523 

8-7 

•8672 

4 

•2500 

•3124 

•3572 

•3364 

10-1 

1-0840 

5 

•3125 

•3905 

•4465             -4205 

13-0 

1-3000 

6 

•3750 

•4686 

•5358             -5046 

152 

1-5176 

7 

•4375 

•5467 

•6251            -5887 

17'3 

1-7344 

8 

•5000 

•6248 

•7144            -6728 

19-5 

1-9512 

9 

•5625 

•7029 

•8037            '7569 

21-7 

2-1680 

10 

•6250 

•7810 

•8930 

•8410 

23-8 

2-3840 

11 

•6875 

•8591 

•9823            -9251 

26-0 

2-6016 

12 

•7500 

•9372 

1-0716 

•0092 

28-2 

2-8184 

13 

•8125 

1-0153 

1-1609 

•0933 

30-4 

3-0350 

14 

•8750 

1-0934 

1-2502 

•1774 

32-5 

3-2520 

15 

•9375 

•1715 

•3395 

•2615 

34-7 

3-4688 

16 

1-0000 

•2496 

•4288 

•3456 

36-9 

3-6856 

17 

1-0625 

•3277 

•5181 

•4297 

39-0 

3-9024 

18 

1-1250 

•4058 

•6074 

•5138 

41-2 

4-1192 

19 

1-1875 

•4839 

•6967 

•5979 

43-3 

4-3360 

20 

1-2500 

•5620 

•7860 

•6820 

The  weight  of  sulphuric  acid  being  given  in  decimals  renders  it 
very  easy  to  arrive  at  the  weight  necessary  for  every  thousand 
gallons  of  liquor,  by  simply  moving  the  decimal  point ;  thus  8-oz. 
liquor  would  require  500  Ib.  of  concentrated  oil  of  vitrol,  625  Ib.  of 
brown  oil  of  vitriol,  or  714J  Ib.  chamber  acid  for  every  1000  gallons, 
and  should  yield  in  all  cases  672'8  (say  673)  Ib.  of  sulphate. 


80  AMMONIACAL   LIQUORS. 

4.     Technically  complete  analysis  of  Ammoniacal  Liquors. 

The  Annual  Reports  of  the  Chief  Inspector  under  the  Alkali,  etc. 
Works  Regulation  Act  have  for  some  years  past  recorded  the 
results  of  investigations  conducted  in  his  laboratory  on  ammoniacal 
liquors  from  various  sources.  The  last  edition  of  this  work 
contained  a  general  summary  of  the  analytical  methods  adopted, 
as  given  in  the  Report  for  1903.  Since  that  date  the  procedure 
there  recommended  has  been  modified  as  the  result  of  further  study 
and  research  in  the  Chief  Inspector's  laboratory  and  elsewhere, 
notably  by  Feld*,  and  Mayer  and  Hempelf. 

The  procedure  now  approved  is  the  following  J  : — 

METHODS  OF  ANALYSIS. — 1.  Free  Ammonia.  («)  By  direct  titration,  to 
determine  approximately  the  volume  of  acid  required  for  distillation  (6) : — 
10  c.c.  of  liquor  are  diluted  to  100  c.c.  and  titrated  with  N/2  H2S04,  Methyl  orange 
indicator. 

(6)  By  distillation  : — 10  c.c.  of  liquor  (more  if  weak)  are  diluted  to  about 
300  c.c.  in  a  round -bottomed  flask  connected  through  a  catch  bulb  to  Liebig 
condenser  and  receiver  containing  excess  of  K/2  H2S04  and  provided  with  outlet 
acid  catch  packed  with  broken  Jena  glass  (some  beads  are  found  to  yield  alkali 
to  N/2  acid,  and  their  use  is  not  recommended).  150  c.c.  of  the  solution  are 
distilled  over  and  the  excess  of  acid  in  the  receiver  is  titrated  with  N/2  Na2C03. 
On  further  distillation  certain  liquors  continue  to  evolve  small  traces  of  ammonia. 
The  presence  of  this  ammonia  is  attributed  to  the  slow  decomposition  of  nitrogenous 
bodies,  and  the  distillation  for  free  ammonia  is  therefore  not  continued  beyond 
150  c.c. 

2.  Fixed  Ammonia.     By   distillation  : — Add    100  c.c.   of  half-normal  caustic 
soda  solution  to  the  residual  liquor  in  flask  (&)  above,  and  proceed  as  before. 

NH3,  grams  per  100  c.c.  of  liquor  =  '0085  xlO  xc.c.  N/2  acid. 

H.E.J  ("Hydrogen  equivalent  ••>  =^  ^-8=^ 

3.  Carbonic  Acid.     10  c.e.  of  liquor  (more,  if  dilute)  are  diluted  to  400  c.c.  in 
a  suitable  flask  provided  with  a  Bun  sen  rubber  valve ;  10  c.c.  of  ammoniacal 
calcium  chloride  (1  c.c.  ='044  gram  CO2)  are  added  and  the  whole  heated  for 
1£  to  2  hours  in  a  water  bath  at  100°  C.     The  calcium  carbonate  obtained  by 
filtration  is  washed  back  into  the  flask,  dissolved  in  N/2   HC1,  and  the  excess  of 
acid  titrated  with  N/2    Na2C03.     The  small  amount  of  calcium  carbonate   left 
on  the  filter  paper  is  best  recovered  by  incineration  and  added  to  the  contents  of 
the  flask. 

C02,  grams  per  100  c.c.  of  liquor  ='011  xlO  xc.c.  N/2  acid. 

w  ,3,      c°2»  grams 
•022 

4.  Chloride.     10  c.c.  of  boiled  liquor  are  diluted  to  150  c.c.,  25  c.c.  of  hydrogen 
peroxide  (10  vols.    "  free  from  chloride  ")||   added,   and   the   whole   boiled    for 
15  minutes  to  oxidize  thiocyanate,  &c.     To  the  hot  solution  are  added  five  or 
six  drops  of  a  10  per  cent,  solution  of  potassium  chromate  and  the  boiling  continued 
for  two  minutes,  then  a  slight  excess  of  sodium  carbonate  with  boiling  for  one 
minute.     The  solution,  which  should  possess  a  clear  lemon  colour,  is  filtered, 
cooled,  and  made  up  to  250  c.c.  ;  an  aliquot,  portion  is  then  titrated  with  N/1O 

*  J.  S.  C.  /.  1903,  p.  1068.  f  Jour,  fur  GasbeleucUung,  1908. 

I  I  am  indebted  to  Mr.  E.  Linder,  Assistant  to  the  Chief  Inspector,  for  an  advance 
proof  of  his  Memorandum  as  it  appears  in  the  Annual  Report  for  1909. 

§  NOTE. — For  method  of  stating  result,  see  paper  by  Lewis  T.  Wright,  Journ. 
Soc.  Chem.  Ind.  1886,  pp.  655-661 ;  also  Appendix,  Annual  Report,  1905,  p.  48, 
example  of  analysis  of  typical  gas  liquor  with  calculation  of  results. 

||  Some  chemists  prefer  to  use  Merck's  pcrhydrol,  a  reagent  free  from  chloride. 


AMMONIACAL   LIQUORS.  81 

AgN03  (potassium  chromate  indicator)  after  neutralizing  with  dilute  nitric  acid. 
A  blank  experiment  is  made  with  10  c.c.  of  N/io  NaCl,  and  the  same  volumes  of 
water,  peroxide,  and  chromate  as  in  the  actual  analysis,  to  determine  the  correction 
for  traces  of  chloride  in  the  reagents  used.  Should  the  organic  matter  in  solution 
resist  oxidation,  further  addition  of  peroxide  must  be  made  and  the  boiling 
continued,  with  addition  of  potassium  chromate  as  before. 

HC1,  grams  per  100  c.c.  =.  -00364  xlO  xc.c.  N/1O  AgN03. 

5.  Sulphur,  (a)  As  sulphate  :• — 250  c.c.  of  the  liquor  are  concentrated  to 
about  10  c.c.  on  the  water  bath,  2  c.c.  of  strong  hydrochloric  acid  added,  and  the 
evaporation  continued  to  dryness  to  decompose  thiosulphate  and  render  organic 
matter  less  soluble.  The  residue  is  extracted  with  water,  and  the  filtered  solution 
made  up  to  250  c.c.  ;  100  c.c.  of  this  solution  are  acidified  with  hydrochloric  acid, 
brought  to  the  boil,  and  barium  chloride  added ;  the  precipitate,  after  standing 
one  night,  is  filtered  and  weighed. 

Sulphur  as  sulphate,  grams  per  100  c.c.  =0'1373  x  grams  BaS04. 
(b)  As  sulphide,  sulphite,  and  thiosulphate. 

10  c.c.  of  liquor  are  diluted  to  500  c.c.,  acidified  with  hydrochloric  acid  and 
titrated  with  N/lo  iodine,  the  flask  being  closed  and  well  shaken  at  the  end  of 
the  titration  to  re-absorb  sulphuretted  hydrogen  gas  above  the  solution.  The 
volume  of  N/io  iodine  required  determines  that  of  the  liquor  taken.  Thus. 

10  c.c.  of  liquor  (or  more)  are  run  into  excess  of  N/5  ammoniacal  zinc  chloride 
solution  diluted  to  about  80  c.c.  ;  the  solution  is  warmed  to  coagulate  the  sulphide, 
filtered,  and  the  precipitate  washed  with  water  at  40°  to  50°  C. 

(i)  Sulphide. — The  zinc  sulphide  on  the  filter  is  washed  into  excess  of  N/io 
iodine  acidified  with  hydrochloric  acid  (the  last  traces  of  sulphide  being  washed 
through  with  cold  dilute  acid).  After  vigorous  shaking  to  complete  the  solution 
of  the  sulphide  (an  important  point  to  attend  to),  the  excess  of  iodine  is  determined 
with  N/io  thiosulphate, — starch  indicator. 

Sulphur  as  sulphide,  grams  per  100  c.c.  =10  x'0016  xc.c.  N/io  iodine. 
Sulphuretted  hydrogen,  „  =10  x'0017  xc.c.  N/io  iodine. 

.u  H2S,  grams. 

•017 

(ii)  Sulphite  and  Thiosulphate. — An  approximate  method  for  the  differentiation 
of  sulphite  and  thiosulphate  was  described  in  Annual  Report,  1903,  p.  36.* 
Mayer  and  Hem  pel  |  describe  a  method  based  upon  the  use  of  strontium 
chloride,  which  they  consider  to  be  sufficiently  accurate  for  the  desired  purpose. 
Prolonged  experience  in  the  analysis  of  ammoniacal  liquors,  however,  has  led 
the  author  to  the  conclusion  that  no  exact  estimation  of  sulphite  and  thiosulphate 
is  possible  in  such  liquors  by  any  method  based  on  titration  with  iodine,  save  in 
quite  exceptional  cases,  and  lie  prefers  to  present  a  united  figure  for  sulphur  as 
sulphite  and  thiosulphate,  reached  by  "  difference  "  :  by  subtracting  from  the 
"  total  sulphur  "  found  by  bromine  oxidation  the  sulphur  present  in  the  form  of 
sulphate,  thiocyanate,  and  sulphide. 

(c)  Sulphur  as  Thiocyanate.  (i)  Ferrocyanide  absent.  50  c.c.  of  the  solution 
are  treated  with  lead  carbonate  to  remove  sulphide,  the  lead  sulphide  and  carbonate 
are  then  removed  by  filtration  and  thoroughly  washed  ;  to  the  filtrate  acid  sulphite 
of  soda,  containing  a  little  free  S02,  is  then  added,  followed  by  distinct  excess 
of  a  10  per  cent,  solution  of  copper  sulphate,  and  the  solution  allowed  to  stand 
for  5  to  10  minutes  at  70°  to  80°  C.  to  coagulate  the  cuprous  thiocyanate.  The 
solution  is  then  filtered  and  thoroughly  washed  with  boiling  water  until  the  final 
washings  remain  colourless  on  addition  of  dilute  potassium  ferrocyanide;  the  residue 
on  the  filter  is  then  washed  back  into  the  flask,  digested  at  30°  to  40°  C.  with 
25  c.c.  of  a  4  per  cent,  solution  of  caustic  soda  (free  from  chloride)  and  filtered. 
To  the  cold  filtrate  are  added  5  c.c.  of  nitric  acid  free  from  oxides  of  nitrogen 
(50  per  cent,  strength)  followed  by  1  c.c.  of  a  saturated  solution  of  iron  alum  ;  the 
solution  is  then  filtered,  if  necessary,  from  separated  organic  matter,  and  titrated 
with  N/1Q  AgN03. 

*See  Polysulphide  Method.    Vol.  Anal.,  9th  edition,  p.  80. 
t  .7".  /Rr  GasbeleucJitung,  1908. 


82  AMMONIAC AL   LIQUORS. 

Sulphur  as  thiocyanate,  grams  per  100  c.c.  =2  x'0032  xc.c.  N/io  AgN03. 
Hydrocyanic  acid  as  thiocyanate,        „         =2  x'0027  xc.c.  N/io  AgN03. 

(ii)  Ferrocyanide  present. — 50  c.c.  of  the  liquor  are  slightly  acidified  with 
sulphuric  acid,  and  ferric  alum  solution  added  in  sufficient  quantity  to  impart 
a  decided  red  coloration  ;  the  solution  is  now  warmed  to  60°  C.,  filtered  from  the 
Prussian  blue,  and  washed  with  water  containing  sodium  sulphate.  The  filtrate 
is  then  treated  as  in  (i)  above. 

(d)  Total  Sulphur. — 50  c.c.  of  liquor  (100  c.c.  if  weak)  are  slowly  dropped  into 
a  flask  containing  excess  of  bromine  (free  from  sulphur)  covered  by  water 
moderately  acidified  with  hydrochloric  acid.  The  oxidized  solution  is  evaporated 
to  dryness  on  the  water  bath,  the  residue  extracted  with  boiling  water,  filtered, 
cooled,  made  up  to  250  c.c.  and  100  c.c.  precipitated  with  barium  chloride  in  the 
usual  way. 

Sulphur,  grams  per  100  c.c.  =5  x'1373  xgrams  BaS04. 

In  the  case  of  certain  liquors,  e.g.,  coke  and  blast  furnace  liquors,  oxidation 
with  bromine  yields  a  heavy  yellow  precipitate  of  brominated  phenols ;  this 
precipitate  may  retain  traces  of  sulphate  in  amount  sufficient  to  affect  the  per- 
centage distribution  figures  unless  it  is  recovered  by  fusion  with  the  smallest 
quantity  of  potassium  carbonate  and  nitrate,  or  sodium  peroxide,  and  included  in 
the  total. 

6.  Ferrocyanide,  by  F eld's  method*.     Mayer  and  Hem  pel  take  250  c.c. 
of  liquor,  acidify  slightly  with  sulphuric  acid,  and  add  ferric  alum  solution  in 
sufficient  excess  to  impart  a  deep  red  coloration  (ferric  thiocyanate).     The  solution 
is  then  heated  to  60°  C.  and  filtered.     Should  the  filtrate  be  coloured  blue,  it  must 
be  returned  to  the  filter  and  the  process  repeated  until  a  small  quantity  shows  no 
blue  colour  after  addition  of  mercuric  chloride  to  destroy  the  ferric  thiocyanate. 
The  precipitate  is  then  washed  two  or  three  times  with  water  containing  sodium 
sulphate.     Filter  and  precipitate  are  next  transferred  to  a  flask  and  the  Prussian 
blue  decomposed  by  boiling  for  5  miriutes  with  10  c.c.  of  normal  caustic  soda,  and 
the  solution  diluted  to  150  c.c.;    15  c.c.    of  magnesium  chloride  solution   (610 
grams  MgCl2  6H20  per  litre)  are  next  added  to  the  boiling  solution  very  slowly 
and  with  constant  agitation  to  avoid  the  formation  of  clots  of  magnesium  hydroxide. 
To  the  boiling  mixture  about  100  c.c.  of  boiling  mercuric  chloride  solution  (27.1 
grams  HgCl2  per  litre)  are  added,  and  the  whole  is  boiled  from  5  to  15  minutes. 
The  liquor  is  then  distilled  for  20  to   30  minutes  with  addition  of  30   c.c.   of 
sulphuric  acid  (196  grams  H2S04  per  litre),  the  hydrocyanic  acid  being  collected 
in  25  c.c.  of  normal  caustic  soda  and  titrated  as  described  in  the  method  for 
estimation  of  hydrocyanic  acid  below. 

1  c.c.  AgN0300=-54  gram  HCy. 

=  •00947  gram  (NH4)4FeCye. 

7.  Hydrocyanic  Acid,  by  Feld's  method.     60  c.c.  of  liquor  are  distilled  with 
excess  of  a  saturated  solution  of  lead  nitrate. 

The  apparatus  employed  is  similar  to  that  used  for  estimation  of  ammonia  and 
ferrocyanide.  It  consists  of  a  500  c.c.  round-bottomed  flask  provided  with  two- 
holed  caoutchouc  stopper  with  inlet  tube  sealed  in  liquor  and  exit  tube  connected 
through  a  catch  bulb  to  Liebig  condenser  and  receiver.  The  exit  tube  of  the 
condenser  is  sealed  in  the  25  c.c.  of  normal  soda  with  which  the  receiver  is  charged, 
residual  gases  escaping  through  a  scrubbing  bulb  containing  broken  glass  moistened 
with  caustic  soda.  At  the  end  of  the  distillation  a  current  of  air  is  drawn  through 
the  apparatus  for  a  few  minutes  as  a  necessary  precaution.  The  distillate  is 
diluted  to  400  c.c.,  a  crystal  of  potassium  iodide  added,  and  the  solution  titrated 
on  */10  AgN03. 

HCy,  grams  per  100  c.c.  =2  x'0054  xc.c.  N/1O  AgN03. 

«HF»  HCy>  &&™s- 

H'E-    =          -027 

*Dr.Skirrow  (Journ.  Soc.  Chem.  Ind.,  1910,  p.  319)  has  recently  questioned  the 
accuracy  of  F  e  1  d  s  method  for  determination  of  ferrocyanide,  but  his  conclusions  are 
challenged  by  D  r .  Colman  (Analyst,  1910,35,  295,). 


ORGANIC  NITROGEN.  83 

Certain  ammoniacal  liquors,  e.g.,  coke  oven  liquors,  froth  considerably  on 
distillation  with  lead  nitrate,  the  flask,  therefore,  should  be  heated  cautiously. 
In  general,  the  distillation  will  be  complete  when  100  c.c.  of  liquor  have  passed 
over  (30  to  40  minutes  gentle  boiling). 

(8)  Phenols. — No  occasion  has  yet  arisen  to  determine  the  amount  of  phenols 
present  in  ammoniacal  liquors.  A  very  suitable  method  for  their  estimation  is 
that  described  by  Dr.  F.  W.  Skirrow  (Jour,  Soc.  Chem.  Ind.,  1908,  p.  58).  The 
phenol  is  distilled  off  with  water  and  then  converted  into  tri-iodophenol  by  means 
of  excess  of  iodine,  which  is  titrated  back  with  thiosulphate. 

5.     Determination  of  Combined  Nitrogen  in  Organic  Substances. 

The  old  process  consists  in  heating  the  dried  substance  in  a  com- 
bustion tube  with  soda  lime,  by  which  the  nitrogen  is  converted 
into  ammonia  ;  and  this  latter,  being  led  into  a  measured  volume 
of  standard  acid  contained  in  a  suitable  bulb  apparatus,  combines 
with  its  equivalent  quantity  ;  the  solution  is  then  titrated  residually 
with  standard  alkali  for  the  excess  of  acid,  and  thus  the  quantity 
of  ammonia  found. 

As  the  combustion  tube  with  its  arrangements  for  organic  analysis 
is  well  known  and  described  in  any  of  the  standard  books  on  general 
analysis,  it  is  not  necessary  to  give  a  description  here. 

6.     K  j  e  1  d  a  h  1 '  s  Method  and  its  developments. 

This  has  met  with  considerable  acceptance  in  lieu  of  the  com- 
bustion method,  on  account  of  its  easy  management  and  accurate 
results.  Unlike  the  combustion  method,  the  ammonia  is  obtained 
free  from  organic  matters  and  colour,  and  moreover  salts  of  ammonia 
and  nitrates  may  be  determined  with  extreme  accuracy.  It  was 
first  described  by  Kjeldahl*,  and  has  since  been  commented  upon 
by  many  operators,  among  whom  are  Warington**,  Pfeiffer 
and  Lehmannf,  Marcker  and  othersj;  Gunning§;  Arnold 
and  Wedermeyer||  ;  and  Bernard  DyerU. 

The  original  process  consisted  in  heating  the  nitrogenous  substance 
in  a  flask  with  concentrated  sulphuric  acid  at  boiling  temperature, 
and,  when  the  oxidation  through  the  agency  of  the  acid  is  nearly 
completed,  adding  finely  powdered  potassium  permanganate  in 
small  quantities  till  a  green  or  pink  colour  remains  constant  (long 
experience  has  shown  that  this  addition  of  permanganate  is  not 
advisable,  as  it  leads  to  loss  of  ammonia)  ;  the  whole  of  the 
nitrogen  is  thus  converted  into  ammonium  sulphate.  The  flask  is 
then  allowed  to  cool,  diluted  with  water,  excess  of  caustic  soda 
added,  the  ammonia  distilled  off  into  standard  acid,  and  the 
amount  found  by  titration  in  the  usual  way. 

Some  practical  difficulties  occurred  in  the  process  as  originally 
devised  ;  moreover,  with  some  organic  substances  a  very  long  time 
was  required  to  oxidize  the  carbon  set  free  by  the  strong  acid. 

Another  difficulty  was  that  if  nitrates  were  present  in  the  com- 

*  Z.  a.  C.  22,  366.  **  C.  N.  52,  162.  t  Z.  a.  (7.24, 388.  J  Z.  a.  C.  23,  553  ;  24, 199,  393  ; 
25,  149,  155;  26,  92;  27,  222,  398.  §  Ibid  28,  188.  ||  Ibid  31,  525.  f  J.  C.  S.  1895,  811. 

G    2 


84  KJELDAHL   METHOD. 

pound  analyzed  their  reduction  to  ammonia  was  neither  certain 
nor  regular,  and  unless  this  difficulty  could  be  overcome  the  value 
of  the  process  was  limited. 

The  various  modifications  of  the  original  process  are. as  follows  : — 
Gunning  proposed  the  addition  of  potassium  sulphate  to  the 
sulphuric  acid,  by  which  means  its  boiling  point  is  raised  and  the 
process  of  oxidation  greatly  facilitated.  Arnold  proposed  the 
use  of  mercury,  as  affording  still  further  assistance  in  the  same 
direction.  Jodlbauer  proposed  the  use  of  a  mixture  of  phenol  or, 
better,  salicylic  acid  and  sulphuric  acid  for  use  with  substances 
containing  nitrates  as  well  as  organic  or  ammoniacal  nitrogen. 

The  experience  of  these  and  many  hundreds  of  other  operators 
since  this  method  was  first  introduced  has  resulted  in  rendering  it 
as  perfect  as  need  be,  and  the  results  of  this  experience  in  all  essential 
particulars  will  now  be  described,  omitting  the  details  as  to  some  of 
the  special  forms  of  apparatus  which  are  not  absolutely  essential. 
The  method  requires  the  following  re-agents  and  apparatus  : — 

1.  Standard  acid,  which  may  be  either  sulphuric  or  hydrochloric  ; 
a  convenient  strength  is  semi-normal  or  fifth-normal. 

2.  Standard  alkali,    either  ammonia,   or  sodium  or  potassium 
hydroxide,  of  corresponding  strength  to  the  acid. 

3.  Concentrated  sulphuric  acid  free  from  nitrates  and  ammonium 
sulphate.* 

4.  Mercuric  oxide  prepared  in  the  wet  way  or  metallic  mercury,  f 

5.  Powdered  potassium  sulphate. 

6.  Granulated  zinc. 

7.  Solution  of  potassium  sulphide  in  water,  40  gm.  in  the  litre. 

8.  A  saturated  solution  of  caustic  soda  free  from  nitrates  or 
nitrites.     This   should    be    tried    as    to   the  amount  required   to 
saturate  20  c.c.  of  the  sulphuric  acid  (3  above)  used  and  marked 
on  the  stock  bottle.     About  5—10  c.c.  more  than  this  amount  is 
added  to  the  distillation  flask  for  each  20  c.c.  of  acid  used. 

9.  An    indicator — litmus,    methyl    orange,    or    cochineal    are 
suitable,  but  phenolphthalein  may  not  be  used. 

10.  Digestion  flasks  with  long  neck  and  round  bottom,  holding 
about  200 — 250  c.c.     These  flasks  should  be  well  annealed,  and  not 

*  Commercial  oil  of  vitriol  frequently  contains  ammonium  compounds,  o\ying  to  the 
fact  that  makers  sometimes  add  ammonium  sulphate  during  concentration  in  order  to 
get  rid  of  nitrous  compounds.  M  e  1  d  o  1  a  and  M  o  r  i  t  z  state  that  any  traces  of 
ammonia  may  be  destroyed  by  digesting  the  acid  for  two  or  three  hours  at  a  tempera- 
ture below  boiling  with  sodium  or  potassium  nitrate  in  the  proportion  of  0'5  gm.  of 
the  salt  to  100  c.c.  of  acid.  Lunge  objected  to  this  treatment,  because  of  the  probable 
formation  of  nitro-sulphuric  acid.  Experiments  have  since  been  madebyMoritz 
which  prove  that  the  objection  is  groundless,  provided  the  digestion  is  carried  on  for 
a  period  sufficient  to  expel  the  nitrous  acid  (J.  S.  C- 1.  9,  443).  The  purification  of  the 
acid  may  of  course  be  obviated  by  ascertaining  once  for  all  the  amount  of  ammonia 
in  any  given  stock  of  acid,  by  making  a  blank  experiment  with  pure  sugar  and  allow- 
ing in  all  cases  for  the  amount  of  NHa  so  found. 

t  C.  A.  M  o  o  e  r  s  (Analyst  28,  44)  states  that  when  using  mercury  with  the  material 
under  examination,  there  is  a  danger  of  the  metal  passing  over  with  the  distillate  and 
forms  an  amalgam  with  the  tin  condensing  tubes  which  absorbs  some  of  the  ammonia. 
He  therefore  advocates  Jeria  glass  instead  of  tin  tubes.  No  such  effects  have  been 
obtained  in  my  laboratory  with  the  apparatus  fig.  31.  It  is  possible  that  the  danger 
alluded  to  is  due  to  impure  tin  or  to  the  form  of  distilling  flask  used. 


KJELDAHL   METHOD.  85 

too  thick,  preferably  made  of  Jena  glass — the  neck  about  f  inch 
wide,  and  3J — 4  inches  long. 

11.  Distillation  flasks  of  hard  Bohemian  or  Jena  glass  of  conical 
shape,  600-850  c.c.  capacity,  fitted  with  a  rubber  stopper  and  a  bulb 
above  with  curved  delivery  tube,  to  prevent  the  spray  of  the  boiling 
alkaline  liquid  from  being  carried  over  into  the  condenser  tubes. 
Copper  distilling  bottles  or  flasks  are  used  by  some  operators  for 
technical  purposes  with  good  results,  but  in  this  case  it  is  advisable 
to  put  the  soda  into  the  vessel  first,  then  to  add  the  acid  liquid. 

12.  The  condenser.     Owing  to  the  undoubted  solubility  of  glass 
in  freshly  distilled  water  containing  ammonia,  it  is  advisable  to 
have  the  condenser  tube  made  of  pure  block  tin.     This  should  be 
about  three-eighths  of  an  inch  wide  externally,  and  is  connected  with 
the  bulb  tube  of  the  distilling  flask  with  stout  pure  rubber  tube. 
It  is  surrounded  by  either  a  metal  or  glass  casing,  through  which 
a  current  of  cold  water  is  passed  in  the  usual  manner.     It  is  very 
easy  to  fit  up  such  an  arrangement  with  the  condenser  tubes  made 
entirely  of  glass  sold  by  the  dealers  in  chemical  apparatus.     The 
end  of  the  condenser  tube  may  be  simply  inserted  into  the  neck  of 
a  flask  containing  the  standard  acid,  or  it  may  have  a  delivery  tube 
connected  by  a  rubber  tube  leading  into  a  beaker.     There  is  no 
necessity  for  dipping  the  delivery  tube  into  the  acid  unless  the 
temperature  of  the  laboratory  is  very  high. 

In  places  where  it 
is  difficult  to  arrange 
for  a  flow  of  water  to 
keep  the  distilling 
tube  cool  the  simple 
apparatus  shown  in 
fig.  30  may  be  service- 
able, and  unless  the 
temperature  of  the 
laboratory  is  exceed- 
ingly high  there  is  no 
loss  of  ammonia.  This 
Fig.  30.  arrangement  is  used 

by  Stutzer,  whose  results  with  it  compare  well  with  others  made 
with  condensers  surrounded  by  flowing  water.  The  explanation  of 
this  is,  no  doubt,  that  ammonia  possesses  a  very  strong  affinity  fdr 
water  and,  when  in  very  minute  quantity,  is  held  tenaciously  even 
at  a  tolerably  high  temperature.  The  tube  should  be  not  less  than 
3  feet  long.  Where  a  large  number  of  determinations  are  being 
carried  on  it  is  oonvenient  to  have  a  special  condenser,  which  will 
allow  of  six  or  more  distillations  being  worked  at  the  same  time. 
Several  forms  of  such  arrangements  have  been  devised,  and  are 
obtainable  from  apparatus  dealers. 

For  use  in  my  own  laboratory,  where  a  large  number  of  agricultural 
samples  are  examined,  the  form  shown  in  fig.  31  has  been  adopted 
and  has  been  found  to  answer  well.  The  body  of  the  condenser 


86  KJELDAHL  METHOD. 

consists  of  an  ordinary  10-gallon  iron  drum  filled  with  water  ;  the 
pure  tin  distilling  tubes  run  through  this  at  equal  distances  from 
each  other,  and  emerge  at  the  bottom  of  sufficient  length  to  dip  into 
the  vessels  containing  the  standard  acid.  With  this  arrangement 
there  is  no  necessity  for  running  water,  and  six  distillations  may  be 
carried  on  simultaneously  without  fear  of  losing  ammonia  ;  the 
body  of  water  is  so  great  that  the  lower  portion  is  quite  cool  after 
the  distillations  are  finished.  In  case  of  extremely  hot  weather  or 
in  a  very  hot  laboratory  the  cover  may  be  removed  and  a  lump  of 
ice  placed  in  the  water  if  a  large  number  of  distillations  are 
required. 

The  distilling  flasks  are  closed  with  rubber  stoppers,  and  fitted 
with  glass  bulb  tops  as  in  figs.  30  and  31.  These  are  connected 
with  the  tin  tubes  by  rubber  joints,  and  supported  on  an  iron  frame 
over  which  is  laid  a  strip  of  wire  gauze.  The  B  u  n  s  e  n  burners  are 
of  Fletcher's  make,  with  nickel  gauze  tops  which  give  a  smokeless 
flame  of  any  desired  size.  So  well  does  this  arrangement  work  that 
during  many  hundreds  of  distillations  not  one  breakage  has 
occurred  due  to  the  heating  or  the  distillation.  The  tin  condensing 
tubes  do  not  in  this  case  dip  into  the  standard  acid,  as  various 


Fig.  31. 


KJELDAHL-GUNNING   METHOD.  87 

experiments  have  proved  it  unnecessary  unless  the  temperature  of 
the  laboratory  is  very  high. 

B.  Dyer  uses  a  tin  condensing  tube  (air  condenser)  rising  15 

18  inches  vertically  from  the  distilling  flask,  then  bent  downwards 
and  fitting  into  a  pear-shaped  adapter  (with  large  expansion  to 
allow  of  varied  pressure),  whose  narrowed  end  dips  actually  into  the 
acid.  The  receiving  flask  should  stand  in  a  tank  of  running  water. 

13.  A  convenient  stand  for  holding 
the  digestion  flasks  is  shown  in  fig.  32. 
They  rest  in  an  oblique  position  and 
heat  is  applied  by  small  Bun  sen 
burners.  With  a  little  care  the  naked 
flame  can  be  applied  directly  to  the 
flask  without  danger.  Some  operators 
prefer  to  close  the  digestion  flasks 
with  a  loosely  fitting  glass  stopper 
elongated  to  a  point,  and  having  Fig.  32. 

a  balloon-sha.ped  top.  This  aids  in  the  condensation  of  any  acid 
which  may  distil,  but  if  the  flasks  are  tolerably  long  in  the  neck, 
there  is  practically  no  loss  of  acid  except  as  S02  which  occurs  in 
any  case.  It  is  almost  needless  to  say  that  the  digestion  should 
be  done  in  a  fume  closet  with  good  draught. 

As  the  acid  fumes  given  off  when  organic  matters  are  boiled  with 
strong  acid  are  very  disagreeable  and  irritating  to  the  lungs, 
especially  in  small  laboratories,  another  method  can  be  adopted  so 
that  no  fume  closet  is  necessary,  and  the  digestion  can  be  carried 
on  in  the  open  air.  The  acid  flask  is  closed  with  a  pear-shaped 
hollow  stopper,  attached  to  which  is  a  bent  glass  tube,  the  end  of 
which  is  passed  through  a  cork  into  a  globe-shaped  adapter  held  in 
the  neck  of  a  conical  flask,  which  contains  a  solution  of  caustic  soda 
into  which  the  end  of  the  adapter  dips.  By  this  method  all  the 
acid  fumes  are  absorbed.* 

Determination  of  Nitrogen  in  the  absence  of  Nitrates. 

THE  K  j  EL  DAHL-G  TINNING  PROCESS:  From  0'5  to  5  gin.  of  the  substance 
according  to  its  nature  is  brought  into  a  digestion  flask  with  approximately 
0'5  gm.  of  mercuric  oxide  or  a  small  globule  of  the  metal  and  20  c.c.  of  sulphuric 
acid  (in  case  of  bulky  vegetable  substances  30  c.c.  or  more  may  be  necessary). 
The  flask  is  placed  on  wire  gauze  over  a  small  Bunsen  burner  in  an  upright 
position,  or  in  the  frame  above  described  in  an  inclined  position,  and  heated 
below  the  boiling-point  of  the  acid  for  from  five  to  fifteen  minutes,  or  until 
frothing  has  ceased.  The  heat  is  then  raised  till  the  acid  boils  briskly,  and  the 
boiling  continued  for  about  fifteen  minutes,  when  about  10  grams  of  potassium 
sulphate  are  added  and  the  boiling  resumed.  Anhydrous  sodium  sulphate  also 
answers  this  purpose,  and  the  same  effect  can  be  obtained  in  the  same  time  by 
sodium  pyrophosphate,  2  gm.  of  the  latter  acting  as  well  as  10  gm.  of  either  of  the 
former.  No  further  attention  is  required  till  the  contents  of  the  flask  have  become 
a  clear  liquid,  which  is  colourless,  or  at  least  has  only  a  very  pale  straw  colour. 
The  flask  is  then  removed  from  the  frame,  and,  after  cooling,  the  contents  are 
transferred  to  the  distilling  flask  with  repeated  quantities  of  water  amounting 

*  A  figure  of  this  apparatus  is  shown  in  Analyst  28,  54. 


88  KJELDAHL-GUNNING-JODLBAUER   METHOD. 

in.  all  to  about  250  c.c.,  and  to  this  25  c.c.  of  potassium  sulphide  solution  are 
added,  soda  solution*  sufficient  in  quantity  to  make  the  reaction  strongly 
alkaline,  and  a  few  pieces  of  granulated  zinc.  The  flask  is  at  once  connected 
with  the  condenser,  and  the  contents  are  distilled  till  all  ammonia  has  passed 
over  into  the  standard  acid,  and  the  concentrated  solution  can  no  longer  be  safely 
boiled.  This  operation  usually  requires  from  twenty  to  thirty  minutes.  The 
distillate  is  then  titrated  with  semi-normal  or  fifth-normal  alkali. 

The  use  of  mercury  or  its  oxide  in  this  operation  greatly  shortens  the  time 
necessary  for  digestion,  which  is  rarely  over  an  hour,  and  in  the  case  of  substances 
most  difficult  to  oxidize  is  more  commonly  less  than  an  hour.  Potassium  sulphide 
removes  all  mercury  from  solution,  and  so  prevents  the  formation  of  mercuro- 
ammonium  compounds  which  are  not  commonly  decomposed  by  soda  solution. 
The  addition  of  zinc  gives  rise  to  an  evolution  of  hydrogen  and  prevents  violent 
bumping. 

Determination  of  Nitrogen  in  the  presence  of  Nitrates. 

THE  K  J  E  L  D  A  H  L-G  u  N  N  i  N  G-Jo  D  L  B  A  u  E  R  PROCESS  :  The'requisite  quantity 
of  substance  to  be  analyzed  (usually  0'5  to  5  gm.)  is  put  into  a  digestion  flask 
and  30  c.c.  of  sulphuric  acid  containing  2  gm.  of  salicylic  acid  are  then  quickly 
poured  over  the  substance  so  as  to  cover  it  at  once.  To  do  this  the  two 
acids  should  be  mixed  together  in  a  beaker  and  the  latter  emptied  into  the 
flask.  The  latter  is  allowed  to  stand  for  10  minutes,  generally  in  the  cold,  with 
occasional  shaking.  2  gm.  of  zinc  dust  are  then  added,  also  a  drop  of  mercury, 
and  the  whole  gently  heated  till  frothing  is  over.  Potassium  sulphate  (10  grams) 
is  then  added  and  the  process  finished  as  described  above. 

(In  the  official  Methods  of  Analysis  issued  by  the  Board  of  Agriculture  and 
Fisheries  on  Nov.  9,  1908,f  the  method  given  is  practically  as  above,  with  the 
exception  that  only  1  gram  of  salicylic  acid  is  used  and  5  grams  of  sodium  thio- 
sulphate  are  used  in  place  of  the  2  gm.  of  zinc  dust.  Copper  sulphate  is  mentioned 
as  an  alternative  to  mercury.) 

A  Hank  determination  should  be  made  with  all  the  materials  used  in  either 
of  the  above  methods.  In  the  case  of  the  Kjeldahl-Gunning  process  1  gram  of 
pure  cane-sugar  should  be  heated  with  20  c.c.  of  sulphuric  acid,  mercury  and 
potassium  sulphate  being  added,  etc.,  and  the  distillate  received  into  5  c.c.  of 
N/5  acid.  To  this  5  c.c.  of  N/5  soda  are  then  added,  with  a  drop  of  methyl  orange, 
and  N/5  acid  run  in  until  a  pink  colour  appears.  With  good  acid  20  c.c.  give 
"  blanks  "  ranging  between  0'2  and  0'6  c.c.  N/5  acid.  This  amount  is  of  course 
deducted  from  the  amounts  of  acid  neutralized  in  actual  determinations.  The 
use  of  the  cane-sugar  is  to  reduce  any  trace  of  nitrates  that  may  be  present  in  the 
sulphuric  acid. 

NOTE. — Some  substances  froth  considerably  during  the  distillation  with  soda 
into  standard  acid.  This  may  generally  be  prevented  by  adding  a  small  piece  of 
solid  paraffin,  when  the  distillation  proceeds  quietly  as  usual.  In  the  case  of 
cheese,  milk,  etc.  Brown}:  recommends  that  the  solution  obtained  after  decom- 
position by  sulphuric  acid  be  diluted  with  water  to  about  100  c.c.  and  boiled 
briskly  in  the  digestion  flask  until  only  about  40  c.c.  remain.  On  transferring 
to  the  distillation  flask,  diluting,  and  adding  NaOH  etc.  as  usual,  the  distillation, 
can  be  carried  out  without  the  least  trouble. 

7.     Method  of  Ronchese.|| 

This  method  depends  on  the  fact  that  in  the  presence  of  a  large 
excess  of  formaldehyde  ammonium  salts  form  hexamethylene- 

•Sorne  operators  prefer  to  close  the  distilling  flask  with  a  caoutchouc  stopper, 
through  which  in  addition  to  the  distilling  tube,  a  funnel  with  tap  is  fixed  for  running 
in  the  alkali.  This  is  to  guard  against  possible  loss  of  ammonia. 

t  Board  of  Agriculture  and  Fisheries.     Statutory  Rules  and  Orders,  1908.    No.  964. 
J  C.  N.  1910, 102,  61.  I!  J.  PJtarm.  CJiem.  1907,  Gil,  and  Analyst,  32,  303. 


RONCHKSE    METHOD.  89 

tetramine,  N4(CH2)6  (see  under  Formaldehyde).  The  liberated 
acid  titrated  directly  with  standard  alkali  gives  a  measure  of  the 
combined  ammonia.  The  method  is  interesting,  and  obviates 
distillation,  but  is  of  limited  scope.  J.  M.  Wilkie*  has  improved 
it,  and  H.  G.  Bennettf  has  applied  it  to  the  determination  of 
hide-substance  in  leather  and  tannery  liquors. 


ACIDIMETRY    OR    THE    TITRATION    OF    ACIDS. 

THIS  operation  is  simply  the  reverse  of  all  that  has  been  said  of 
alkalies,  and  depends  upon  the  same  principles  as  have  been 
explained  in  alkalimetry. 

With  liquid  acids,  such  as  hydrochloric,  sulphuric,  or  nitric, 
the  strength  is  generally  taken  by  means  of  the  hydrometer  or 
specific-gravity  bottle,  and  the  amount  of  real  acid  in  the  sample 
ascertained  by  reference  to  the  tables  constructed  by  Lunge  and 
his  pupils. 

In  the  case  of  titrating  concentrated  acids  of  any  kind  it  is 
preferable  in  all  cases  to  weigh  accurately  a  small  quantity,  dilute 
to  a  definite  volume,  and  take  an  aliquot  portion  for  titration. 


Delicate  End-reaction  in  Acidimetry. 

If  an  alkali  iodate  or  bromate  be  added  to  a  solution  of  an  alkali 
iodide  in  the  presence  of  a  mineral  acid,  iodine  is  set  free  and  remains 
dissolved  in  the  excess  of  alkali  iodide,  giving  the  solution  the 
well-known  colour  of  iodine.  This  reaction  has  long  been  known, 
and  is  capable  of  being  used  with  excellent  effect  as  an  indicator  for 
the  delicate  titration  of  acids,  and  therefore  of  alkalies,  by 
the  residual  method.  K  j  e  1  d  a  h  1,  for  instance,  uses  it  in  his  ammonia 
process,  where  the  distillate  contains  necessarily  an  excess  of 
standard  acid.  The  reaction  is  definite  in  character,  and  may  be 
used  in  various  ways  in  volumetric  processes.  For  instance, 
potassium  bromate  liberates  iodine  in  exact  proportion  to  its 
contained  oxygen  in  the  presence  of  excess  of  dilute  mineral  acid, 
and  the  iodine  so  liberated  may  be  accurately  titrated  with  sodium 
thiosulphate.  In  acidimetry,  however,  the  method  is  simply  used 
for  its  exceeding  delicacy  as  an  end-reaction,  one  drop  of  N/ioo 
sulphuric,  nitric,  or  hydrochloric  acid  being  quite  sufficient  to 
cause  a  deep  blue  colour  in  the  presence  of  starch. 

The  adjustment  of  the  standard  liquids  is  made  as  follows  :— 
2  or  3  c.c.  of  N/10  acid  are  run  into  a  flask,  diluted  somewhat  with 
water,  and  a  crystal  or  two  of  potassium  iodide  thrown  in.  1  or 
2  c.c.  of  a  5  per  cent,  solution  of  potassium  iodate  are  then 
added,  which  at  once  produces  a  brown  colour,  due  to  the 
liberation  of  iodine.  A  solution  of  sodium  thiosulphate  is  added 

*  J.  8.  C.  I.  1910,  29,  6.  t  J.  S.  C.  /.  1909,  28,  291. 


90  ACIDIMETRY. 

from  a  burette,  with  constant  shaking,  until  the  colour  is  nearly 
discharged  ;  a  few  drops  of  clear  freshly  prepared  starch  solution 
are  now  poured  in,  and  the  blue  colour  removed  by  the  very 
cautious  addition  of  thiosulphate.  The  quantity  of  thiosulphate 
used  represents  the  comparative  strengths  of  it  arid  of  the  standard 
acid,  and  is  used  as  the  basis  of  calculation  in  other  titrations.  The 
first  discharge  of  the  blue  colour  must  be  taken  in  all  cases  as  the 
correct  ending,  because  on  standing  a  few  minutes  the  blue  colour 
returns,  due  to  some  obscure  reaction  from  the  thiosulphate. 
This  has  been  probably  regarded  as  one  of  the  drawbacks  of  the 
process,  and  another  is  the  instability  of  the  thiosulphate  solution  ; 
but  these  by  no  means  invalidate  its  accuracy,  moreover,  it  possesses 
the  advantage  of  being  applicable  to  excessively  dilute  solutions, 
and  may  be  used  by  artificial  light.  The  organic  acids  cannot  be 
determined  by  this  method,  the  action  not  being  regular.  Neutral 
alkali  and  alkaline  earthy  salts  do  not  interfere,  but  salts  of  the 
organic  acids  and  borates  must  be  absent. 


ACETIC    ACID. 

C2H4O2= 60-03. 

IN  consequence  of  the  discrepancies  existing  between  the  sp.  gr. 
of  strong  acetic  acid  and  its  actual  strength,  the  hydrometer 
readings  are  not  reliable  ;  but  the  volumetric  determination  is  now 
rendered  extremely  accurate  by  using  phem)lphthalein  as  indicator, 
acetates  of  the  alkalies  and  alkaline  earths  having  a  perfectly  neutral 
behaviour  to  this  indicator.  Even  coloured  vinegars  may  be  titrated 
when  highly  diluted.  Where,  however,  the  colour  is  too  dark  for 
this  method  to  succeed,  Pettenkofer's  method  of  procedure  is 
the  best,  and  this  is  endorsed  by  A.  R.  Leeds.*  The  latter  takes 
50  c.c.  of  the  vinegar  and  50  c.c.  of  water  with  a  drop  of 
phenolphthalein,  then  adds  N/10  baryta  to  slight  excess.  This 
causes  the  organic  colouring  matters  to  separate  either  in  the  cold 
or  on  warming,  and  the  excess  of  baryta  is  then  found  by  titration 
with  N/10  acid  and  turmeric  paper. 

Several  processes  have  at  various  times  been  suggested  for  the 
accurate  and  ready  determination  of  acetic  acid,  among  which  is 
that  of  G  r  e  v  i  1 1  e  W  i  1 1  i  a  m  s,  by  means  of  a  standard  solution  of  lime 
syrup.  The  results  obtained  were  very  satisfactory. 

C.  Mohr's  process  consists  in  adding  to  the  acid  a  known 
excessive  quantity  of  precipitated  neutral  and  somewhat  moist 
calcium  carbonate.  When  the  decomposition  is  as  nearly  as 
possible  complete  in  the  cold,  the  mixture  must  be  heated  to  expel 
the  CO2  and  to  complete  the  saturation  ;  the  residual  carbonate  is 
then  brought  upon  a  filter,  washed  with  boiling  water,  treated  with 
excess  of  normal  acid  and  titrated  back  with  alkali. 

*J.  Am.  C.  S.  17,  741. 


ACETATES.  91 

In  testing  the  impure  brown  pyroligneous  acid  of  commerce, 
this  method  has  given  fairly  accurate  results.* 

The  titration  of  acetic  acid  or  vinegar  may  also  be  performed  by 
the  ammonio-cupric  solution  described  on  p.  56. 

1.  Free  Mineral  Acids  in  Vinegar. — Hehner  has  devised  an 
excellent  method  for  this  purpose,  f 

Acetates  of  the  alkalies  are  always  present  in  commercial  vinegar  ; 
and  when  such  vinegar  is  evaporated  to  dryness,  and  the  ash  ignited, 
the  alkalies  are  converted  into  carbonates  having  a  distinctive 
alkaline  reaction  on  litmus  ;  if,  however,  the  ash  has  a  neutral  or 
acid  reaction,  some  free  mineral  acid  must  have  been  present. 
The  alkalinity  of  the  ash  is  diminished  in  exact  proportion  to  the 
amount  of  mineral  acid  added  to  the  vinegar  as  an  adulterant. 

METHOD  or  PROCEDURE  :  50  c.c.  of  the  vinegar  are  mixed  with  25  c.c.  of  N/io 
soda  or  potash,  evaporated  to  dryness,  and  ignited  at  a  low  red  heat  to  convert 
the  acetates  into  carbonates ;  when  cooled,  25  c.c.  of  N/io  acid  are  added ;  the 
mixture  heated  to  expel  C02,  and  filtered  ;  after  washing  the  residue,  the  filtrate 
and  washings  are  exactly  titrated  with  N/io  alkali ;  the  volume  so  used  equals 
the  amount  of  mineral  acid  present  in  the  50  c.c.  of  vinegar. 

1  c.c.  N/io  alkali  =0'0049  gm.  H2SO4  or  0'003647  gm.  HC1. 


If  the  vinegar  contains  more  than  0'2  per  cent,  of  mineral  acid, 
more  than  25  c.c.  of  N/10  alkali  must  be  used  to  the  50  c.c.  vinegar 
before  evaporating  and  igniting. 

2.  Acetates  of  the  Alkalies  and  Earths. — These  salts  are  converted 
by  ignition  into  carbonates,  and  can  be  then  residually  titrated 
with  normal  acid  ;  no  other  organic  acids  must  be  present,  nor 
must  nitrates,  or  similar  compounds  decomposable  by  heat.     1  c.c. 
normal  acid =0*06  gm.  acetic  acid. 

3.  Metallic  Acetates. — Neutral  solutions  of  lead  and  iron  acetates  may  be 
precipitated  by  an  excess  of  normal  sodium  or  potassium  carbonate,  the  pre- 
cipitate well  boiled,  filtered,  and  washed  with  hot  water,  the  filtrate  and  washings 
made  up  to  a  definite  volume,  and  an  aliquot  portion  titrated  with  N/io  acid ; 
the  difference  between  the  quantity  so  used  and  calculated  for  the  original  volume 
of  alkali  will  represent  the  acetic  acid. 

If  such  solutions  contain  free  acetic  or  mineral  acids,  they  must 
be  exactly  neutralized  previous  to  treatment. 

If  salts  other  than  acetates  are  present,  the  process  must  be 
modified  as  follows  : — 

METHOD  OF  PROCEDURE  :  Precipitate  with  alkali  carbonate  in  excess,  exactly 
neutralize  with  hydrochloric  acid,  evaporate  the  whole  or  part  to  dryness,  ignite 
to  convert  the  acetates  into  carbonates,  then  titrate  residually  with  normal  acid. 
Any  organic  acid  other  than  acetic  will,  of  course,  record  itself  in  terms  of  acetic 
acid. 

*  A.  R.  L  ee  ds  (loc.  dt.)  has  not  found  this  method  to  answer,  which  I  think  must 
be  due  to  using  dried  calcium  carbonate.  I  have  only  \ised  it  for  commercial  wood 
acid,  and  the  figures  obtained  by  me  were  the  highest  among  several  other  methods ; 
but  it  is  important  to  mention  that  the  CaCOa  should  not  be  thoroughly  dried,  and 
that  its  alkalinity  should  be  known. 

t  Analyst  1, 105. 


ACETATES. 

4.  Commercial  Acetate  of  Lime. — The  methods  just  described 
are  often  valueless  in  the  case  of  this  substance,  owing  to  the 
presence  of  tarry  matters,  which  readily  produce  an  excess  of 
carbonates. 

Fresenius*  adopts  the  following  process  for  tolerably  pure 
samples  : 

METHOD  or  PROCEDURE  :  5  gm.  are  weighed  and  transferred  to  a  250  c.c. 
flask,  dissolved  in  about  150  c.c.  of  water,  and  70  c.c.  of  normal  oxalic  acid  added  ; 
the  flask  is  then  well  shaken,  and  filled  to  the  mark,  2  c.c.  of  water  are  added  to 
allow  for  the  volume  occupied  by  the  precipitate,  the  whole  is  again  well  shaken 
and  left  to  settle.  The  solution  is  then  filtered  through  a  dry  filter  into  a  dry 
flask ;  the  volume  so  filtered  must  exceed  200  c.c. 

100  c.c.  are  first  titrated  with  normal  alkali  and  litmus  ;  or,  if  highly  coloured, 
by  help  of  litmus  or  turmeric  paper ;  the  volume  used  multiplied  by  2*5  will  give 
the  volume  for  5  gm. 

Another  100  c.c.  are  precipitated  with  solution  of  pure  calcium  acetate  in  slight 
excess,  warmed  gently,  the  precipitate  allowed  to  settle  somewhat,  then  filtered, 
well  washed,  dried,  and  strongly  ignited,  in  order  to  convert  the  oxalate  into 
calcium  carbonate  or  oxide,  or  a  mixture  of  both.  The  residue  so  obtained  is 
then  decomposed  with  excess  of  normal  acid,  and  titrated  residually  with  normal 
alkali.  By  deducting  the  volume  of  acid  used  to  neutralize  the  precipitate  from 
that  of  the  alkali  used  in  the  first  100  c.c.,  and  multiplying  by  2'5,  there  is  obtained 
the  volume  of  alkali  expressing  the  weight  of  acetic  acid  in  the  5  gm.  of  acetate. 

In  the  case  of  very  impure  and  highly  coloured  samples  of  acetate, 
it  is  only  possible  to  determine  the  acetic  acid  by  repeated 
distillations  with  phosphoric  acid  and  water  to  incipient  dryness, 
and  then  titrating  the  acid  direct  with  /10  alkali,  each  c  c.  of  which 
represents  0*006  gm.  acetic  acid. 

The  distillation  is  best  arranged  as  suggested  by  Still  well  and 
Gladding,  or  later  by  Harcourt  Phillips.  | 

METHOD  OF  PROCEDURE  :  A  100  to  120  c.c.  retort,  the  tubulure  of  which  carries 
a  small  funnel  fitted  in  with  a  rubber  stopper,  and  the  neck  of  the  funnel  stopped 
tightly  with  a  glass  rod  shod  with  elastic  tube,  is  supported  upon  "a  stand  in  such 
a  way  that  its  neck  inclines  upwards  at  about  forty-five  degrees  ~.  the  end  of  the 
neck  is  drawn  out,  and  bent  so  as  to  fit  into  the  condenser  by  help  of  an  elastic 
tube.  The  greater  part  of  the  retort  neck  is  coated  with  flannel,  so  as  to  prevent 
too  much  condensation. 

1  gm.  of  the  sample  being  placed  in  the  retort,  10  c.c.  of  a  40  per-cent.  solution 
of  P205  are  added,  together  with  as  much  water  as  will  make  about  50  c.c.  A 
small  naked  flame  is  used,  and  if  carefully  manipulated  the  distillation  may  be 
carried  on  nearly  to  dryness  without  endangering  the  retort.  After  the  first 
operation  the  retort  is  allowed  to  cool  somewhat,  then  50  c.c.  of  hot  water  added 
through  the  funnel,  another  distillation  made  as  before,  and  the  same  repeated 
a  third  time,  which  will  suffice  to  carry  over  all  the  acetic  acid.  The  distillate  is 
then  titrated  with  alkali  and  phenolphthalein. 

By  this  arrangement  the  frothing  and  spirting  is  of  no  consequence, 
and  the  whole  process  can  be  completed  in  less  than  an  hour.  The 
results  are  excellent  for  technical  purposes. 

Web  erf  has  devised  a  ready  and  fairly  accurate  method  of 
determining  the  real  acetic  acid  in  samples  of  acetate  of  lime,  based 
on  the  fact  that  acetate  of  silver  is  insoluble  in  alcohol. 

*  Z.  a.  C.  13,  153.  t  C.  AT.  53,  181.  J  Z.  a.  C.  24,  614. 


ACETATES.  93 

METHOD  OF  PROCEDURE  :  10  gm.  of  the  sample  in  powder  are  placed  in  a  250 
c.c.  flask,  a  little  water  added,  and  heated  till  all  soluble  matters  are  extracted, 
cooled,  and  made  up  to  the  mark ;  25  c.c.  are  then  filtered  through  a  dry  filter, 
put  into  a  beaker,  50  c.c.  of  absolute  alcohol  added,  and  the  acetic  acid  at  once, 
precipitated  with  an  alcoholic  solution  of  silver  nitrate.  The  silver  acetate, 
together  with  any  chloride,  sulphate,  etc.,  separates  free  from  colour.  The 
precipitate  is  brought  on  a  filter,  well  washed  with  60  per-cent.  alcohol  till  the 
free  silver  is  removed  ;  precipitate  is  then  dissolved  in  weak  nitric  acid,  and  titrated 
with  N/i0  salt  solution.  Each  c.c.  represents  0*006  gm.  acetic  acid. 

Several  trials  made  in  comparison  with  the  distillation  method 
with  phosphoric  acid  gave  practically  the  same  results. 

A  good  technical  method  has  been  devised  by  Grimshaw.* 

METHOD  OF  PROCEDURE  :  10  gm.  of  the  sample  are  treated  with  water  and 
an  excess  of  sodium  bisulphate  (NaHS04),  the  mixture  diluted  to  a  definite  volume, 
filtered,  and  a  measured  portion  of  the  filtrate  titrated  with  standard  alkali  ; 
a  similar  portion  meanwhile  is  evaporated  to  dryness,  with  repeated  moistening 
with  water  to  drive  off  all  free  acetic  acid.  The  residue  is  dissolved  and  titrated 
with  standard  alkali,  when  the  difference  between  the  volume  now  required  and 
that  used  in  the  original  solution  will  correspond  to  the  acetic  acid  in  the  sample. 
Litmus  paper  is  the  proper  indicator. 


BORIC    ACID    AND    BORATES. 

Boric  anhydride  B2O3  =  70. 

THE  soda  in  borax  may,  according  to  Thomson,  be  very 
accurately  determined  by  titrating  the  salt  with  standard  H2SO4 
and  methyl  orange  or  lacmoid  paper.  Litmus  and  phenacetolin 
give  very  doubtful  end-reactions  :  phenolphthalein  is  utterly 
useless. 

EXAMPLE  :  1*683  gm.  sodium  pyroborate  in  50  c.c.  of  water  required  in  one 
case  16'7  c.c.  normal  acid,  and  in  a  second  16*65  c.c.  The  mean  of  the  two 
represents  0'517  gm.  Na20.  Theory  requires  0'516  gm. 

The  determination  of  boric  acid,  as  such,  formerly  presented  great 
difficulties,  and  no  volumetric  method  of  any  value  was  available. 

R.  T.  Thomsonf  has  removed  this  difficulty  by  finding  a  method 
easy  of  execution  and  of  fair  accuracy. 

METHOD  OF  PROCEDURE  :  To  determine  boric  acid  in  articles  of  commerce 
it  is  necessary  to  use  methyl  orange,  to  which  indicator  boric  acid  is  perfectly 
neutral.  In  the  case  of  boric  acid  in  borax  1  gm.  is  dissolved  in  water,  methyl 
orange  added,  and  then  dilute  sulphuric  acid  till  the  pink  colour  just  appears. 
Boil  for  a  short  time  to  expel  C02,  cool,  and  add  normal  or  fifth-normal  soda  till 
the  pink  colour  of  the  methyl  orange  (a  little  more  of  which  should  be  added  if 
necessary)  just  assumes  a  pure  yellow  tinge.  At  this  stage  all  the  boric  acid  will 
exist  in  the  free  state.  Add  glycerin  in  such  proportion  that  the  total  solution 
after  titration  will  contain  30  per  cent,  at  least,  then  add  a  little  phenolphthalein, 
and  lastly  normal  or  fifth-normal  soda  (free  from  C02)  from  a  burette  until  a  per- 
manent pink  colour  is  produced.  More  glycerin  may  be  added  during  the  deter- 
mination if  it  is  found  necessary.  The  proportion  of  boric  acid  present  is  calculated 
from  the  number  of  c.c.  of  soda  consumed. 

*  A 1 1  e  n '  s  Organic  Analysis,  1,  3rd  edition,  479. 
t.7.  S.  C.  7.12,432. 


94  BORIC  ACID. 

1  c.c.  normal  NaOH  =0'0620  gm.  H3B03 

1  c.c.        „  „      =0-0505  gm.  Na2B407 

1  c.c.       „  „     =0-0955  gm.  Na2B407+10H20 

In  the  case  of  boric  acid  of  commerce,  which  generally  contains  salts 
of  ammonium,  1  gm.  may  be  dissolved  in  hot  water,  a  slight  excess  of  sodium 
carbonate  added,  and  the  solution  boiled  down  to  about  half  its  bulk  to  expel 
ammonia.  Any  precipitate  which  appears  may  then  be  filtered  off,  and  the 
filtrate  titrated  as  already  described. 

The  method  may  also  be  applied  to  boracite  and  borate  of  lime  by  dissolving 
1  gm.  of  either  of  these  minerals  in  dilute  hydrochloric  acid  with  the  aid_of  heat, 
nearly  neutralizing  with  caustic  soda,  boiling  to  expel  C02,  cooling,  exactly 
neutralizing  to  methyl  orange,  and  continuing  the  determination  as  in  borax. 
If  much  iron  is  present,  however,  it  should  be  removed  by  a  preliminary  treat- 
ment with  sodium  carbonate  and  removal  of  oxide  of  iron  as  well  as  the  carbonates 
of  calcium  and  magnesium  by  filtration. 

L.  C.  Jones*  has  based  a  method  of  titrating  boric  acid  upon  the 
fact  that  when  a  solution  of  the  acid  is  mixed  with  one  of  mannitol, 
a  much  stronger  acidic  character  is  developed  from  the  boric  acid 
than  it  naturally  possesses  (a  similar  effect  occurs  with  glycerin), 
and  further,  that  boric  acid  alone  in  solution  in  moderate  amount 
has  not  the  slightest  action  on  a  solution  containing  potassium 
iodide  and  iodate.  Therefore,  if  a  given  solution  containing  boric 
acid  be  mixed  with  iodide  and  iodate,  the  acid  set  free  by  addition 
of  a  mineral  acid,  and  the  free  iodine  so  produced  exactly  destroyed 
by  thiosulphate,  there  results  a  colourless  liquid  containing  the 
boric  acid  in  a  free  state  and  ready  to  be  titrated  by  any  convenient 
method. 

METHOD  OF  PROCEDURE  :  The  solution,  about  50  c.c.,  containing  the  boric 
compound  and  about  O'l  gm.  of  boric  acid,  is  acidified  distinctly  with  hydrochloric 
acid,  but  any  large  excess  must  be  removed  with  soda.  5  c.c.  of  a  10  per-cent, 
solution  of  barium  chloride  are  then  added.  In  a  separate  beaker  the  iodide  and 
iodate  mixture,  say  10  c.c.  of  a  25  per-cent.  solution  of  iodide  and  the  same  volume 
of  a  saturated  solution  of  iodate,  together  with  starch  indicator  is  placed — the 
quantity  must  be  sufficient  to  liberate  an  amount  of  iodine  equivalent  at  least  to 
the  free  HC1  in  the  boric  solution  ;  the  colour  of  the  starch  iodide  which  is  usually 
liberated  from  this  mixture  is  removed  by  a  dilute  solution  of  sodium  thiosulphate. 
To  this  neutral  solution  of  iodide  and  iodate  a  single  drop  of  the  boric  solution 
is  added  by  a  glass  rod ;  if  a  blue  colour  appears  it  is  evident  that  the  boric  solution 
is  acid  with  free  HC1  and  the  boric  acid  is  in  a  free  condition.  The  solutions  are 
then  mixed  and  the  free  iodine  removed  by  cautious  addition  of  thiosulphate. 
The  mixture  is  then  colourless  and  contains  only  starch,  neutral  chloride,  potassium 
tetrathionate,  iodide  and  iodate,  with  all  the  B203  in  a  free  state.  Any  C02  will 
have  been  removed  by  the  barium  chloride. 

The  titration  is  now  begun  by  adding  a  few  drops  of  phenolphthalein  and 
N/s  caustic  soda  run  in  from  the  burette  until  a  strong  red  colour  is  shown  ; 
a  pinch  of  mannitol  is  then  thrown  in  which  bleaches  the  colour,  more  soda  and 
more  mannitol  are  in  turn  adddd  until  the  colour  is  permanent.  As  a  rule  1  or 
2  gm.  of  mannitol  suffice  for  a  determination.  The  amount  of  B203  is  calculated 
on  the  assumption  that  under  the  above  mentioned  conditions  1  mol.  of  the  acid 
requires  2  mol.  of  sodium  hydroxide.  Test  analyses  on  calcium  borate  and 
colemanite  gave  satisfactory  results  and  within  a  very  short  time.  Silicates  and 
fluorides  do  not  interfere,  but  ammonium  salts'must  be  removed  by  boiling  with 
alkali  previous  to  adopting  the  process,  owing  to  their  well  known  effect  on  the 
indicator. 

*Am.J.  S.,  1898,  147-153. 


BORIC   ACID.  95 

Schwartz*  recommends  the  glycerin  method  in  the  case  of 
boracite  to  be  carried  out  as  follows  : — 

1  or  2  gm.  of  the  finely  powdered  material  are  mixed  with  5  to  10  c.c.  of  strong 
hydrochloric  acid,  made  up  to  about  50  or  100  c.c.  with  water,  and  digested  with 
stirring  for  several  hours  at  ordinary  temperature.  The  process  may  be  hastened 
by  heating,  but  in  that  case  a  reflux  condenser  may  be  necessary  to  avoid  loss  of 
boric  acid.  In  either  case  the  liquid  is  filtered,  residue  washed,  and  the  filtrate 
rendered  exactly  neutral  to  methyl  orange  with  N/5  soda  (free  from  C03).  The 
volume  is  made  up  to  100  or  200  c.c.,  then  25  or  50  c.c.  mixed  with  the  same 
volume  of  absolutely  neutral  glycerin,  diluted  somewhat,  then  titrated  with 
phenolphthalein  and  N/5  soda. 

Boric  Acid  in  Milk,  Butter,  and  other  Foods.— R.  T.  Thomsonf.  1  to  2  gm. 
of  sodium  hydrate  are  added  to  100  c.c.  of  milk,  and  the  whole  evaporated  to 
dryness  in  a  platinum  dish.  The  residue  is  cautiously  but  thoroughly  charred, 
heated  with  20  c.c.  of  water,  and  hydrochloric  acid  added  drop  by  drop  until  all 
but  the  carbon  is  dissolved.  The  whole  is  transferred  to  a  100  c.c.  flask,  the 
bulk  not  being  allowed  to  exceed  50  or  60  c.c.,  and  0'5  gm.  dry  calcium  chloride 
added.  To  this  mixture  a  few  drops  of  phenolphthalein  are  added,  then 
a  10  per-cent.  solution  of  caustic  soda,  till  a  permanent  slight  pink  colour  is  per- 
ceptible, and  finally  25  c.c.  of  lime-water.  In  this  way  all  the  P205  is  precipitated 
as  calcium  phosphate.  The  liquid  is  made  up  to  100  c.c.  thoroughly  mixed  and 
filtered  through  a  dry  filter.  To  50  c.c.  of  the  filtrate  (equal  to  50  c.c.  of  the  milk) 
normal  sulphuric  acid  is  added  till  the  pink  colour  is  gone,  then  methyl  orange,  and 
the  addition  of  the  acid  continued  until  the  yellow  is  just  changed  to  pink.  N/5 
caustic  soda  is  now  added  till  the  liquid  assumes  the  yellow  tinge,  excess  of  soda 
being  avoided.  At  this  stage  all  acids  likely  to  be  present  exist  as  salts  neutral 
to  phenolphthalein,  except  boric  acid  (which,  being  neutral  to  methyl  orange,  exists 
in  the  free  condition),  and  a  little  C02,  which  it  is  absolutely  necessary  to  expel 
by  boiling  for  a  few  minutes.  The  solution  is  cooled,  a  little  phenolphthalein 
added,  and  as  much  glycerin  as  will  give  at  least  30  per  cent,  of  that  substance 
in  the  solution,  then  titrated  with  N/5  caustic  soda  till  a  distinct  permanent  pink 
colour  is  produced.  Each  c.c.  of  the  soda  is  equal  to  0'0124  gm.  crystallized 
boric  acid.  A  series  of  experiments  with  this  process  showed  that  no  boric  acid 
was  precipitated  with  the  phosphate  of  lime  so  long  as  the  solution  operated 
upon  did  not  contain  more  than  0'2  per  cent,  of  crystallized  boric  acid,  but  when 
stronger  solutions  were  tested,  irregular  results  were  obtained.  The  charring 
of  the  milk  is  apt  to  drive  off  boric  acid,  but  by  carefully  carrying  the  incinera- 
tion only  so  far  as  is  necessary  to  secure  a  residue  which  will  yield  a  colourless 
solution,  no  appreciable  loss  occurs. 

There  is  no  doubt  that  C02  must  be  got  rid  of  in  titrating  boric  acid  with 
phenolphthalein,  and  hence  it  is  necessary  to  boil  the  solution.  Some  operators 
therefore  do  this  in  a  flask  with  upright  condenser  to  avoid  the  loss  of  boric  acid. 
It  is  doubtful,  however,  whether  by  this  confined  escape,  the  gas  is  got  rid  of  as 
easily  as  is  thought.  L.  de  KoninghJ  gives  the  results  of  experiments  made 
in  this  manner,  and  shows  that  a  dilute  solution  of  the  acid  may  be  boiled  even  up 
to  fifteen  minutes  in  an  open  vessel  (which  is  longer  than  necessary),  with  the  loss 
of  a  very  faint  trace  of  the  acid.  The  same  operator  also  advocates  the  removal 
of  phosphoric  acid,  which  is  nearly  always  present  in  foods,  by  adding  a  slight 
excess  of  sodium  carbonate  to  the  boric  acid  liquid,  then  cautiously  adding  calcium 
chloride  ;  this  precipitates  any  phosphate  and  the  excess  of  carbonate,  while 
the  borate  in  very  dilute  solution  is  not  affected.  On  now  adding  a  solution 
of  ammonium  carbonate  containing  an  excess  of  free  ammonia  the  excess  of 
lime  is  precipitated.  By  boiling  the  clear  solution  with  excess  of  sodium 
carbonate  the  ammonium  compounds  are  quickly  expelled,  and  the  titration  may 
be  carried  on  as  before  described. 

A  new  process  is  also  described  in  the  same  article  by  which  the  boric  acid  may 

*  Chem.  Zeit.,  1899,  497.  t  Glasgow  City  Anal.  Soc.  Repts.,  1895,  p.  3. 

%J.  Am.  C.  S.  1897,  385. 


96  BORIC  ACID. 

be  determined  after  the  removal  of  the  P205  by  means  of  magnesium  mixture  ;  the 
nitrate  is  mixed  with  excess  of  sodium  carbonate  and  heated,  the  precipitate  of 
magnesia  is  removed  by  filtration,  the  nitrate  evaporated  to  dryness  to  render  the 
rest  of  the  magnesia  insoluble,  and  the  residue  is  then  treated  with  a  little  water 
and  filtered.  The  boric  acid  may  then  be  titrated  according  to  Thomson's 
glycerin  method.  As  a  test  experiment,  O'l  gm.  of  boric  acid  was  dissolved  in 
aqueous  soda,  then  mixed  with  100  gm.  of  oatmeal  and  incinerated  ;  from  the  ash, 
0*095  gm.  of  boric  acid  was  recovered. 

A  rapid  method  for  determining  boric  acid  in  BUTTER  has  been  worked  out  by 
H.  DroopRichmond  and  J.  B.  P.  H  a  r  r  i  s  o  n.  *  25  gm.  of  the  butter  are  weighed 
out  into  a  beaker,  and  25  c.c.  of  a  solution  containing  6  gm.  of  milk  sugar  and  4  c.c. 
of  normal  sulphuric  acid  in  100  c.c.  are  added.  The  beaker  is  placed  in  the  water 
oven  until  the  fat  has  just  melted  and  the  mixture  is  well  stirred.  After  allowing 
the  aqueous  portion  to  settle  for  a  few  minutes  20  c.c.  are  pipetted  out,  a  little 
phenolphthalein  added,  brought  to  ths  boiling  point,  and  titrated  with  semi-normal 
caustic  soda  until  a  faint  pink  colour  just  appears.  12  c.c.  of  neutral  glycerin 
are  now  added,  and  the  further  titration  carried  on  till  a  pink  colour  appears. 
The  difference  between  the  two  titrations  multiplied  by  0'0368  gives  the  amount 
of  boric  acid  in  20  c.c.,  and  this  multiplied  by 

100  +  percentage  of  water  in  the  butter 
30 

will  give  the  percentage  of  boric  acid.  The  determination  is  not  affected  by  the 
phosphoric  or  butyric  acid,  or  milk  sugar  present  in  the  butter. 

For  the  determination  of  boric  acid  in  meat  C.  Fresenius  and 
G.  Poppf  adopt  the  following  method  with  good  results  : — 

10  gm.  of  the  chopped  meat  is  triturated  in  a  mortar  with  40  to  80  gm.  of 
anhydrous  sodium  sulphate,  and  dried  in  the  water  oven  ;  the  mass  is  then  finely 

Eowdered,  if  necessary  with  the  addition  of  more  sodium  sulphate,  introduced 
ito  a  300-c.c.  Erlenmeyer  flask,  and  100  c.c.  of  methylic  alcohol  added.  After 
standing  for  twelve  hours,  the  alcohol  is  distilled  off  ;  50  c.c.  more  methylic 
alcohol  are  poured  on  to  the  residue,  and  this  is  again  distilled  off.  The  distillate 
is  finally  made  up  with  methylic  alcohol  to  150  c.c.,  and  50  c.c.  of  this  are  mixed 
with  50  c.c.  of  water  and  50  c.c.  of  50  per-cent,  glycerin  solution  containing 
phenolphthalein,  and  carefully  neutralized  with  soda  ;  after  thoroughly  mixing 
the  liquid,  it  is  titrated  with  N/2O  soda,  1  c.c.  of  soda  =0'0031  gm.  of  crystallized 
boric  acid. 


CARBONIC    ACID    AND    CARBONATES. 

ALL  carbonates  are  decomposed  by  strong  acids  ;  the  carbonic 
acid  which  is  liberated  splits  up  into  water  and  carbonic  anhydride 
(C02),  which  latter  escapes  in  the  gaseous  form. 

It  will  readily  be  seen,  from  what  has  been  said  previously  as  to 
the  determination  of  the  alkaline  earths,  that  carbonic  acid  in 
combination  can  be  determined  volumetrically  with  a  very  high 
degree  of  accuracy  (see  p.  72). 

The  carbonic  acid  to  be  determined  may  be  brought  into  combi- 
nation with  either  calcium  or  barium,  these  bases  admitting  of  the 
firmest  combination  as  neutral  carbonates. 

If  the  carbonic  acid  exist  in  a  soluble  form  as  an  alkali  mono- 

*  Analyst  27,  179.  t  Chcm.  Centr.,  1897,  2,  69. 


CARBONIC   ACID.  97 

carbonate,  the  decomposition  is  effected  by  the  addition  of  barium 
or  calcium  chloride  as  before  directed ;  if  as  bicarbonate,  or 
a  compound  between  the  two,  ammonia  must  be  added  with  either 
of  the  chlorides. 

As  solution  of  ammonia  frequently  contains  traces  of  C02,  this 
must  be  removed  by  the  aid  of  barium  or  calcium  chloride  previous 
to  use. 


1.     Carbonates  Soluble  in  Water. 

It  is  necessary  to  remember  that  when  calcium  chloride  is  used 
as  the  precipitant  in  the  cold  amorphous  calcium  carbonate  is  first 
formed  ;  and  as  this  compound  is  sensibly  soluble  in  water,  it  is 
necessary  to  convert  it  into  the  crystalline  form.  In  the  absence  of 
free  ammonia  this  can  be  accomplished  by  boiling.  When  ammonia 
is  present,  the  same  end  is  obtained  by  allowing  the  mixture  to 
stand  for  eight  or  ten  hours  in  the  cold,  or  by  heating  for  an  hour 
or  two  to  70-80°  C.  With  barium  the  precipitation  is  regular. 

Another  fact  is  that  when  ammonia  is  present,  and  the  precipita- 
tion occurs  at  ordinary  temperatures,  ammonium  carbamate  is 
formed  and  the  barium  or  calcium  carbonate  is  only  partially 
precipitated.  This  difficulty  is  overcome  by  heating  the  mixture 
nearly  to  boiling  for  a  couple  of  hours,  and  is  best  done  by  passing 
the  neck  of  the  flask  through  a  retort  ring,  and  immersing  the  flask 
in  boiling  water. 

When  caustic  alkali  is  present  in  the  substance  to  be  examined, 
it  is  advisable  to  use  barium  as  the  precipitant ;  otherwise,  for  all 
volumetric  determinations  of  C02  calcium  is  to  be  preferred,  because 
the  precipitate  is  much  more  quickly  and  perfectly  washed  than  the 
barium  compound. 

EXAMPLE  :  1  gm.  of  pure  anhydrous  sodium  carbonate  was  dissolved  in  water, 
precipitated  while  hot  with  barium  chloride,  the  precipitate  allowed  to  settle  well, 
the  clear  liquid  decanted  through  a  moist  filter,  more  hot  water  containing  a  few 
drops  of  ammonia  poured  over  the  precipitate,  this  treatment  being  repeated  so 
that  the  bulk  of  the  precipitate  remained  in  the  flask,  being  washed  by  decantation 
through  the  filter  ;  when  the  washings  showed  no  trace  of  chlorine,"  the  filter  was 
transferred  to  the  flask  containing  the  bulk  of  the  precipitate,  and  20  c.c.  of 
normal  nitric  acid  added,  then  titrated  back  with  normal  alkali,  of  which  1'2  c.c. 
was  required  =  18'8  c.c.  of  acid  ;  this  multiplied  by  0'022,  the  coefficient  for  carbonic 
acid,  gave  0-4136  gm.  C02=41'36  per  cent.,  or  multiplied  by  0*053,  the  coefficient 
for  sodium  carbonate,  gave  0'9964  gm.  instead  of  1  gm. 


2.    Carbonates  Soluble  in  Acids. 

It  sometimes  occurs  that  substances  have  to  be  examined  for 
carbonic  acid  which  do  not  admit  of  being  treated  as  above  described, 
such,  for  instance,  as  white  lead,  calamine,  carbonates  of  magnesia, 
iron,  and  copper,  cements,  mortar,  and  many  other  substances.  In 
these  cases  the  carbonic  acid  must  be  evolved  from  the  combination 

H 


98 


CARBONATES    SOLUBLE    IN    ACIDS. 


by  means  of  a  stronger  acid,  and  conducted  into  an  absorption 
apparatus  containing  ammonia,  then  precipitated  with  calcium 
chloride,  and  titrated  as  before  described. 

The    following   form  of  apparatus  (fig.   33)  affords  satisfactory 
results. 


Fig.  33 

METHOD  OF  PROCEDURE  :  The  weighed  substance  from  which  the  carbonic 
acid  is  to  be  evolved  is  placed  in  6  with  a  little  water ;  the  tube  d  contains  strong 
hydrochloric  acid,  and  c  broken  glass  wetted  with  ammonia  free  from  carbonic 
acid.  The  flask  a  is  about  one-eighth  filled  with  the  same  ammonia ;  the  bent 
tube  must  not  enter  the  liquid.  When  all  is  ready  and  the  rubber  stoppers  tight, 
warm  the  flask  a  gently  so  as  to  fill  it  with  vapour  of  ammonia,  then  open  the  clip 
and  allow  the  acid  to  flow  gradually  upon  the  material,  which  may  be  heated  until 
all  carbonic  acid  is  apparently  driven  off ;  then  by  boiling  and  shaking  the  last 
traces  can  be  evolved,  and  the  operation  ended.  When  cool,  the  apparatus  may 
be  opened,  the  end  of  the  bent  tube  washed  into  a,  and  also  a  good  quantity  of 
boiled  distilled  water  passed  through  c,  so  as  to  carry  down  any  ammonium 
carbonate  that  may  have  formed.  Then  add  solution  of  calcium  chloride,  boil, 
filter,  and  titrate  the  precipitate  as  before  described. 

During  the  filtration,  and  while  ammonia  is  present,  there  is  a  great  avidity  for 
carbonic  acid,  therefore  boiling  water  should  be  used  for  washing,  and  the  funnel 
kept  covered  with  a  small  glass  plate. 

In  many  instances  CO2  may  be  determined  by  its  equivalent  in 
chlorine  with  N/10  silver  and  chromate,  as  on  page  143» 


CARBONIC   ACID    IN    WATER.  99 

3.     Carbonic  Acid  Gas  in  Waters,  etc. 

The  carbonic  acid  existing  in  waters  as  neutral  carbonates  of  the 
alkalies  or  alkaline  earths  may  very  elegantly  and  readily  be  titrated 
directly  by  N/10  acid  (see  p.  73). 

Well  or  spring  water,  and  also  mineral  waters,  containing  free 
carbonic  acid  gas,  can  be  examined  by  collecting  measured  quantities 
of  them  at  their  source,  in  bottles  containing  a  mixture  of  calcium 
and  ammonium  chlorides,  afterwards  heating  the  mixture  in  boiling 
water  for  one  or  two  hours,  and  titrating  the  precipitate  as  before 
described. 

Pettenkofer's  method  with  caustic  baryta  or  lime  is  in 
general  use.  Lime  water  may  be  used  instead  of  baryta  with  equally 
good  results,  but  care  must  be  taken  that  the  precipitate  is  crystal- 
line. 

The  principle  of  the  method  is  that  of  removing  all  the  carbonic 
acid  from  a  solution,  or  from  a  water,  by  excess  of  baryta  or  lime 
water  of  a  known  strength  ;  and,  after  absorption,  finding  the  excess 
of  baryta  or  lime  by  titration  with  N/10  acid  and  turmeric  paper. 

The  following  course  is  the  best  to  be  pursued  in  this  method 
for  ordinary  drinking  waters  not  containing  large  quantities  of 
carbonic  acid  : — 

METHOD  OF  PROCEDURE  :  100  c.c.  of  the  water  are  put  into  a  flask  with  3  c.c. 
of  strong  solution  of  calcium  or  barium  chloride,  and  2  c.c.  of  saturated  solution 
of  ammonium  chloride  ;  45  c.c.  of  baryta  or  lime  water,  the  strength  of  which  has 
been  previously  ascertained  by  means  of  decinormal  acid,  are  then  added,  the  flask 
well  corked  and  put  aside  to  settle ;  when  the  precipitate  has  fully  subsided,  take 
out  50  c.c.  of  the  clear  liquid  with  a  pipette,  and  titrate  it  with  decinormal 
acid.  The  quantity  required  must  be  multiplied  by  3,  there  being  50  c.c.  only 
taken ;  the  number  of  c.c.  so  found  must  be  deducted  from  the  original 
quantity  required  for  the  baryta  or  lime  solution  added  ;  the  remainder 
multiplied  by  0'0022  will  give  the  weight  of  carbonic  acid  existing  free  and  as 
bicarbonate  in  the  100  c.c. 

The  addition  of  the  barium  or  calcium  chloride  and  ammonium  chloride  is 
made  to  prevent  any  irregularity  which  might  arise  from  alkaline  carbonates  or 
sulphates,  or  from  magnesia. 

A  more  accurate  method  of  determining  CO2  in  its  various 
states  of  existence  in  drinking  waters  has  been  used  for  some  years 
past.  It  is  described  by  C.  A.  Seyler.* 

Whatever  may  really  be  the  condition  under  which  C02  exists 
in  natural  waters,  and  there  is  difference  of  opinion  on  the  point, 
it  is  sufficient  for  all  practical  purposes  to  assume  that  it  occurs 
in  three  forms,  namely  :  first,  as  monocarbonates  of  alkalies  or 
alkaline  earths  ;  second,  as  bicarbonates  of  the  same ;  and 
third,  as  free  CO2.  Seyler  proposes  to  distinguish  the  first  as 
fixed  and  the  two  others  as  volatile  C02,  inasmuch  as  the  gas 
existing  as  bicarbonate  may  be  almost,  and  the  free  gas  completely, 
dispelled  by  boiling.  On  the  assumption  that  the  half-bound 
acid  (i.e.,  as  bicarbonate)  is  equal  to  the  combined,  the  free  CO2 

*  C.  N.,  1894 ;  Analyst,  1897,  p.  312. 

II    2 


100  CARBONIC   ACID    IN   WATER. 

may  be  determined  by  subtracting  the  combined  from  the  volatile 
as  found  by  Pettenkofer's  process — this,  however,  is  inaccurate 
with  small  quantities  and  tedious.  What  is  required  is  a  method 
of  determining  the  free  CO2  independently. 

Pettenkofer's  method  has  been  modified  by  Trillich,  Lunge,  and 
Seyler,  and  the  modifications  have  been  carefully  investigated  by  Ellms  and 
Beneker,*  who  have  come  to  the  conclusion  that  Seyler's  method  is  the  most 
accurate. 

The  essential  details  of  the  process  are  as  follows  : — 

The  free  carbonic  acid  is  determined  by  placing  100  c.c.  of  the  sample  in  a  glass 
cylinder  with  25  to  30  drops  of  a  neutral  solution  of  phenol phthalein.  To  the 
sample  is  then  added  N/go  sodium  carbonate,  stirring  carefully  and  thoroughly 
until  a  faint  permanent  pink  colour  is  obtained. 

METHOD  OF  PROCEDURE  :  1.  The  titration  can  conveniently  be  performed  in 
a  Nessler  cylinder,  approximately  18  cm.  long  by  3  cm.  in  diameter,  graduated 
for  50  and  100  c.c.  The  stirring  rod  is  bent  at  its  lower  end  into  the  form  of 
a  circle,  and  then  turned  so  as  to  stand  at  right-angles  to  the  rod.  A  comparison 
cylinder  containing  the  same  amount  of  water  as  the  titrating  cylinder  is  found  to 
aid  in  the  determination  of  the  end-point. 

2.  The  larger  part  of  the  sodium  carbonate  solution  should  be  added  quickly, 
and  the  strong  pink  colour  formed  should  be  discharged  by  stirring  and  mixing 
with  the  rod.     The  titration  can  then  be  cautiously  completed,  until  colour 
remains  permanent.     The  sodium  carbonate  solution  should  be  prepared  with 
freshly  ignited  sodium  carbonate,  and  with  air-free  water.     The  exposure  of  this 
solution  to  the  air  should  be  avoided  as  much  as  possible,  as  sodium  bicarbonate 
is  readily  formed,  which  renders  it  useless  for  this  titration  where  accurate  results 
are  desired. 

3.  With  waters  that  are  high  in  free  and  half-bound  carbonic  acid  it  is  better 
to  use  less  than  100  c.c.  for  the  titration.     With  such  a  water,  care  is  necessary 
in  transferring  the  sample  to  the  cylinder  in  order  to  avoid  loss  of  C02.     Too 
vigorous  stirring  of  the  water  is  also  to  be  avoided  for  the  same  reason. 

The  fixed  carbonic  acid,  from  which  the  half-bound  acid  is  estimated,  is 
determined  according  to  the  method  of  Hehner  (p.  74).  Seyler  uses  methyl 
orange  as  the  indicator  for  this  titration,  but  lacmoid  is  preferable. 

In  the  absence  of  free  C02  in  a  water,  the  half-bound  may  equal  the  fixed,  in 
which  case  it  would  be  neutral  to  phenolphthalein.  If,  however,  the  water  is 
alkaline  to  phenolphthalein,  the  half-bound  C02  does  not  equal  the  fixed  ;  or,  in 
other  words,  a  portion  of  the  carbonates  of  the  bases  exist  in  solution  without  the 
assistance  of  any  half-bound  C02.  In  such  a  case  the  half-bound  acid  is  obtained 
by  first  determining  the  fixed  C02  by  means  of  lacmoid.  From  this  is  deducted 
an  amount  of  C02  equal  to  twice  the  quantity  indicated  by  the  acid  required  to 
discharge  the  pink  colour  produced  by  phenolphthalein.  The  difference  is  the 
amount  of  half-bound  C02  which  is  present.  These  titrations  may  be  made  on 
the  same  sample,  in  which  case  the  "  phenolphthalein  alkalinity  "  is  first  determined 
and  then  followed  by  the  titration  with  lacmoid  ;  or  they  may  be  made  on  separate 
samples. 

The  principles  upon  which  the  above  procedure  is  based  have  been  pointed 
out  above. 

These  titrations  involve  no  especial  difficulties,  and  can  be  easily  and  quickly 
carried  out.  N/2O  solutions  of  sulphuric  acid  and  sodium  carbonate  were  used 
by  the  experimenters.  Seyler  has  prepared  a  series  of  formula}  for  calculating 
the  results,  which  simplifies  the  work  somewhat.  If  results  are  obtained  with 
100  c.c.  of  the  sample  and  the  reagents  employed  are  N/2O>  the  following  formulae 
express  the  results  in  parts  per  million : — 

I.     For  waters  acid  or  neutral  to  phenolphthalein  : — 

*  J.  Am.  C.  S.  23,  405. 


CARBONIC    ACID    IN    WATER.  ;  101 

Free  carbonic  acid     ..  ..  .*      I       .'.",  *=4'4^>  -    «  %•  '• !  "  ': 

Fixed  or  half-bound  carbonic  acid         . .  .  .      ^4'4  m 

Volatile  carbonic  acid  . .  . .  . .      =4'4  (m  +p) 

Total  carbonic  acid  .  .         •     . .  . .      =4'4  (2m  +p) 

p  =c.c.  N/20  sodium  carbonate~solution  required  to  produce  a  pink  colour  with 

phenolphthalein  in  100  c.c.  of  the  water ;  and 

m  =c.c.  N/20  sulphuric  acid  solution  required  to  obtain  the  end-point  with  methyl 
orange  or  lacmoid  in  the  same  volume  of  water. 
II.     For  waters  alkaline  to  phenolphthalein  : — 
Fixed  carbonic  acid  . .  . .  . .  . .      =4*4  m 

Half-bound  or  volatile  carbonic  acid     . .  . .      =4*4  (m  —2p') 

Total  carbonic  acid  . .  . .  . .  . .      =4'4  (2m  —p') 

m  =c.c.  N/2O  sulphuric  acid  solution  required  to  obtain  end-point  with  methyl 

orange  or  lacmoid  in  100  c.c.  of  the  sample. 
//  =c.c.  N/2o  sulphuric  acid  required  to  discharge  the  pink  colour  produced  by 

phenolphthalein  in  100  c.c.  of  the  sample. 

There  is  a  third  case  in  which  free  C02  might  exist  in  a  solution  containing 
free  mineral  acid,  and  for  which  S  ey  ler  has  given  a  method  with  its  corresponding 
formulae  for  calculating  the  results.  But  such  a  condition  would  seldom  be  found 
in  natural  waters. 

The  errors  affecting  the  accuracy  of  Sey  ler's  method  are  those  which  arise  in 
part  from  the  determination  of  the  free  C0a.  The  end-point  in  the  tit-ration  of 
the  sample  with  sodium  carbonate  and  phenolphthalein  is  not  entirely  satisfactory. 
The  results  obtained  are  usually  low,  but  with  care  and  practice  the  error  from 
this  source  should  be  less  than  2  to  3  parts  per  million,  even  with  considerable 
amounts  of  C02,  and  on  small  amounts  it  is  less  still. 

The  error  due  to  the  determination  of  the  fixed  carbonic  acid,  from  which  the 
half-bound  is  derived,  arises  from  those  errors  involved  in  the  carrying  out  of 
Hehner's  method,  which  in  good  work  ought  not  to  exceed  1  to  2  parts 
per  million. 

4.     Carbonic  Acid  in  Aerated  Beverages,  etc. 

For  ascertaining  the  quantity  of  CO2  in  bottled  aerated  waters, 
such  as  soda,  seltzer,  potass,  and  others,  the  following  apparatus 
is  useful. 

Fig.  34  is  a  brass  tube  made  like  a  cork-borer,  about  five  inches  long,  having 
four  small  holes,  two  on  each  side,  and  about  two  inches  from  its  cutting  end  ; 
the  upper  end  is  securely  connected  with  the  bent  tube  from  the  absorption  flask 
(fig.  35)  by  means  of  a  vulcanized  tube ;  the  flask  contains  a  tolerable  quantity 
of  pure  ammonia,  into  which  the  delivery  tube  dips  ;  the  tube  a  contains  broken 
glass  moistened  with  ammonia. 

Everything  being  ready  the  brass  tube  is  greased,  and  the  bottle  being  held  in 
the  right  hand,  the  tube  is  screwed  a  little  aslant  through  the  cork  by  turning 
the  bottle  round,  until  the  holes  appear  below  the  cork  and  the  gas  escapes  into 
the  flask.  When  all  visible  action  has  ceased,  after  the  bottle  has  been  well 
shaken  two  or  three  times  to  evolve  all  the  gas  that  can  possibly  be  eliminated, 
the  vessels  are  quietly  disconnected,  the  tube  a  washed  out  into  the  flask,  and  the 
contents  of  the  bottle  added  also ;  the  whole  is  then  precipitated  with  calcium 
chloride  and  boiled,  and  the  precipitate  titrated  as  usual.  This  gives  the  total 
C02  free  and  combined. 

To  find  the  quantity  of  the  latter,  another  bottle  of  the  same  manufacture 
must  be  evaporated  to  dry  ness,  and  the  residue  gently  ignited,  then  titrated  with 
normal  acid  and  alkali ;  the  amount  of  C02  in  the  monocarbonate,  deducted  from 
the  total,  will  give  the  weight  of  free  gas  originally  present. 

The  volume  may  be  found  as  follows : — 1000  c.c.  of  C02  at  0°,  and  760  mm. 
weigh  1-9709  gm.  (Rayleigh).  Suppose,  therefore,  that  the  total  weight  of 
C02  found  in  a  bottle  of  ordinary  soda  water  was  2*8  gm.,  and  the  weight 
combined  with  alkali  0'42  gm.,  this  leaves  2'38  gm.  CO2  in  a  free  state — 

1-9769   :   2-38   :   :   1000  :  x  -1204  c.c. 


102 


AERATED    WATERS. 


If  «;he  number  of  c.c.  of  carbonic  acid  found  is  divided  by  the 
number  of  c.c.'  of  soda  water  contained  in  the  bottle  examined,  the 
quotient  will  be  the  volume  of  gas  compared  with  that  of  the  soda 
water.  The  volume  of  the  contents  of  the  bottle  is  ascertained  by 
marking  the  height  of  the  fluid  previous  to  making  the  experiment  ; 
the  bottle  is  afterwards  filled  to  the  same  mark  with  water,  emptied 
into  a  graduated  cylinder  and  measured ;  say,  the  volume  was  292 
c.c.,  therefore 

1204 

292  '  C°2' 


•P 
Fig.  34. 


Fig.  35. 


5.     Carbonic  Acid  in  Air. 

A  dry  glass  globe  or  bottle  capable  of  being  securely  closed  by 
a  rubber  stopper,  and  holding  4  to  6  litres,  is  filled  with  the  air  to 
be  tested  by  means  of  a  bellows  aspirator  ;  baryta  or  lime  water, 
containing  a  little  barium  chloride,  is  then  introduced  in  convenient 
quantity  and  of  known  strength  as  compared  with  N/i0o  acid.  The 
vessel  is  securely  closed,  and  the  liquid  allowed  to  flow  round 
the  sides  at  intervals  during  half  an  hour  or  more.  When  absorption 
is  judged  to  be  complete,  the  alkaline  solution  is  emptied  out 
quickly  into  a  stoppered  bottle,  and  the  excess  at  once  ascertained 
in  a  measured  portion  by  N/i0o  oxalic  or  hydrochloric  acid  and 
turmeric  paper  as  described  on  p.  55.  The  final  calculation  is  of 
course  made  on  the  total  alkali  originally  used,  and  upon  the  exact 
measurement  of  the  air-collecting  vessel. 

It  is  above  all  things  necessary  to  prevent  the  absorption  of  C02 
from  extraneous  sources  during  the  experiment,  especially  from  the 


CARBONIC   ACID   IN    AIR.  103 

breath  of  the  operator.  The  error  may  be  reduced  to  a  minimum 
by  carrying  on  the  titration  in  the  vessel  itself,  which  is  done  by 
fixing  an  accurately  graduated  pipette  through  the  cork  or 
caoutchouc  stopper  of  the  air  vessel,  to  the  upper  end  of  which  is 
attached  a  stout  piece  of  elastic  tube,  .closed  with  a  pinch-cock  ; 
and  this  being  filled  to  the  0  mark  with  dilute  standard  acid  acts 
as  a  burette.  The  baryta  or  lime  solution  tinted  with  phenolph- 
thalein  is  placed  in  the  air  bottle,  which  must  be  of  colourless  glass, 
and  after  the  absorption  of  all  C02  the  excess  of  alkali  is  found  by 
running  in  the  acid  until  the  colour  disappears.* 

The  cork  or  stopper  must  have  a  second  opening  to  act  as 
ventilator  ;  a  small  piece  of  glass  tube  does  very  well. 

If  a  freshly  made  solution  of  oxalic  acid  is  used  containing  2*8636 
gm.  per  litre,  each  c.c.  represents  1  mgm.  CO2.  The  liquid  holds 
its  strength  correctly  for  a  day,  and  can  be  made  as  required  from 
a  strong  solution,  say  28*636  per  litre. 

Another  method  of  calculation  is  to  convert  the  volume  of  baryta 
solution  decomposed  into  its  equivalent  volume  in  N/10  acid, 
1  c.c.  of  wilich  =0*0022  gm.  CO2  or  by  measurement  at  0°  C.  and 
760  mm.  pressure  represents  1*119  c.c.  The  method  above 
described  is  a  combination  of  those  of  Pettenkofer  and  Dal  ton, 
and,  though  much  used,  is  liable  to  considerable  error  from  various 
causes. 

These  errors  have  been  examined  by  Letts  and  Blake,|  more 
especially  as  to  absorbing  the  CO2  by  baryta  from  a  sample  of 
air  collected  in  a  glass  vessel  and  titrating  with  acid,  and  they 
show  that,  in  addition  to  the  more  obvious  sources  of  error,  the 
action  of  the  alkaline  absorbent  on  the  glass  is  one  of  importance. 

In  order  to  avoid  it,  they  coat  both  the  receiver  containing  the 
air  sample  and  the  bottle  holding  the  stock  of  standard  solution 
of  baryta  with  paraffin  wax.  By  this  means  they  at  once  obtained 
more  concordant  results  in  a  series  of  determinations.  They  then 
proceeded  to  test  the  degree,  both  of  accuracy  and  of  delicacy,  of 
Pettenkofer's  process  if  carried  out  with  all  the  available  pre- 
cautions which  suggested  themselves.  For  this  purpose  they  em- 
ployed paraffined  receiving  vessels,  an  apparatus  for  performing 
the  titrations  in  a  vacuum,  and  burettes  of  special  construction. 
In  addition,  an  apparatus  was  used  for  delivering  very  accurately 
measured  volumes  of  pure  carbonic  anhydride  into  known  volumes 
of  air  previously  freed  from  that  gas. 

Experimenting  with  such  mixtures  of  the  two  as  occur  in  air 
containing  about  3  vols.  of  CO2  in  10,000,  the  authors  show  that 
with  careful  work  the  mean  error  in  the  determinations  need  not 
exceed— 0*04  part.  The  actual  quantity  of  CO2  added  to  each 

*  Some  operators  prefer  a  standard  mixture  of  caustic  soda  or  potash  containing 
some  barium  chloride  to  the  baryta  or  lime  solution.  This  is  adopted  by  S  y  m  o  n  s 
and  Stephens  with  acetic  acid  as  control.  The  method  used  by  them,  which  gives 
excellent  results,  is  explained  in  their  voluminous  paper  contributed  to  J.C.S.  Trans., 
1896,  pp.  869-881. 

t  Proc.  Chem.  Soc.  1896,  192, 


104  CARBONIC   ACID    IN    AIR, 

receiver  full  of  air,  in  a  series  of  five  experiments,  amounted  to 
0'927  c.c.,  and  the  mean  amount  found  to  0'916  c.c.,  giving, 
therefore,  a  mean  error  of —0*011  c.c. 

They  thus  show  that  Pettenkofer's  process,  if  properly 
performed,  is  one  of  great  accuracy  and  delicacy. 

A.  H.  Gill  in  a  report  from  the  Sanitary  and  Gas  Analysis 
Laboratory  of  the  Technical  Institute  at  Boston,  U.S.A.,*  gives 
a  somewhat  modified  arrangement  of  the  Pettenkofer  method. 
Ordinary  green  glass  bottles  of  one  or  two  gallons  capacity  are 
measured  by  filling  them  with  water,  and  the  contents  in  c.c. 
ascertained,  preferably  by  weighing  on  a  good  balance. 

The  bottles  are  dried  before  being  used.  This  may  easily  be 
done  by  rinsing  first  with  alcohol  or  methylated  spirit,  draining, 
then  rinsing  with  ether,  and  after  again  draining  the  bottle  is  quickly 
dried  by  blowing  air  through  it  with  the  ordinary  laboratory  bellows. 
If  this  plan  is  not  used  they  must  be  dried,  after  draining  well,  in 
a  warm  place.  A  special  form  of  bellows  for  filling  the  bottle  with 
air  is  used  by  Gill,  but  the  usual  aspirator  made  on  the  accordion 
pattern  suffices,  or  a  small  fan  blower,  the  driving  parts  of  which 
are  connected  by  rubber  bands  to  render  it  noiseless,  may  be  used. 

The  bottle  is  fitted  with  a  rubber  stopper  carrying  a  glass  tube, 
closed  by  a  plug  of  solid  rubber. 

The  air  to  be  tested  is  drawn  into  the  bottle  by  repeated  use  of 
the  aspirator  so  as  to  collect  a  representative  sample,  and  if  the  test 
is  made  in  a  room  everything  should  be  quiet,  and  care  must  be 
taken  to  avoid  draughts  or  the  proximity  of  a  number  of  persons. 

METHOD  OF  PROCEDURE  :  50  c.c.  of  the  standard  barium  hydrate  are  rapidly 
run  into  the  bottle  from  a  burette  (the  jet  passing  entirely  through  the  tube  in 
the  stopper),  the  cap  replaced,  and  the  solution  spread  completely  over  the  sides 
of  the  bottle  while  waiting  three  minutes  for  the  draining  of  the  burette,  before 
reading,  unless  it  be  graduated  to  deliver  50  c.c.  The  bottle  is  now  placed  upon 
its  side,  and  shaken  at  intervals  for  forty  to  sixty  minutes,  taking  care  that  the 
whole  surface  of  the  bottle  is  moistened  with  the  solution  each  time.  The 
absorption  of  the  last  traces  of  C02  is  very  slow  indeed,  half  an  hour  in  many 
cases  being  insufficient. 

At  the  time  at  which  the  barium  solution  is  added  the  temperature  and  pressure 
should  be  noted.  At  the  end  of  the  above  period,  shake  well  to  ensure  homo- 
geneity of  the  solution,  remove  the  cap  from  the  tube,  and  invert  the  large  bottle 
quickly  over  a  60  or  70  c.c.  glass  stoppered  bottle,  so  that  the  solution  shall  come 
in  contact  with  the  air  as  little  as  possible.  Without  waiting  for  the  bottle  to 
drain,  withdraw  a  portion  of  15  or  25  c.c.  with  a  narrow-stemmed  spherical-bulbed 
pipette  and  titrate  with  sulphuric  acid  f  (1  c.c.  =1  mgm.  C02),  using  rosolic  acid 
as  an  indicator.  The  difference  between  the  number  of  c.c.  of  standard  acid 
required  to  neutralize  the  amount  of  barium  hydrate  (e.g.,  50  c.c.)  before  and 
after  absorption  gives  the  number  of  milligrams  of  C02  present  in  the  bottle. 

*  Analyst  17,  184. 

t  Sulphuric  acid,  in  distinction  to  oxalic  acid,  enables  one  to  determine  the  excess  of 
barium  hydrate  in  presence  of  the  suspended  barium  carbonate,  and  also  of  caustic 
alkali,  which  is  a  frequent  impurity  of  commercial  barium  hydrate.  Professor 
Johnson,  in  the  American  edition  of  Fresenius'  Quantitative  Analysis,  calls 
attention  to  the  fact  that  the  normal  alkali  oxalates  decompose  the  alkaline  earthy 
carbonates,  so  that  the  reaction  continues  alkaline  if  the  least  trace  of  soda  or  potash 
be  present.  The  sulphuric  acid  may  be  prepared  by  diluting  4  6 '51  c.c.  normal 
sulphuric  acid  to  a  litre. 


SCHEIBLER'S  CALCIMETER.  105 

This  is  expressed  in  cubic  centimetres  under  standard  conditions,  and  divided 
by  the  capacity  of  the  bottle  under  standard  conditions,  and  the  results  reported 
in  parts  per  10,000.  To  reduce  the  air  in  the  bottle  to  standard  conditions, 
a  hygrometric  measurement  of  the  air  in  the  room  from  which  the  sarnple^was 
taken  is  necessary.  This  in  ordinary  cases  is  usually  omitted,  as  the  object  of 
the  investigation  is  comparative  results,  as  regards  the  efficiency  of  ventilation, 
and  the  rooms  in  the  same  building  would  not  vary  appreciably  in  the  amount  of 
moisture  in  the  atmosphere.  This  correction  may  make  a  difference  of  about 
0-15  parts  per  10,000. 

Another  method  on  the  same  principle  is  to  attach  a  bulb 
apparatus,  containing  a  measured  quantity  of  baryta  or  lime  water, 
to  an  aspirator  bottle  filled  with  water ;  the  tap  of  the  bottle  is 
opened  to  such  an  extent  as  to  allow  the  air  to  bubble  through  the 
test  solution  at  a  moderate  rate.  The  process  of  titration  is  the 
same  as  above.  This  method  takes  longer  time,  and  the  volume 
of  air,  which  should  not  be  less  than  five  or  six  litres,  is  ascertained 
by  measuring  the  volume  of  water  allowed  to  run  out  of  the 
aspirator,  the  rate  of  flow  being  regulated  so  that  from  two  to  three 
hours  are  required  to  pass  the  above  volume  of  air.  If  a  flask, 
fitted  with  tubes,  is  used  in  place  of  the  bulb  apparatus,  the  titration 
may  be  done  without  transferring  the  test  solution. 

6.     Scheibler's  Galcimeter  for  the  determination  of 
Carbonic  Acid  by  Volume. 

This  apparatus  is  adapted  for  the  determination  of  the  CO2 
contained  in  native  carbonates,  as  well  as  in  artificial  products, 
and  has  been  specially  contrived  for  the  purpose  of  readily  deter- 
mining the  C02  in  the  bone-black  used  in  sugar  refining.  The 
principle  upon  which  the  apparatus  is  founded  is  simply  this  : — 
That  the  quantity  of  CO2  contained  in  calcium  carbonate  may  be 
used  as  a  measure  of  the  quantity  of  that  salt  itself  ;  and  instead  of 
determining,  as  has  usually  been  the  case,  the  quantity  of  gas  by 
weight,  this  apparatus  admits  of  its  determination  by  volume  ; 
and  it  is  by  this  means  possible  to  perform,  in  a  few  minutes, 
operations  which  would  otherwise  take  hours  to  accomplish,  while, 
moreover,  the  operator  need  possess  scarcely  any  knowledge  of 
chemistry.  The  results  obtained  by  this  apparatus  are  said  to  be 
correct  enough  for  technical  purposes. 

The  apparatus  is  shown  in  fig.  36,  and  consists  of  the  following 
parts  : — 

The  glass  vessel,  A,  serves  for  the  decomposition  of  the  material  to  be  tested 
for  C02,  which  for  that  purpose  is  treated  with  dilute  HC1 ;  this  acid  is  contained, 
previous  to  the  experiment,  in  the  gutta  percha  vessel  s.  The  glass  stopper  of  A, 
is  perforated,  and  through  it  firmly  passes  a  glass  tube,  to  which  is  fastened  the 
india-rubber  tube  r,  by  means  of  which  communication  is  opened  with  B,  a  bottle 
having  three  openings  in  its  neck.  The  central  opening  of  this  bottle  contains 
a  glass  tube  (r)  firmly  fixed,  which  is  in  communication,  on  the  one  hand,  with 
A,  by  means  of  the  flexible  india-rubber  tube  already  alluded  to,  and,  on  the 
other  hand,  inside  of  B,  with  a  very  thin  india-rubber  bladder,  K.  The  neck  (q) 
of  the  vessel  B  is  shut  off  during  the  experiment  by  means  of  a  piece  of  india- 
rubber  tubing,  kept  firmly  closed  with  a  spring  clamp.  The  only  use  of  this 


106 


SCHEIBLER'S  CALCIMETER. 


opening  of  the  bottle  B,  arranged  as  described,  is  to  give  access  of  atmospheric 
air  to  the  interior  of  the  bottle,  if  required.  The  other  opening  is  in  communication 
with  the  measuring  apparatus  C,  a  very  accurate  cylindrical  ^glass  tube  of  150  c.c. 
capacity,  divided  into  0'5  c.c.  ;  the  lower  portion  of  this  tube  Cfis  in  communication 
with  the  tube  D,  serving  the  purpose  of  controlling  the  pressure  of  the  gas. 
The  lower  part  of  this  tube  D  ends  in  a  glass  tube  of  smaller  diameter,  to  which 


Fig.  36. 

is  fastened  the  india-rubber  tube  p,  leading  to  E,  but  the  communication  between 
these  parts  of  the  apparatus  is  closed,  as  seen  at  p,  by  means  of  a  spring  clamp. 
E  is  a  water  reservoir,  and  on  removal  of  the  clamp  at  p,  the  water  contained 
in  C  and  D  runs  off  towards  E  ;  when  it  is  desired  to  force  the  water  contained  in 
E  into  C  and  D,  this  can  be  readily  done  by  blowing  with  the  mouth  into  V,  and 
opening  the  clamp  at  p. 

Precise  directions  for  the  use  of  this  instrument  are  issued  by  the  makers  and 
need  not  be  repeated  here.  It  has  been  considerably  used  for  technical  purposes, 
but  is  liable  to  serious  errors,  for  which  various  corrections  have  to  be  made,  but 
even  then  there  is  room  for  considerable  improvement. 


IMPROVED    CALCIMETER. 


107 


This  improvement  has  been  made  by  A.  Marshall,*  and  the 
apparatus  is  shown  in  fig.  37. 


Fig.  37. 

It  consists  of  a  gas  reduction  tube  M,  and  a  measuring  tube  E,  which  both  pass 
through  corks  to  the  bottom  of  the  Woulff's  bottle  H,  which  is  so  fitted  that 
the  pressure  of  air  in  it  can  be  accurately  adjusted.  It  contains  some  refined 
petroleum  oil  of  high  boiling  point,  which  can  be  forced  into  the  tubes  M  and  E. 
M  contains  such  a  quantity  of  air  that,  if  it  were  reduced  to  0°  C.  and  760  mm. 
pressure,  it  would  just  fill  the  tube  down  to  a  certain  mark  on  it.  The  tube  E 
is  graduated  in  cubic  centimetres,  and  is  fitted  at  the  top  with  a  three-way  cock 
of  special  design,  by  means  of  which  it  can  be  brought  into  communication 
either  with  the  atmosphere  or  with  the  tube  G,  which  leads  to  the  generating 
vessel  A.  Branching  out  of  G  is  the  mercury  manometer  D,  which  enables  one 
to  adjust  the  pressure  inside  A,  G,  and  E  till  it  is  equal  to  the  atmospheric  pressure. 
The  generating  vessel  A  is  fitted  with  a  well-ground  tubulated  stopper,  and 
contains  a  small  glass  tube  B  to  hold  the  acid.  It  is  inserted  in  the  glass  water- 
bath  C,  which  should  contain  cold  water. 

Briefly  stated,  a  determination  is  conducted  as  follows  : — The  carbon  dioxide 
is  generated  by  the  action  of  a  small  volume,  1  c.c.,  of  concentrated  hydrochloric 
acid  on  a  weighed  quantity  of  the  substances  to  be  tested  ;  O'l  to  0'8  or  more 
gm.  should  be  taken,  according  to  the  percentage  of  carbonic  acid  it  contains. 
A  mixture  of  air  and  carbon  dioxide  passes  over  into  the  measuring  tube  E. 
When  the  action  is  complete,  the  pressure  is  adjusted,  till  the  manometer  D 
shows  that  it  is  equal  to  that  of  the  atmosphere.  The  cock  is  then  turned  off, 
and  the  pressure  is  again  adjusted  till  the  liquid  in  M  stands  at  the  highest 
graduation.  The  volume  in  E  is  then  read  off.  This,  without  any  correction 
whatever,  is  the  volume  of  carbon  dioxide  contained  in  the  substance  taken. 
The  whole  operation  does  not  take  more  than  ten  to  fifteen  minutes. 

The  gas  reduction  tube  E  acts  on  the  same  principle  as  that  in  Lun ge'  s  well- 
known  "  gas  volumeter."  To  give  absolutely  accurate  results  the  level  of  liquid 
in  M  and  E  should  be  the  same.  The  density  of  the  petroleum  is,  however,  so 

*  J.  S.  C.  I.,  1898,  1106. 


108  IMPROVED    CALCIMETEE. 

small  that  the  error  from  this  cause  never  amounts  to  more  than  0'3  c.c.  with  an 
apparatus  having  the  dimensions  selected  by  the  inventor. 

Carbon  dioxide  is  slightly  soluble  even  in  heavy  petroleum  oil,  but  the  solution 
proceeds  very  slowly.  In  the  case  of  this  apparatus,  if  the  printed  instructions 
be  followed,  only  a  dilute  mixture  of  carbon  dioxide  can  come  in  contact  with 
the  petroleum.  The  error  due  to  this  cause  therefore  falls  well  within  the  limits 
of  experimental  error  due  to  other  causes. 

The  error  due  to  the  solubility  of  carbon  dioxide  in  hydrochloric  acid 
is  reduced  to  a  minimum  by  employing  a  small  quantity  of  concentrated 
acid  ;  using  1  c.c.  of  acid  of  I'll  sp.  gr.  it  does  not  amount  to. more  than  0'5  c.c.  This 
error  is  in  the  opposite  direction  to  that  due  to  the  inequality  of  the  levels  in  the 
tubes  M  and  E.  Consequently  it  is  to  a  great  extent  neutralized  by  the  latter. 
Concentrated  hydrochloric  acid  dissolves  less  carbon  dioxide  than  the  same  volume 
of  dilute  acid. 

If  the  generating  vessel  A  be  not  kept  cool  a  notable  quantity  of  hydrogen 
chloride  is  expelled  from  it,  and  is  slowly  reabsorbed  as  the  apparatus  cools  down 
again.  This,  of  course,  would  interfere  with  the  accuracy  of  the  process. 
During  the  action  the  vessel  should,  therefore,  be  kept  immersed  in  cold  water. 
The  cold-water  bath  also  tends  to  prevent  the  temperature  of  the  generating 
apparatus  varying  to  any  perceptible  extent.  Any  error  due  to  the  latter  cause 
is,  in  addition,  greatly  reduced  by  the  small  volume  of  the  generating  apparatus, 
which  is  not  more  than  100  c.c. 

The  following  are  the  chief  advantages  of  the  apparatus  described  : — 

1.  The  quantity  of  carbon  dioxide  dissolved  in  the  acid  is  reduced  to  a  minimum 
by  using  a  small  quantity  of  concentrated  acid. 

2.  No  corrections  have  to  be  made  for  temperature  and  pressure  ;  consequently 
no  reading  of  thermometer  or  barometer  need  be  taken. 

3.  The  total  volume  of  the  generating  and  measuring  apparatus  being  less 
than    100   c.c.,    and   the   generating   vessel   being   immersed   in   a   considerable 
quantity  of  cold  water,  the  volume  of  the  air  inside  it  cannot  change  during 
a  determination  to  an  extent  sufficient  to  introduce  a  perceptible  error. 

4.  The  apparatus  is  quite  simple,  and  although  no  barometer  or  thermometer 
is  required  the  results  are  considerably  more  accurate  than  those  obtained  with 
Scheibler's. 

To  determine  the  percentage  of  CaC03  in  any  substance,  weigh  out  accurately 
0-224  gm.  and  proceed  as  above.  The  volume  found,  multiplied  by  2,  gives  the 
per  cent,  of  CaCO3. 


CITRIC    ACID. 

C3H4  (OH)  (COOH)3+H20  =  210-08. 

THIS  acid  in  the  free  state  may  readily  be  titrated  with  normal 
soda  and  phenolphthalein.  1  c.c.  normal  alkali  =  0*07  gm.  crystal- 
lized citric  acid. 

1.  Citrates  of  the  Alkalies  and  Earths. — These  citrates  may  be  treated  with 
neutral  solution  of  lead  nitrate  or  acetate,  in  the  absence  of  other  acids  precipitable 
by  lead.     The  lead  citrate  is  washed  with  a  mixture  of  equal  parts  alcohol  and 
water,  the  precipitate  suspended  in  water,  and  H2S  passed  into  it  till  all  the  lead 
is  converted  into  sulphide  ;  the  clear  liquid  is  then  boiled  to  remove  H2S,  and  titrated 
with  normal  alkali. 

2.  Fruit    Juices,    etc. — If    tartaric    is    present,    together    with 
free  citric  acid,  the  former  is  first  separated  as  potassium  bitartrate, 
which  can  very  well  be  done  in  the  presence  of  citric  acid. 


CITRIC    ACID.  109 

METHOD  OF  PROCEDURE  :  A  cold  saturated  solution  of  potassium  acetate  in 
proof  spirit  is  added  to  a  somewhat  strong  solution  of  the  mixed  acids  in  proof 
spirit  in  sufficient  quantity  to  separate  all  the  tartaric  acid  as  bitartrate,  the 
mixture  after  stirring  well  being  allowed  to  stand  some  hours.  The  precipitate 
is  then  transferred  to  a  filter,  and  washed  with  proof  spirit,  then  rinsed  off  the 
filter  with  a  cold  saturated  solution  of  potassium  bitartrate,  and  allowed  to  stand 
some  hours,  with  occasional  stirring  ;  this  treatment  removes  any  adhering  citrate. 
The  bitartrate  is  again  brought  on  to  a  filter,  washed  once  with  proof  spirit,  then 
dissolved  in  hot  water,  and  titrated  with  normal  alkali,  1  c.c.  of  which  =0*15  gm. 
tartaric  acid. 

The  first  filtrate  may  be  titrated  for  the  free  citric  acid  present  after  evaporating 
the  bulk  of  the  alcohol. 

3.  Lime  and  Lemon  Juices. — The  citric  acid  contained  in  lemon, 
lime,  and  similar  juices,  may  be  very  fairly  determined  by 
Warington's  method.* 

METHOD  OF  PROCEDURE  :  15  or  20  c.c.  of  ordinary  juice,  or  3  —4  c.c.  of 
concentrated  juice,  are  first  exactly  neutralized  with  pure  normal  soda,  made  up, 
if  necessary,  to  about  50  c.c.,  heated  to  boiling  in  a  salt  bath,  and  so  much 
solution  of  calcium  chloride  added  as  to  be  slightly  in  excess  of  the  organic  acids 
present.  The  mixture  is  kept  at  the  boiling  point  for  about  half  an  hour,  the 
precipitate  collected  on  a  filter  and  washed  with  hot  water,  filtrate  and  washings 
concentrated  to  about  15  c.c.,  and  a  drop  of  ammonia  added  :  this  will  produce 
a  further  precipitate,  which  is  collected  separately  on  a  very  small  filter  by  help 
of  the  previous  filtrate,  then  washed  with  a  small  quantity  of  hot  water.  Both 
filters,  with  their  precipitates,  are  then  dried,  ignited  at  a  low  red  heat,  and  the' 
ash  titrated  with  normal  or  N/io  acid,  each  c.c.  of  which  represents  respectively 
0-07  or  0-007  gm.  H3Ci  +H20. 


FORMIC    ACID. 

HCOOH= 46-02. 

H.  C.  JONES!  has  worked  out  a  method  which  though  not 
acidimetric  may  be  quoted  here.  It  is  based  on  a  process  originally 
devised  by  Pea u  deSaint-Gilles,  viz.,  titration  with  permanga- 
nate in  the  presence  of  an  alkali  carbonate.  Lieben  confirmed 
this,  using  a  more  elaborate  process.  The  method  is  on  the  same 
principle,  but  the  procedure  differs  from  that  of  Lieben. 

METHOD  OF  PROCEDURE  :  The  solution  containing  the  formic  acid  is  made 
alkaline  with  Na?C03,  warmed,  and  an  excess  of  standard  permanganate  added. 
All  the  formic  acid  is  thus  oxidized,  and  a  precipitate  of  manganese  hydroxide 
thrown  down.  The  solution  is  acidified  with  H2S04,  and  a  measured  volume  of 
oxalic  acid  run  in  until  all  the  precipitate  has  dissolved  and  the  permanganate 
disappeared.  The  excess  of  oxalic  acid  is  then  titrated  with  standard  perman- 
ganate. A  volume  of  oxalic  acid  equal  to  that  taken  is  also  titrated  with  the 
permanganate  solution,  and  the  difference  between  the  result  and  the  total 
permanganate  used  gives  the  quantity  of  permanganate  required  to  oxidize  the 
formic  acid.  The  experimental  results  agree  well  among  themselves  and  with 
those  obtained  by  other  methods. 

The  author  further  shows  that  Saint-Gi lies'  statement  that 

*  J.  C.  S.  1875,  934.  t  Amer.  Chem.  Jour.  17,  539-541. 


110  FORMIC   ACID. 

oxalic  acid  can  be  titrated  in  acid  solution  in  the  presence  of  formic 
acid  is  unreliable,  since  formic  acid  is  also  oxidized  to  some  extent 
by  the  permanganate  under  these  conditions. 

F.  Freyer*,  having  occasion  to  determine  the  formate  in  a 
mixture  of  calcium  acetate  and  formate,  has  devised  the  following 
method. 

METHOD  OF  PROCEDURE  :  The  mixed  calcium  salts  are  distilled  witn  dilute 
sulphuric  acid  in  a  current  of  steam  until  the  distillate  is  no  longer  acid  ;  an 
aliquot  portion  of  the  distillate  is  titrated  with  alkali  to  determine  the  total  acid, 
whilst  another  portion  is  evaporated,  if  necessary,  with  excess  of  caustic  soda  to 
concentrate  it,  and  is  treated  as  follows  :  10  to  20  c.c.,  containing  about  0*5  gm. 
of  formic  acid,  are  heated  for  half  an  hour  to  an  hour  with  50  c.c.  of  a  6  per  cent, 
solution  of  potassium  dichromate  and  10  c.c.  of  concentrated  sulphuric  acid  in 
a  flask  provided  with  a  reflux  condenser.  The  liquid  is  now  made  up  to  200  c.c., 
and  the  unaltered  chromic  acid  determined  in  10  c.c.  of  it.  For  this  purpose, 
1  to  2  gm.  of  pure  potassium  iodide,  10  c.c.  of  a  25  per  cent,  solution  of 
phosphoric  acid,  and  some  water  are  added  ;  and  after  five  minutes  the  solution 
is  diluted  to  about  100  c.c.  with  boiled  water,  and  titrated  with  N/1O  thiosulphate 
solution  in  the  usual  manner.  The  phosphoric  acid  is  added  according  to 
M  e  i  n  e  k  e '  s  recommendation,  and  is  for  the  purpose  of  rendering  the  change 
from  the  blue  colour  of  the  iodide  of  starch  to  the  green  of  the  chromium  salt 
more  visible  ;  the  commercial  glacial  acid  may  be  dissolved  in  water,  oxidized  by 
potassium  permanganate  until  it  has  a  faint  rose  colour,  and  filtered  before  being 
used. 

The  dichromate  solution  used  for  the  oxidation  is  titrated  in  the  same  way. 
One  mol.  potassium  dichromate  is  equivalent  to  three  mols.  formic  acid. 

The  results  quoted  by  the  author  show  that  the  method  is  fairly 
accurate,  both  in  the  absence  and  in  the  presence  of  acetic  acid 


HYDROFLUORIC     ACID,     HYDROFLUOSILICIC     (SILICO- 
FLUORIC)    ACID,    AND    FLUORIDES. 

1  c.c.  of  */!    alkali  =0-02  gm.  of  HF =0*024  gm.  of  H2SiF6. 

COMMERCIAL  hydrofluoric  acid  is  as  a  rule  far  from  pure.  It 
generally  contains  hydrofluosilicic  acid,  sulphuric  acid,  sulphurous 
acid,  and  frequently  traces  of  iron  and  lead.  Two  analyses  of 
commercial  acid  gave  the  following  figures  : — 

1  2 

Hydrofluoric  acid     48 '00     45*80 

Hydrofluosilicic  acid    13 '05     9*49 

Sulphuric  acid 4*07     3*23 

Sulphurous  acid 0*49     1*06 

Left  on  evaporation     0*16     .... 

Water  by  difference    34*23     40*42 

100*00  100*00 

If  it  is  desired  to  prepare  pure  acid,  the  best  way  is  to  add  to  the 
commercial  acid  peroxide  of  hydrogen  till  it  ceases  to  decolorize  iodine, 

*  Chem.  Zeit.  19,  1184. 


HYDROFLUORIC   ACID.  Ill 

and  then  potassic  liydric  fluoride  sufficient  to  fix  all  the  hydrofluo- 
silicic  and  sulphuric  acids.  Re-distillation  in  a  lead  retort  with 
a  platinum  condenser  will  then  give  perfectly  pure  acid. 

The  total  amount  of  free  acid  may  be  determined  with  normal 
alkali  (preferably  potash),  using  phenolphthalein  or  litmus,  the 
former  being  best.  Methyl  orange  and  lacmoid  do  not  give  good 
'results.  In  the  case  of  pure  acid,  each  c.c.  of  N/x  alkali  indicates 
0'02  gm.  of  HF,  and  the  reaction  when  phenolphthalein  is  employed 
is  very  sharp.  When,  however,  commercial  acid  is  thus  titrated 
a  difference  is  observed  ;  the  pink  colour  obtained  on  adding  the 
alkali  only  endures  for  a  second  or  so  and  then  fades  away,  and  this 
may  be  repeated  for  some  time  till  at  last  a  permanent  pink  is 
produced.  The  cause  of  this  is  the  presence  of  hydrofluosilicic 
acid.  The  first  appearance  of  pink  ensues  when  the  reaction 
+  K2O==K2SiF6  +  H2O  occurs.  Then  another  reaction  sets  in 


K2SiF6  +  2K2O  =  6KF  -f  SiO2, 

but  from  the  slight  solubility  of  the  potassium  silicofluoride  some 
time  elapses  before  it  is  complete. 

The  sulphuric  and  sulphurous  acids  must  also  be  determined  if 
the  real  amount  of  HF  is  required. 

Determination  of  Sulphuric  Acid  in  Presence  of  Hydrofluoric  Acid  (  W.  B.  Giles) 
Long  experience  has  convinced  the  author  of  this  new  process  that  all  methods 
depending  upon  the  supposed  solubility  of  barium  fluoride,  and  the  corresponding 
insolubility  of  the  sulphate,  in  either  hot  or  cold  diluted  hydrochloric  acid  give 
most  erroneous  results.  For  instance,  a  sample  of  hydrofluoric  acid  known  to 
contain  4  %  of  H«S04  was  treated  in  the  way  described  by  Fresenius,  using 
a  large  volume  of  hot  dilute  hydrochloric  acid,  and  the  precipitate  was  copiously 
washed  with  the  same  weak  acid.  The  barium  precipitate  obtained  was  equal 
to  6'08  %  of  H2S04  or  over  50  %  more  than  was  present,  and  it  was  found  that 
on  repeatedly  moistening  the  precipitate  with  dilute  H2S04,  and  re-igniting,  that 
the  weight  increased  materially,  showing  co-precipitation  of  barium  fluoride. 
The  author  therefore  devised  the  following  process  for  the  determination  of  the 
SO3,  which  gives  accurate  results.  Its  basis  is  — 

1.  The  conversion  of  HF  into  H2SiF6,  which  is  easily  accomplished. 

2.  The  precipitation  of  the  S03  from  this  solution  by  means  of  lead  silicofluoride. 

3.  The  total  insolubility  of  PbS04  in  a  solution  containing  an  excess  of  the 
said  lead  salt. 

METHOD  OF  PROCEDURE  :  A  convenient  weight  of  the  hydrofluoric  acid  is 
placed  in  a  platinum  dish,  about  half  its  volume  of  water  is  added,  and  then 
precipitated  silica  in  evident  excess,  and  the  whole  is  allowed  to  stand  with  occasional 
stirring  for  a  few  hours.  It  is  then  filtered,  using  an  ebonite  funnel,  into  another 
suitable  platinum  basin,  and  the  excess  of  silica  thoroughly  washed.  The  filtrate 
and  washings  are  then  evaporated  to  a  convenient  bulk,  and  solution  of  lead 
silicofluoride  is  added  in  excess.  If  the  least  trace  of  sulphuric  acid  was  originally 
contained  in  the  acid,  an  almost  immediate  precipitate  of  PbS04  will  form,  as  it  is 
exceedingly  insoluble  in  the  presence  of  the  lead  silicofluoride.  The  solution  is 
allowed  to  stand  an  hour  or  two,  and  the  PbS04  separated  by  filtration,  when  it 
can  of  course  be  treated  in  any  convenient  volumetric  way  for  the  determination  of 
the  lead,  or  it  may  be  weighed  direct. 

Lead  silicofluoride  is  easily  prepared  by  saturating  commercial  HF  with  coarsely 
powdered  flint  in  a  lead  basin,  and  then  agitating  with  powdered  litharge.  Its 
solubility  is  very  great,  and  the  specific  gravity  of  the  solution  may  reach  2-000 


112  HYDROFLUORIC   ACID. 

EXAMPLE  :  To  37'89  gm.  of  chemically  pure  HF  of  1250  sp.  gr.  there  was 
added  25  c.c.  of  normal  acid  (  =1-0  gm.  S03.)  The  mixture  was  then  treated  as 
described  above,  and  gave  PbS04  3*782  gm.  =1-0002  gm.  of  S03. 

Determination  of  the  Hydrofluosilicic  Acid. — To  a  convenient  quantity  of 
the  acid  contained  in  a  platinum  dish  a  solution  of  potassium  acetate  in  strong 
methylated  spirit  is  added  in  excess,  and  then  more  spirit  is  added,  so  that  there 
may  be  about  equal  volumes  of  liquid  and  spirit.  Allow  to  stand  for  several 
hours,  and  then  filter  and  wash  with  a  mixture  of  half  spirit  and  half  water.  The 
resulting  potassium  silicofluoride  may  then  be  titrated  with  normal  alkali  according 
to  the  equation : 

K2SiF6  +2  K30  =6  KF  +Si02, 
or  if  the  filter  was  a  weighed  one,  it  may  be  dried  at  100°  C.  and  weighed  direct. 

EXAMPLE  :  2  gm.  of  chemically  pure  precipitated  silica  were  dissolved  in  a  large 
excess  of  pure  diluted  HF.  Treated  as  above  described,  it  yielded  7 '35  gm.  of 
K2SiF6  which  equals  2-004  gm.  of  silica ;  2  gm.  of  some  powdered  flint  treated  in 
the  same  way  with  50  gm.  of  pure  HF  (of  40  %)  gave  7-168  gm.  of  K2SiF6  = 
1-958  gm.  of  silica. 

Sulphurous  Acid. — This  is  easily  determined  by  taking  the  solution  which 
results  from  the  total  acidity  determination  and  titrating  with  decinormal  iodine. 
Commercial  hydrofluoric  acid  generally  contains  from  0'5  to  2-0  %. 

The  amount  of  each  of  the  impurities  being  thus  known,  the 
percentage  of  real  HF  is  easily  calculated;  e.g.,  10  gm.  of  an  acid 
was  found  to  neutralize  276*0  c.c.  of  normal  alkali.  It  was  found 
to  give  the  following  results  :. — 

c.c.normal  alkali     8'0  =  3*23  SO3 

„        „       39-0=  9-36  H2SiF6 
276-47  =  229  c.c.  xO'02  =  45*80%  HF. 

41-61%  H20  by  difference 


100-00 

In  this  instance  the  amount  of  S02  was  not  allowed  for. 

Bifluorides. — These  salts  may  be  titrated  in  the  same  way  as 
the  acid  with  phenolphthalein.  They  generally  contain  some 
silicofluoride.* 

The  determination  of  fluorine  in  soluble  fluorides  has  been  done 
volumetrically  by  Knoblochf.  The  process  is  based  on  the 
following  facts  : — • 

When  a  solution  of  ferric  chloride  is  mixed  with  its  equivalent 
quantity  of  potassium  fluoride  the  decomposition  is  complete,  and 
the  resulting  ferric  fluoride  solution  is  colourless.  In  this  state  the 
iron  is  not  detectable  by  such  tests  as  thiocyanate,  salicylic  acid, 
etc.  Still  more  interesting  is  the  fact  that  ferric  fluoride  does  not 
liberate  iodine  from  iodides. 

The  following  standard  solutions,  etc.,  are  required  : — 

N/io  potassium  fluoride  ;  5*809  gm.  of  the  pure  ignited  salt  in 
a  litre  of  water. 

*  The  whole  of  this  section,  to  this  point,  is  kindly  contributed  by  W.  B.   Giles, 
F.I.O.,  who  has  had  large  practical  experience  in  the  subjects  treated. 

t  Pharm.  Zeitschrift  39,  558. 


HYDROFLUORIC    ACID.  113 

N/eo  solution  of  ferric  chloride,  which  the  author  prepared  by 
diluting  19  gm.  of  the  officinal  ferric  chloride  of  the  Prussian 
pharmacopoeia  to  a  litre. 

N/30  sodium  thiosulphate  solution. 

Zinc  iodide  solution,  made  by  mixing  10  gm.  of  iodine,  5  gm.  of 
zinc  powder,  and  25  c.c.  of  water  in  a  flask,  and  warming  till  the 
violent  action  is  over  and  the  solution  colourless,  then  diluting  to 
40  c.c.  and  filtering. 

METHOD  OF  PROCEDURE  :  The  liquid  containing  the  fluorides  in  solution  is 
mixed  with  a  known  excess  of  ferric  chloride  solution,  then  with  excess  of  zinc 
iodide,  and  allowed  to  remain  in  a  closed  vessel  at  35  —40°  C.  for  half  an  hour ; 
the  liberated  iodine  is  then  titrated  with  thiosulphate.  The  volume  of  the  latter 
used  is  deducted  from  that  of  the  ferric  chloride — the  difference  being  the  measure 
of  the  fluorine.  1  c.c.  thiosulphate  =0-0019  gm.  F. 

The  author  states  that  calcium  and  strontium  in  their  soluble 
salts  may  also  be  determined  by  the  same  method  by  acidifying 
their  solutions  with  hydrochloric  acid,  adding  equal  volumes,  first 
of  potassium  fluoride  and  then  of  ferric  chloride  solution  in  excess, 
adding  excess  of  zinc  iodide,  and  digesting  at  35—40°  C.  The 
liberated  iodine  is  ascertained  as  before.  1  c.c.  of  thiosulphate  = 
0-002  Ca. 

None  of  these  reactions  have  been  verfied  by  me,  but  the  method 
as  given  here  is  novel,  and  probably  capable  of  being  developed 
with  further  experience. 

A  very  interesting  paper  on  the  acidimetry  of  hydrofluoric  acid 
is  contributed  by  Haga  and  Osaka*,  being  the  results  of 
independent  experiments  made  by  them  in  the  laboratory  of 
the  Imperial  University,  Japan. 

The  authors  examined  the  behaviour  of  the  usual  indicators  in 
the  neutralization  of  hydrofluoric  acid.  That  its  alkali  salts  blue 
litmus  and  that  its  avidity  number  places  it  among  the  vegetable 
acids  rather  than  with  the  strong  mineral  acids,  appear  to  be  the 
only  two  facts  yet  recorded  bearing  upon  its  acidimetry. 

To  get  uniform  indications  it  was  found  necessary  to  have  not 
only  the  acid  pure,  but  the  titrating  solutions  also  ;  a  little  silica, 
alumina,  or  carbon  dioxide  affecting  the  titration  more  than  it 
would  in  the  case  of  the  ordinary  mineral  acids. 

Phenolphthalein  is  the  best  indicator,  and  leaves  nothing  to  be 
desired  when  potash  or  soda  is  used  for  the  titration.  Rosolic  acid 
is  almost  equal  to  it,  and  can  be  used  also  with  ammonia.  With 
both  indicators  the  change  of  colour  has  the  advantage  of  being 
very  evident  in  platinum  vessels.  Methyl  orange  is  useless.  Litmus, 
lacmoid  and  phenacetolin  are  all  capable  of  being  made  to  yield 
accurate  results  in  the  hands  of  an  experienced  operator. 

The  fact  that  accurate  results  can  only  be  obtained  with  very 
pure  acids  and  reagents  militates  against  the  value  of  any  acidi- 
metric  process,  and  therefore  the  indirect  method  by  Giles, 
described  above,  is  of  greater  technical  value. 

*J.  C.  S.  17, 18,  251.. 


114  OXALIC   ACID.       PHOSPHORIC   ACID. 

OXALIC    ACID. 

C2H2O42H2O  =  126-05. 

THE  free  acid  can  be  accurately  titrated  with  normal  alkali  and 
phenolphthalein. 

PROCEDURE  WHEN  IN  COMBINATION  WITH  ALKALIES  :  The  acid  can  be  pre- 
cipitated with  calcium  chloride  as  calcium  oxalate,  where  no  other  matters 
precipitable  by  calcium  are  present.  If  acetic  acid  is  present  in  slight  excess  it  is 
of  no  consequence,  as  it  prevents  the  precipitation  of  small  quantities  of  sulphates. 
The  precipitate  is  well  washed,  dried,  ignited,  and  the  carbonate  titrated  with 
normal  acid,  1  c.c  of  which  =0'063  gm.  0. 

Acid  oxalates  are  titrated  direct  for  the  amount  of  free  acid. 
The  reaction  continues  to  be  acid  until  alkali  is  added  in  such 
proportion  that  1  molecule  acid  =  2  atoms  alkali  metal. 

The  combined  acid  may  be  found  by  igniting  the  salt,  and 
titrating  the  residual  alkaline  carbonate  as  above. 

PHOSPHORIC    ACID. 

P2O5  =  142-08. 

Thomson  has  shown  in  his  researches  on  the  indicators  that 
phosphoric  acid,  either  in  the  free  state  or  in  combination  with  soda 
or  potash,  may  with  very  fair  accuracy  be  determined  by  the  help 
of  methyl  orange  or  phenolphthalein.  If,  for  instance,  normal 
potash  be  added  to  a  solution  of  phosphoric  acid  until  the  pink 
colour  of  methyl  orange  is  discharged,  KH2P04  is  formed  (112 
KHO  =  142  P2O5.)  If  now  phenolphthalein  is  added,  and  the 
addition  of  potash  continued  until  a  red  colour  appears,  K2HP04  is 
formed.  (Again  112  KHO  =  142  P2O5.)  On  adding  standard 
hydrochloric  or  sulphuric  acid  until  the  pink  colour  of  methyl 
orange  reappears,  the  titration  with  standard  potash  may  be 
repeated.  In  each  case  1  c.c.  normal  potash  =  -071  gm.  P2O5  or 
•098  gm.  H3P04,  each  titration  acting  as  a  check  upon  the  others. 

Or  the  titration  may  be  made  with  phenolphthalein  only.  But 
to  obtain  a  sharp  end-reaction  the  standard  alkali  must  be  free 
from  carbonate  and  the  solution  should  be  cold  and  concentrated 
(preferably  with  the  addition  of  sodium  chloride),  in  order  to 
prevent  dissociation  of  the  dibasic  salt  formed,  whereby  it  tends 
to  react  alkaline  to  *  the  indicator.  H3PO4  +  2KOH=K2HP04  + 
2H2O.  1  c.c.  normal  alkali- -0355  gm.  P2O5  or  -049  gm.  H3PO4. 

Many  attempts  have  been  made  to  utilize  these  reactions  for  the 
accurate  determination  of  P2O5  in  manures,  etc.,  but,  so  far  as  my 
experience  goes,  without  adequate  success. 

Titration  as  Ammonio-magnesium  Phosphate. — Stolba*  adopts 
an  alkalimetric  method,  which  depends  upon  the  fact  that  one 
molecule  of  the  double  salt  requires  two  molecules  of  a  mineral 
acid  for  saturation. 

*  Chem.  Cent.  1866,  727,  728. 


PHOSPHORIC   ACID.  115 

METHOD  OF  PROCEDURE  :  The  precipitation  is  made  with  magnesia  mixture, 
the  precipitate  well  washed  with  ammonia,  and  the  latter  completely  removed  by 
washing  with  alcohol  of  50  or  60  per  cent.  The  precipitate  is  then  dissolved  in 
a  measured  excess  of  N/IO  acid,  methyl  orange  added,  and  the  amount  of  acid 
required  found  by  titration  with  N/1O  alkali.  Care  must  be  taken  that  all  free 
ammonia  is  removed  from  the  filter  and  precipitate,  and  that  the  whole  of  the 
double  salt  is  decomposed  by  the  acid  before  titration,  which  may  always  be 
ensured  by  using  a  rather  large  excess  and  warming.  The  titration  is  carried 
out  in  the  cold. 

This  method  has  given  me  very  good  results  in  comparison  with 
the  gravimetric  method.  The  same  method  is  applicable  to  the 
determination  of  arsenic  acid,  and  also  of  magnesia. 

1  c.c.  of  N/10  acid  -0-00355  gm.  P2O5 
-0-00575  gm.  As2O3 
=0-002      gm.  MgO 

The  reaction  in  the  case  of  phosphoric  acid  may  be  expressed  as 
follows  :  — 

Mg  (NH4)  P04+2HC1  =  (NH4)  H2P04+MgCl2. 

Determination  of  Phosphoric  Acid  in  its  Pure  Solutions.—  R.  Segalle*  has 
investigated  various  methods  for  the  above  purpose  with  the  following  result  :  — 

By  far  the  most  accurate  results  are  obtained  byGliicksmann's  method.  In 
this,  the  phosphoric  acid  is  precipitated  by  an  excess  of  magnesia  mixture  of 
known  strength  in  free  ammonia,  the  precipitate  filtered  off,  and  the  free  ammonia 
left  in  solution  is  titrated  by  standard  acid.  From  the  equation  — 


•H3P04  +MgS04  +  3NH3  =  MgNH4P04  +(NH4)2S04 
it  will  be  seen  that  H3P04=3NH3. 

The  following  modification  is  recommended  as  being  more  convenient  and 
simple.  To  the  phosphoric  acid  solution,  contained  in  a  graduated  flask,  an 
excess  of  standard  ammonia  (preferably  normal)  is  added,  followed  by  an  excess 
of  a  saturated  neutral  solution  of  magnesium  sulphate.  The  liquid  is  then  diluted 
to  the  mark,  well  shaken,  and  filtered,  and  the  residual  ammonia  titrated  in  an 
aliquot  part  of  the  filtrate.  "  Methyl  red  "  (see  p.  41)  is  especially  suitable  as 
indicator  in  this  process,  as  in  ammonia  titrations  of  all  kinds. 

On  account  of  its  simplicity,  the  modified  method  is  well  adapted  for  ascertaining 
the  strength  of  the  solutions  of  phosphoric  acid  employed  in  pharmacy 

J..  M.  Wilkief  has  devised  a  method  for  the  direct  titration  of 
free  tri-basic  phosphoric  acid  in  the  same  manner  as  other  acids. 
To  the  phosphoric  acid  solution  is  added  silver  nitrate  and  sodium 
acetate,  and  the  liberated  acetic  acid  is  titrated  with  standard 
baryta  in  the  presence  of  phenolphthalein. 

From  the  equation, 

H3P04  +  3  AgNO3  +  3  CH3COONa  =  Ag3P04  +  3NaN03  +  3CH3COOH 
it  is  seen  that  H3PO4  =  3CH3COOH. 

METHOD  OF  PROCEDURE  :  A  suitable  amount  of  the  free  acid  is  diluted  to  about 
50  c.c.,  excess  of  approximately  decinormal  silver  nitrate  is  then  added  and 
finally  a  large  excess  of  3  %  sodium  acetate.     After  adding  a  few  drops  of  phenol- 
phthalein, baryta  is  run  in  until  a  permanent  pink  coloration  is  developed. 
1  c.c.  N/io  Ba  (OH)2  =0-00327  gr.  H3P04 
=0-00237  gr.  P205. 

*  Z.  a.  C.  34,  33-39.  f./-  S.  C.  I.  1909,  68. 


116  PHOSPHORIC   ACID. 

Unless  an  excessive  amount  of  silver  is  added  the  end  point  is  sharp  and  easy  of 
recognition,  since  the  precipitated  silver  phosphate  readily  settles  leaving  the 
supernatant  liquid  nearly  clear.  The  method  is  also  available  for  the  mono- 
and  di-alkali  phosphates  (ammonium  if  present  must  be  removed  by  evaporation 
or  boiling  with  standard  alkali),  but  in  the  presence  of  alkali  carbonate  it  is 
necessary  also  to  determine  the  amount  of  silver  actually  precipitated  as  silver 
phosphate.*  (See  under  Phosphoric  Acid  and  Phosphates). 

For  example,  if  a  weight  w  of  Na2HP04  requires  b.  c.c.  N/io  Ba(OH)2  and  a  c.c. 
N/io  AgN03,  we  have 


or  in  terms  of  Po05  and  total  Na20. 

%PA=^ 

o/0Na2oJ<^ 

Precautions. — All  water  used  must  be  free  from  C02.  In  the  case  of  Na2HPO4 
it  is  necessary  to  free  from  C02  by  boiling  with  a.t  least  sufficient  standard  acid 
to  convert  to  NaH2P04,  due  allowance  being  made  in  the  baryta  titration.  The 
silver  nitrate  should  be  tested  for  neutrality  by  treating  with  excess  of  sodium 
chloride  and  then  adding  phenolphthalein  and  one  drop  of  N/io  NaOH.  The 
sodium  acetate  likewise  must  be  neutral  and  free  from  carbon  dioxide.  Alkali 
chloride  if  present  has  no  effect  in  the  baryta-titration  but  must  be  allowed  for 
in  the  silver-titration. 

Other  methods  are  described  under  Phosphoric  Acid  and 
Phosphates. 

FUMING  SULPHURIC  ACID. 
Nordhausen  Oil  of  Vitriol. 

This  consists  of  a  mixture  of  SO3  and  H2SO4.  When  rich  in  SO3 
it  exists  in  a  solid  form,  and  being  very  hygroscopic  cannot  be 
weighed  in  the  ordinary  manner.  Accordingly,  it  is  weighed  either 
in  glass  bulbs  or  in  a  Lunge  and  Key's  glass-tap  pipette.  The 
bulbs  used  for  this  purpose  are  of  about  2  c.m.  diameter,  with  two 
capillary  tubes  fused  in.  The  acid,  if  solid,  is  first  melted  and 
when  quite  homogeneous  is  sucked  up  into  the  bulb  by  means  of  an 
indiarubber  tube  attached  to  one  capillary  tube,  the  other  being 
dipped  in  the  acid.  About  3-5  gm.  are  taken  and  the  bulb  should 
be  about  half-filled.  The  wet  tube  is  carefully  cleaned  outside,  and 
one  of  the  capillary  ends  is  sealed.  The  bulb  is  then  weighed,  best 
by  supporting  it  on  a  platinum  crucible  with  two  hicks,  on  which 
the  ends  of  the  bulb  rest.  In  case  of  fracture,  the  acid  runs  into 
the  crucible  instead  of  on  the  balance.  When  weighed,  put  the 
Bulb,  open  end  downward,  into  a  small  conical  flask,  into  the  neck 
of  which  it  fits  exactly,  and  containing  sufficient  water  to  cov  er  well 
the  lower  part  of  the  tube.  Break  off  the  other  point,  allow  the 
acid  to  run  out,  blow  a  few  drops  of  water  into  the  upper  capillary. 

*J.  S.C.I.  1910,  794. 


SULPHURIC   ANHYDRIDE.  117 

and  finally  rinse  the  whole  bulb  tube  by  repeated  aspiration  of 
water.  Dilute  the  liquid  to  500  c.c.  and  titrate  50  c.c.  of  it  with 
N/5  sodium  carbonate  solution,  using  methyl  orange  as  indicator. 
From  the  acidity  thus  found  must  be  deducted  that  due  to  S02, 
which  is  determined  by  titrating  another  portion  with  N/10  Iodine. 
For  each  c.c.  of  the  latter  0'05  c.c.  normal  sodium  carbonate  is 
subtracted,  since  with  methyl  orange  the  colour  changes  when  S02 
has  passed  into  NaHS03. 

Let  n=no.  of  c.c.  of  sodium  carbonate  used, 

m=  ,,          ,,         N/10  iodine  solution  required  for  the  same 

quantity  of  acid, 
the  acidity  due  to  H2S04  +  S03  is 

(n  -0-05  m)x  0-040035  in  terms  of  S03. 

To  the  S03  thus  found  add  the  SO2  (calculated =0*0032035  m) 
and  assume  the  residue,  in  the  absence  of  solid  impurities,  to  be 
water.  By  multiplying  this  water  by  4-443  we  obtain  the  quantity 
of  SO3  combined  with  it  to  form  H2S04,  and  by  deducting  this 
from  the  total  acidity  that  of  the  free  S03. 

TARTARIC    ACID. 

C4H606  =  150-05  (Dibasic.) 

THE  free  acid  may  be  readily  titrated  with  normal  alkali  and 
phenolphthalein. 

1  c.c.  normal  alkali  =0'075  gm.  tartaric  acid. 

The  amount  of  tartaric  acid  existing  in  tartaric  acid  liquors  is 
best  determined  by  precipitation  as  potassium  bitartrate  ;  the  same 
is  also  the  case  with  crude  argols,  lees,  etc.  Manufacturers  are 
indebted  to  Warington  and  Grosjean  for  most  exhaustive 
papers  on  this  subject,  to  which  reference  should  be  made  by  all 
who  desire  to  study  the  nature  and  analysis  of  all  commercial 
compounds  of  citric  and  tartaric  acids.* 

Without  entering  into  the  copious  details  and  explanations  given 
by  these  authorities,  the  methods  may  be  summarized  as  follows  : — 

Commercial  Tartrates. 

In  the  case  of  good  clean  tartars,  even  though  they  may  contain  sulphates  and 
carbonates,  accurate  results  may  be  obtained  by  indirect  methods. 

(a)  The  very  finely  powdered  sample  is  first  titrated  with  normal  alkali,  and 
thus  the  amount  of  tartaric  acid  existing  as  bitartrate  is  found ;  another  portion 
of  the  sample  is  then  calcined  at  a  moderate  heat,  and  the  ash  titrated.  By 
deducting  from  the  volume  of  acid  so  used  the  volume  used  for  bitartrate,  the 
amount  of  base  corresponding  to  neutral  tartrates  is  obtained. 

(6)  The  whole  of  the  tartaric  acid  is  exactly  neutralized  with  caustic  soda, 
evaporated  to  dryness,  calcined,  and  the  ash  titrated  with  normal  acid ;  the  total 

*  Warington,  J.C.S.  1875,925-994;  Grosj  ean,  J.C.S.  1879,341-356. 


118  TARTRATES. 

tartaric  acid  is  then  calculated  from  the  volume  of  standard  acid  used  ;  any  other 
organic  acid  present  will  naturally  be  included  in  this  amount.  In  the  case  of 
fairly  pure  tartars,  etc.,  this  probable  error  may  be  disregarded 

METHOD  OF  PROCEDURE  :  5  gm.  of  the  finely  powdered  tartar  are  heated 
with  a  little  water  to  dissolve  any  carbonates  that  may  be  present.  If  it  is  wished 
to  guard  against  crystalline  carbonates,  5  c.c.  of  standard  HC1  are  added  in  the 
first  instance,  and  the  heating  is  conducted  in  a  covered  beaker.  Standard  alkali 
is  next  added  to  the  extent  of  about  three-fourths  of  the  amount  required  by 
a  good  tartar  of  the  kind  examined,  plus  that  equivalent  to  the  acid  used,  and  the 
whole  is  brought  to  boiling :  when  nearly  cold,  the  titration  is  finished.  From 
the  amount  of  alkali  consumed,  minus  that  required  by  the  HC1,  the  tartaric  acid 
present  as  acid  tartrate  is  calculated. 

2  gm.  of  the  powdered  tartar  are  next  weighed  into  a  platinum  crucible  with 
a  well-fitting  lid  ;  the  crucible  is  placed  over  an  argand  burner ;  heat  is  applied, 
very  gently  at  first,  to  dry  the  tartar,  and  then  more  strongly  till  inflammable 
gas  ceases  to  be  evolved.  The  heat  should  not  rise  above  very  low  redness.  The 
black  ash  is  next  removed  with  water  to  a  beaker.  If  the  tartar  is  known  to  be 
a  good  one,  20  c.c.  of  standard  H2S04  are  now  run  from  a  pipette  into  the  beaker, 
a  portion  of  the  acid  being  used  to  rinse  the  crucible.  The  contents  of  the  beaker 
are  now  brought  to  boiling,  filtered,  and  the  free  acid  determined  with  standard 
alkali.  As  the  charcoal  on  the  filter  under  some  circumstances  retains  a  little 
acid,  even  when  well  washed,  it  is  advisable  when  the  titration  is  completed  to 
transfer  the  filter  and  its  contents  to  the  neutralized  fluid,  and  add  a  further 
amount  of  alkali  if  necessary.  From  the  neutralizing  power  of  a  gram  of  burnt 
tartar  is  subtracted  the  acidity  of  a  gram  of  unburnt  tartar,  both  expressed  in  c.c. 
of  standard  alkali,  the  difference  is  the  neutralizing  power  of  the  bases  existing 
as  neutral  tartrates,  and  is  then  calculated  into  tartaric  acid  on  this 
assumption.* 

If  the  tartar  is  of  low  quality,  5  c.c.  of  solution  of  hydrogen  peroxide  (1  volume 
=  10  volumes  02)  are  added  to  the  black  ash  and  water,  and  immediately  after- 
wards the  standard  acid  ;  the  rest  of  the  analysis  proceeds  as  already  described  ; 
the  small  acidity  usually  belonging  to  the  peroxide  solution  must,  however,  be 
known  and  allowed  for  in  the  calculation.  By  the  use  of  hydrogen  peroxide  the 
sulphides  formed  during  ignition  are  reconverted  into  sulphates,  and  the  error  of 
excess  which  their  presence  would  occasion  is  avoided. 

The  above  method  does  not  give  the  separate  amounts  of  acid 
and  neutral  tartrates  in  the  presence  of  carbonates,  but  it  gives 
the  correct  amount  of  tartaric  acid  ;  it  is  also  correct  in  cases  where 
free  tartaric  acid  exists,  so  long  as  the  final  results  show  that  some 
acid  existed  as  neutral  salt.  Whenever  this  method  shows  that 
the  acidity  of  the  original  substance  is  greater  than  the  neutralizing 
power  of  the  ash,  it  will  be  necessary  to  use  the  method  b,  which  is 
the  only  one  capable  of  giving  good  results  when  the  sample  contains 
much  free  tartaric  acid. 

Methods   adopted   by  the   Seventh  International   Congress   of 
Applied  Chemistry,  London,  1909.1 

The  crude  tartrate  should  be  carefully  sampled,  ground,  and 
passed  through  a  sieve  having  a  mesh  of  0*5  mm. 

*  It  is  obvious  that  the  neutralizing  power  of  the  ash  of  an  acid  tartrate  is  exactly 
the  same  as  the  acidity  of  the  same  tartrate  before  burning.  In  making  the  calcula- 
tions, it  must  be  remembered  that  the  value  of  the  alkali  in  tartaric  acid  is  twice  as 
great  in  the  calculation  made  from  the  acidity  of  the  unburnt  tartar  as  in  the 
calculation  of  the  acid  existing  as  neutral  tartrates. 

t  P.  Carles,^wn.  Chim.  analyt.,  1910,15,  231. 


TARTRATES.  119 

DETERMINATION  OF  THE  BITARTRATE. — A  weighed  portion  of  2 '35  gm.  of  the 
sample  is  boiled  for  5  minutes  with  400  c.c.  of  water  in  a  500  c.c.  flask,  then  cooled, 
diluted  to  the  mark,  mixed  and  filtered  ;  250  c.c.  of  the  filtrate  are  heated  to 
boiling,  and  titrated  with  w/4  potassium  hydroxide  solution,  using  litmus 
paper  as  indicator.  The  potassium  hydroxide  solution  is  standardized  under 
the  same  conditions  with  pure  potassium  bitartrate. 

DETERMINATION  OF  TOTAL  TARTARIC  ACID. — In  cases  of  tartrates  containing 
upwards  of  45  per  cent,  of  total  tartaric  acid,  6  gm.  of  the  sample  are  taken 
for  the  determination  ;  where  the  tartaric  acid  falls  below  45  per  cent.,  12  gm. 
of  the  sample  are  used,  this  quantity  being  also  taken  for  the  analysis  of  calcium 
tartrates.  The  weighed  portion  of  the  sample  is  thoroughly  mixed  with  18  c.c. 
of  hydrochloric  acid  of  sp.  gr.  1*1  ;  after  the  lapse  of  15  minutes,  the  mixture 
is  rinsed  into  a  200  c.c.  flask  with  water  and  diluted  to  the  mark.  The  solution 
is  then  poured  through  a  filter,  100  c.c.  of  the  filtrate  are  heated  to  boiling  and 
10  c.c.  of  66  per  cent,  potassium  carbonate  solution  are  added  slowly.  The 
mixture  is  heated  for  about  20  minutes  and  the  liquid  together  with  the  pre- 
cipitate is  transferred  to  a  200  c.c.  flask,  cooled,  and  diluted  to  volume.  After 
filtering,  100  c.c.  of  the  filtrate  are  evaporated  to  a  volume  of  15  c.c.,  3 '5  c.c. 
of  glacial  acetic  acid  are  added,  drop  by  drop,  and  the  mixture  is  stirred  for 
5  minutes  ;  at  the  end  of  10  minutes,  100  c.c.  of  95  per  cent,  alcohol  are  added, 
the  stirring  is  continued  for  5  minutes,  and,  after  the  lapse  of  a  further  10 
minutes,  the  liquid  portion  is  poured  through  a  filter,  the  precipitate  is  washed 
three  or  four  times  by  decantation  with  a  little  alcohol,  then  brought  on  to  the 
filter  and  washed  with  alcohol  until  the  washings  are  free  from  acidity.  The 
filter  together  with  the  precipitate  is  then  placed  in  a  flask,  boiled  with  300  c.c. 
of  water  for  1  minute,  and  the  hot  solution  titrated  with  N/4  potassium  hydroxide 
solution,  using  litmus  paper  as  indicator.  The  following  corrections  are  made 
for  the  volume  of  the  insoluble  matter: — For  tartrates  containing  20  per  cent,  of 
total  tartaric  acid,  0'8  is  deducted  from  the  total  percentage  of  acid  found  ; 
for  30  per  cent,  tartrates,  0'70,  and  for  40  per  cent,  tartrates,  0'6.  In  the  case 
of  tartrates  containing  50,  60,  and  80  per  cent,  of  tartaric  acid,  0'25,  0'15,  and 
O'l  per  cent,  is  deducted  respectively. 


Cream  of  Tartar. 

When  prepared  by  boiling  crude  tartar  or  "  argol  "  with  water, 
filtering,  and  crystallizing  the  salt  from  the  clear  liquid,  cream  of 
tartar  always  contains  more  or  less  calcium  tartrate,  which,  though 
nearly  insoluble  in  cold  water,  dissolves  with  moderate  facility  in 
a  hot  solution  of  acid  tartrate  of  potassium.  From  experiments 
made,  as  well  as  from  numerous  analyses,  Allen*  concluded  that 
the  commercial  article  should  not  contain  more  than  9  or  10  per  cent, 
of  calcium  tartrate.  After  a  consideration  of  possible  impurities, 
he  recommended  the  following  process  : — 

1.  Dissolve  1-881  gm.  of  the  moisture-free  sample  in  hot  water 
and  titrate  with  N/10  caustic  alkali  and  phenolphthalein.      In  the 
absence  of  acid  potassium  sulphate  (and  of  free  tartaric  acid)  each 
c.c.   of  alkali  required  represents   1   per  cent,   of  acid  potassium 
tartrate  in  the  sample. 

2.  Ignite  1-881  gm.  of  the  moisture-free  sample  at  a  dull  red 
heat  for  10  minutes,  without  attempting  to  burn  off  all  the  carbon. 
Boil  the  product  with  water,  filter,  and  wash  the  insoluble  carbon- 
aceous residue. 

*  On  the  composition  and  analysis  of  commercial  cream  of  tartar,   The  Analyst, 
1896,21,  174  and  209. 


120  CREAM    OF   TARTAR. 

(a)  Titrate  the  filtrate  with  N/10  HC1  and  methyl- orange.  In 
a  pure  sample  the  volume  of  acid  required  will  exactly  equal  that  of 
the  alkali  consumed  in  process  1.  The  presence  of  calcium  tartrate 
in  the  sample  does  not  affect  the  results.  Each  c.c.  of  deficiency  of 
acid  represents  0*36  per  cent  of  calcium  sulphate  (CaS04),  or  0*72 
per  cent',  of  KHS04.  Any  excess  of  acid  required  points  to  the 
presence  of  neutral  potassium  tartrate,  each  c.c.  of  difference 
representing  O60  per  cent  of  that  salt. 

(6)  Ignite  the  carbonaceous  residue,  dissolve  in  20  c.c.  of  N/10 
acid,  filter  if  necessary,  wash,  and  titrate  the  filtrate  with  N/10  alkali 
and  methyl-orange.  Each  c.c.  required  corresponds  to  0*50  per 
cent,  of  calcium  tartrate  or  0'36  per  cent,  of  CaSO4. 


OXIDIZING   AND    REDUCING   AGENTS.  121 

PART    III. 
ANALYSIS    BY    OXIDATION    OR    REDUCTION. 

THE  number  of  methods  of  analysis  based  on  oxidation  or 
reduction  is  very  great,  and  not  a  few  of  them  possess  extreme 
accuracy,  such  accuracy,  in  fact,  as  it  is  not  possible  to  attain  by 
any  gravimetric  method.  The  completion  of  the  various  processes 
is  generally  shown  by  a  distinct  change  of  colour,  e.g.,  the  beautiful 
rose-red  tint  of  permanganate  and  the  blue  colour  of  iodide  of 
starch  :  and  as  the  smallest  quantity  of  these  substances  suffices 
to  colour  distinctly  large  volumes  of  liquid,  the  slightest  excess  of 
the  oxidizing  agent  is  sufficient  to  produce  a  distinct  effect. 

The  principle  involved  in  the  process  is  extremely  simple. 
Substances  having  a  strong  affinity  for  oxygen  are  brought  into 
solution,  and  titrated  with  an  oxidizing  solution  of  known  strength. 
For  example,  ferrous  salts  rapidly  absorb  oxygen,  and  when  a 
solution  of  permanganate  is  gradually  added  to  a  solution  of  a  ferrous 
salt  the  latter  instantly  discharges  the  colour  of  the  drops  at 
first  run  in,  but  after  the  whole  of  the  ferrous  salt  has  been  con- 
verted into  the  ferric  state  the  liquid  becomes  rose-coloured  on  the 
further  addition  of  a  few  drops  of  permanganate,  the  appearance 
of  this  colour  indicating  the  completion  of  the  change.  The  re- 
action is,  in  its  simplest  form,  as  follows  : — 

lOFeO  +K2Mn208  =  5Fe203  +  2MnO  +K2O. 

The  titration  is  carried  out  in  the  presence  of  dilute  sulphuric 
acid,  and  sulphates  are  formed.  Similarly,  oxalic  acid,  in  the 
presence  of  sulphuric  acid,  is  readily  oxidized  by  permanganate, 
carbon  dioxide  being  formed.  The  reaction  is  : — 

5H2C204 + K2Mn208 + 3H2S04  =  10CO2 + 2MnSO4 +K2SO4 + 8H2O. 

Here,  again,  the  appearance  of  a  rose  tint  in  the  solution  shows 
that  the  oxidation  is  complete. 

The  strength  of  many  oxidizing  agents  is  determined  by  adding 
a  known  quantity  of  a  reducing  agent  in  excess,  then  ascertaining 
the  amount  of  this  excess  by  residual  titration  with  a  standard 
oxidizing  solution.  The  strength  of  the  reducing  solution  being 
known,  the  quantity  required  is  a  measure  of  the  substance  which 
has  been  reduced  by  it. 

The  oxidizing  agents  frequently  used  are  potassium  permanganate, 
iodine,  potassium  dichromate,  and  potassium  ferricyanide. 

The  reducing  agents  employed  are  sulphurous  acid,  sodium 
hyposulphite  (Schiitzenberger's),  sodium  thiosulphate,  oxalic 
acid,  ferrous  salts,  arsenious  oxide,  stannous  chloride,  potassium 
ferrocyanide,  zinc  and  magnesium.  Titanium  chloride,  a  very 
powerful  reducing  agent,  will  be  referred  to  later. 


122  ANALYSIS    BY    OXIDATION    OR   REDUCTION. 

*    i 

The  most  commonly  used  combinations  of  the  above  are  :— - 

1.  Permanganate  and  ferrous  salts  :  permanganate  and  oxalic 
acid.     Both  used  in  sulphuric   acid  solution,   the  appearance  of 
a  rose  colour  being  the  indicator. 

2.  Potassium   dichromate    and   ferrous    salts,    with   potassium 
ferricyanide  as  indicator. 

3.  Iodine  and  sodium  thiosulphate  ;  iodine  and  sodium  arsenite, 
with  starch  as  indicator  in  each  case. 


PREPARATION    OF    STANDARD    SOLUTIONS. 
PERMANGANIC  ACID  AND  FERROUS  OXIDE. 

1.    Potassium  Permanganate. 

K2Mn208  =  316-06.     Decinormal  Solution  =  3' -161  gm.  per  litre. 
1  c.c.  =0-0008  gram  Oxygen. 

THE  solution  of  this  salt  is  best  prepared  for  analysis  by  dissolving 
the  pure  crystals  in  freshly  distilled  water,  and  should  be  of  such 
a  strength  that  17 '9  c.c.  will  oxidize  1  decigram  of  iron.  The 
solution  is  then  decinormal.  If  the  salt  can  be  had  perfectly  pure 
and  dry,  3*161  gm.  dissolved  in  a  litre  of  water  at  15°  C.  will  give 
an  exactly  decinormal  solution  ;  but,  nevertheless,  it  is  always  well 
to  verify  it  as  described  below.  Fairly  pure  permanganate,  in 
large  crystals,  may  now  be  obtained  in  commerce,  and  if  this  salt 
is  recrystallized  twice  from  hot  distilled  water  and  dried  thoroughly 
at  100°  C.,  it- will  be  found  practically  pure.  If  kept  in' the  light  in 
ordinary  bottles  it  will  retain  its  strength  for  several  months,  if  in 
bottles  covered  with  black  paper  much  longer ;  nevertheless,  it 
should  from  time  to  time  be  verified  by  titration  in  one  of  the 
following  ways  : — 

2.     Titration  of  Permanganate. 

(a)  With  Metallic  Iron. — There  is  no  difficulty  in  obtaining  iron 
of  99'8  per  cent,  purity.  It  is  sold  in  the  form  of  thin  wire,  each 
piece  of  which  should  be  drawn  between  two  pieces  of  fine  emery 
cloth  and  then  wiped  with  a  dry  cloth  before  use ;  this  treatment 
removes  rust. 

METHOD  OF  PROCEDURE  :  Fit  a  tight  cork  or  rubber  stopper,  with  bent 
delivery  tube,  into  a  flask  holding  about  300  c.c.,  and  clamp  it  in  a  retort  stand 
in  an  inclined  position,  the  tube  being  so  bent  as  to  dip  into  a  small  beaker 
containing  pure  water.  Fill  the  flask  one-third  with  pure  dilute  sulphuric  acid, 
and  add  a  few  grains  of  sodium  carbonate  in  crystals  ;  the  C02  so  produced  will 
drive  out  the  air.  While  this  is  being  done  weigh  about  O'l  gram  of  the  wire  ;  put 
it  quickly  into  the  flask  when  the  soda  is  dissolved,  and  apply  a  gentle  heat  till  the 
iron  is  completely  in  solution  ;  a  few  black  specks  of  carbon  are  of  no  consequence. 
The  flask  is  then  rapidly  cooled  under  a  stream  of  cold  water,  diluted  if  necessary 
with  some  recently  boiled  and  cooled  water,  and  the  permanganate  run  in 
cautiously  from  a  tap  burette,  with  constant  shaking,  until  a  faint  rose- colour 


STANDARDIZATION    OF   PERMANGANATK.  123 

+  •,  permanent.  Instead  of  this  arrangement  for  dissolving  the  iron  the  apparatus 
shown  in  fig.  44,  may  be  used. 

The  decomposition  which  ensues  from  titrating  ferrous  oxide  by 
permanganic  acid  may  be  represented  as  follows  : — 

lOFeO  and  Mn207  =  2MnO  and  5Fe2O3. 

The  weight  of  wire  taken,  multiplied  by  0'998,  will  give  the 
actual  weight  of  pure  iron  upon  which  to  calculate  the  strength  of 
the  permanganate. 

(b)  With  Ferrous-ammonium  Sulphate. — In  order  to  ascertain 
the  strength  of  the  permanganate,  it  may  be  titrated  with  a  weighed 
quantity  of  this  substance  instead  of  metallic  iron. 

This  salt  is  a  convenient  one  for  titrating  the  permanganate,  as  it  saves  the 

«'  time  and  trouble  of  dissolving  the  iron,  and  when  perfectly  pure  it  can  be 
depended  on  without  risk.  To  prepare  it,  139  parts  of  the  purest  crystals  of 
.  ferrous,  sulphate,  and  66  parts  of  pure  crystallized  ammonium  sulphate  are 

,;  separately  dissolved  in  the  least  possible  quantity  of  distilled  water  at  about 
40°  C.  (if  the  solutions  are  not  perfectly  clear  they  must  be  filtered) ;  mix  them 
at  the  same  temperature  in  a  porcelain  dish,  adding  a  few  drops  of  pure  sulphuric 
acid,  and  stir  till  cold.  During  the  stirring  the  double  salt  will  fall  in  a  finely 
granulated  form.  Set  aside  for  a  few  hours,  then  pour  off  the  supernatant  liquid, 
and  empty  the  salt  into  a  clean  funnel  with  a  little  cotton  wool  stuffed  into  the 
neck,  so  that  the  mother-liquor  may  drain  away ;  the  salt  may  then  be  quickly 
and  repeatedly  pressed  between  fresh  sheets  of  clean  filtering  paper.  Lastly, 
place  in  a  current  of  air  to  dry  thoroughly,  so  that  the  small  grains  adhere  no 

,  longer  to  each  other,  or  to  the  paper  in  which  they  are  contained,  then  preserve 
in  a  stoppered  bottle  for  use.  This  salt  is  useful  for  many  purposes,  and  should 
be  made  by  the  analyst  himself,  as  it  is  difficult  to  buy  the  pure  ferrous  salt. 
Only  a  few  ounces  should  be  made  at  a  time,  according  to  the  directions  above, 
as  if  large  quantities  are  made  it  is  difficult  to  dry  the  granular  salt  in  a  purely 
ferrous  state. 

The  formula  of  the  salt  is— Fe  (NH4)2  (S04)a,  6H2O  =  392'17. 
Consequently  it  contains  almost  exactly  one-seventh  of  its  weight  of 
iron  ;  0*7022  gm.  represents  O'l  gm.  Fe,  and  this  is  a  convenient 
quantity  to  weigh  for  the  purpose  of  titrating  the  permanganate. 

,  , 

1  METHOD  OF  PROCEDURE  :  0-7022  gm.  being  brought  into  dilute  cold  solution 
in  a  flask  or  beaker,  and  20  c.c.  of  dilute  milphuric  acid  (I  to  5)  added  (the 
titration  of  permanganate,  or  any  other  substance  by  it,  should  always  take  place 
in  the  presence  of  free  acid,  and  preferably  sulphuric),  ,the  permanganate  is 
delivered  from  a  burette  with  glass  tap,  as  before  described,  until  a  point  occurs 
when  the  rose  colour  no  longer  disappears  on  shaking. , 

(c)  With  Oxalic  Acid.— This  is  a  very  quick  method  of  titrating  permanganate, 
if  the  exact  value  of  the  solution  of  pure  oxalic  acid  is  known.     10  c.c.  of  normal 
solution  are  brought  into  a  flask  with  dilute  sulphuric  acid,  as  in  the  case  of  the 
iron  salt,  and  considerably  diluted  with  water,  then  warmed  to  about  60°  C., 
and  the  permanganate  added  from  the  burette.     The  colour  disappears  slowly 
at  first,  but  afterwards  more  rapidly,  becoming  first  brown,  then  yellow,  and  so  on 

:  to  colourless.  More  care  must  be  exercised  in  this  case  than  in  the  titration 
with  iron,  as  the  action  is  not  momentary.  100  c.c.  should  be  required  to  be 
strictly  decinormal.  The  chemical  change  which  occurs  is  explained  on  p.  121. 

(d)  With  Sodium  Oxalate. — The  method  of  titration  is  the  same  as  with 
oxalic  acid,  but  is  preferable  since  the  salt  may  easily  be  obtained  pure,  and  being 


124  TITRATIONS   WITH   PERMANGANATE. 

anhydrous  may  be  weighed  with  great  exactness.  A  decinorinal  solution  may  be 
made  by  dissolving  6'7  gm.  per  litre.  This  is  one  of  the  best  methods  of  ascertaining 
the  exact  strength  of  permanganate. 

3.    Precautions  in  Titrating  with  Permanganate. 

It  must  be  borne  in  mind  that  free  acid  is  always  necessary  in 
titrating  a  substance  with  permanganate,  in  order  to  keep  the 
resulting  manganous  oxide  in  solution.  Sulphuric  acid,  in  a  dilute 
form,  has  no  prejudicial  effect  on  the  pure  permanganate,  even  at 
a  high  temperature.  With  hydrochloric  acid  the  solution  to  be 
titrated  must  be  very  dilute  and  at  a  low  temperature,  otherwise 
chlorine  will  be  liberated  and  the  analysis  spoiled.  This  acid  acts 
as  a  reducing  agent  on  permanganate  in  concentrated  solution, 
thus — 

Mn207  +  14HC1  =  7HaO  +  5C12  +  2MnCl2. 

The  irregularities  due  to  this  reaction  may  be  entirely  obviated 
by  the  addition  of  a  few  grams  of  manganous  or  ammonium  sulphate 
before  the  titration,  which  must  be  performed  slowly. 

In  spite  of  the  manifest  advantages  of  standard  dichromate 
where  there  are  reasons  for  titrating  iron  in  hydrochloric  acid 
solution,  many  attempts  have  been  made  to  work  out  a  method 
with  permanganate  which  shall  be  accurate  and  reliable  under 
practical  conditions.*  For  details  of  some  of  these  methods  see 
under  Iron. 

Organic  matter  of  any  kind  decomposes  the  permanganate,  and 
the  solution  therefore  cannot  be  filtered  through  paper,  nor  can  it 
be  used  in  a  clip  burette,  because  it  is  decomposed  by  the  india- 
rubber  tube.  It  may,  however,  be  filtered  through  gun  cotton  or 
glass  wool. 

TITRATION    OF    FERRIC    SALTS    BY    PERMANGANATE. 

ALL  ferric  compounds  requiring  to  be  determined  by  permanganate 
must,  of  course,  be  reduced  to  the  ferrous  state.  This  is  best 
accomplished  by  metallic  zinc  or  magnesium  in  sulphuric  acid 
solution.  Hydrochloric  acid  may  also  be  used  with  the  precautions 
mentioned. 

The  reduction  occurs  on  simply  adding  to*the  warm  diluted 
solution  small  pieces  of  zinc  (free  from  iron,  or  at  least  with  a  known 
quantity  present)  or  coarsely  powdered  magnesium  until  colourless  ; 
or  until  a  drop  of  the  solution  brought  in  contact  with  a  drop  of 
potassium  thiocyanate  produces  no  red  colour.  All  the  zinc  or 
magnesium  must  be  dissolved  previous  to  the  titration. 

The  reduction  may  be  hastened  considerably  as  shown  under 
Iron  2. 

When  the  reduction  is  complete,  no  time  should  be  lost  in 
titrating  the  solution. 

*Fresenius,  Zeit.  Anal.  Chem.,  1862,  361;  Zimmermann,^nna,7ew,  1882,305; 
Reinhardt,  Chem.  Zeit.,  1889,323;  Brandt,  Chem  .Zeit.,  1908,  8 1 2,  etc. ;  F  r i e  nd, 
Chem.Soo.  Trans.,  1909,95, 1228;  Jones  and  J  e  ff  e  r  y,  Analyst,  1909,34,  306. 


TITRATIONS 


WITH   PERMANGANATE.  125 


CALCULATION  OF  THE  RESULTS  OF  ANALYSES  MADE  WITH 
PERMANGANATE    SOLUTION. 

THE  calculation  of  the  results  of  analyses  with  permanganate, 
if  the  solution  is  not  strictly  decinormal,  may  be  made  by  ascertain- 
ing its  coefficient,  reducing  the  number  of  c.c.  used  for  it  to 
decinormal  strength,  and  multiplying  the  number  of  c.c.  thus 
found  by  Tonoo  °f  ^ne  equivalent  weight  of  the  substance  sought ; 
for  instance — 

Suppose  that  15  c.c.  of  permanganate  solution  have  been  found 
to  equal  0*1  gm.  iron  ;  it  is  required  to  reduce  the  15  c.c.  to 
decinormal  strength,  which  would  require  1000  c.c.  of  permanganate 
to  every  5-585  gm.  iron,  therefore  5'585:  1000  :  :  0*1  :  #  =  17'9c.c.; 
17-9x0-005585=0-09997  gm.  iron,  which  is  as  near  to  O'l  gm. 
as  can  be  required.  Or  the  coefficient  necessary  to  reduce  the 
number  of  c.c.  used  may  be  found  as  follows  : — O'l  :  15  :  : 

100 
5-585  :  x  =  83*8  c.c.,  therefore  ^07^  =  1*194.     Consequently   1*194  is 

oo  *o 

the  coefficient  by  which  to  reduce  the  number  of  c.c.  of  that 
special  permanganate  to  decinormal  strength,  whence  the  weight 
of  substance  sought  may  be  found  in  the  usual  way. 

Another  plan  is  to  find  the  quantity  of  iron  or  oxalic  acid  repre- 
sented by  the  permanganate  used  in  any  given  analysis,  and  this 
being  done  the  following  simple  equation  gives  the  required  result  : — 

Fe  (55-85)        eq.  weight  of         the  weight          the  weight  of 

Por       :      the  substance  :  :     of  Fe  or     :  substance 

O  (63)  sought  <T  found  sought 

In  other  words,  if  the  equivalent  weight  of  the  substance  analyzed 
be  divided  by  55-85  or  63  (the  respective  equivalent  weights  of  iron  or 
oxalic  acid),  a  coefficient  is  obtained  by  which  to  multiply  the  weight 
of  iron  or  oxalic  acid  equal  to  the  permanganate  used,  and  the 
product  is  the  weight  of  the  substance  titrated. 

For  example  :  sulphuretted  hydrogen  is  the  substance  sought, 
the  eq.  weight  of  H2S  corresponding  to  2  eq.  Fe  is  17 '04 ;  let  this 

number  be  divided  by  55'85;  enir5-  =0'3051 ;  therefore,  if  the  quantity 

55*85 

of  iron  represented  by  the  permanganate  used  in  a  determination  of 
H2S  be  multiplied  by"0'3051,  the  product  will  be  the  weight  of  the 
sulphuretted  hydrogen  sought. 

Again  :  in  the  case  of  manganese  peroxide,  of  which  the 
equivalent  weight  is  43'46 ; 

SS-™ 

The  weight  of  iron,  therefore,  found  by  permanganate  in  any 
analysis  multiplied  by  the  coefficient  0*7782  will  give  the  amount  of 
peroxide  MnO2.  Again  :  if  ra  gm.  iron  =  fc  c.c.  permanganate,  then 


126 


FACTORS    FOR   PERMANGANATE. 


1  c.c.  permanganate  =  ^rgrn.  metallic  iron. 

The  equivalents  here  given  are  on  the  hydrogen  scale,  in 
accordance  with  the  normal  system  of  solutions  adopted  ;  and  thus 
it  is  seen  that  two  equivalents  of  iron  are  converted  from  the 
ferrous  to  the  ferric  state  by  the  same  quantity  of  oxygen  as  suffices 
to  oxidize  one  equivalent  of  oxalic  acid,  sulphuretted  hydrogen, 
or  manganese  peroxide. 

1  c.c.  decinormal  permanganate  is  equivalent  to 

0-005585  gm.  Fe  determined  in  the  ferrous  state 

0-007185  FeO 

0-008  Fe203 

0-003733  Fe  from  FeS 

0-00595  Sn  „      SnCl2 

0-00298  Sn  „      SnS2 

0-00318  Cu  determined  from  CuS 

0-00275  Mn          „  „     MnS 

0-00318  Cu  „  „     Cu+FeaCle 

0-00636  Cu  „  „     CuO+Fe 

0-0017  H2S 

0-0008  Q 

0-0063  0 

0-002  Ca  from  CaC204 

0-0120  Ur    „     UrO,  etc.,  etc. 

When  possible,  the  necessary  coefficients  will  be  given  in  the 
tables  preceding  any  leading  substance. 


DETERMINATION    OF    FERROUS    OXIDE 
BY    POTASSIUM    DICHROMATE. 

(Penny's  Method.) 

POTASSIUM  dichromate,  as  a  reagent  for  the  determination  of 
ferrous  iron,  possesses  the  advantages  over  permanganate  that  it 
may  easily  be  obtained  in  the  purest  state,  it  is  absolutely  permanent 
in  solution,  and  its  solution  may  be  used  in  a  Mo hr's  burette. 
On  the  other  hand,  the  end  of  the  reaction  can  only  be  ascertained 
by  means  of  an  external  indicator.  For  this  purpose  a  freshly- 
made  and  very  dilute  solution  of  potassium  ferricyanide  is  spotted 
on  a  white  tile  and  drops  of  the  ferrous  solution  are  from  time  to 
time  brought  into  contact  with  the  spots  of  the  indicator.  At 
first  a  deep-blue  colour  is  produced  where  the  drops  meet,  but  as 
the  addition  of  dichromate  is  continued  this  gives  place  to  a  bluish- 
green,  then  green,  shade,  and  the  titration  is  completed  when  a  drop 
of  the  iron  solution  placed  on  the  tile  appears  of  the  same  colour 
as  that  of  the  mixed  drops  of  solution  and  ferricyanide. 

The  reaction  may  be  simply  expressed  as  follows  : — 


REDUCTION    OF   FERRIC   SALTS.  127 

The  decomposition  takes  place  immediately,  and  at  ordinary 
temperatures,  in  the  presence  of  free  hydrochloric  or  sulphuric 
acid.  Nitric  acid  is,  of  course,  inadmissible. 

The  reduction  of  ferric  compounds  to  the  ferrous  state  may  be 
effected  by  stannous  chloride,  sodium  sulphite,  ammonium  bisul- 
phite, sulphurous  acid,  or  magnesium.  Zinc  is  not  so  good  for  this 
purpose,  as  the  zinc  ferricyanide  somewhat  obscures  the  end- 
reaction.  In  the  analysis  of  iron  ores  stannous  chloride  is  most 
useful,  as  it  very  rapidly  reduces  the  ferric  salt  and  causes  the 
yellow  colour  to  disappear  almost  immediately.  The  reduction  is 
carried  out  as  follows  : — 

The  hydrochloric  acid  solution  of  iron,  containing  a  large  excess 
of  free  acid,  is  heated  to  boiling  in  a  flask  and  fairly  strong  stannous 
chloride  solution  dropped  into  it  from  a  burette*  or  a  dropping  tube 
till  the  yellow  colour  of  the  solution  has  nearly  gone.  The  reduction 
is  then  finished  by  adding  a  more  dilute  solution  of  stannous  chloride 
a  drop  at  a  time,  with  agitation  of  the  liquid  after  each  addition, 
till  the  last  trace  of  colour  has  disappeared.  A  good  operator  can 
do  this  with  the  greatest  accuracy.  But  as  any  stannous  chloride 
added  in  excess  of  the  amount  required  to  reduce  the  whole  of  the 
ferric  salt  present  would  reduce  the  dichromate  afterwards  run  in 
and  so  give  a  high  result,  the  following  procedure  may  be  adopted. 
The  reduced  solution  is  poured  into  a  beaker,  diluted  with  water 
that  has  been  boiled  and  cooled,  and  some  mercuric  chloride  solution 
added.  This  converts  any  stannous  chloride  into  the  stannic 
compound  with  the  precipitation  of  mercurous  chloride,  which  does 
not  interfere  with  the  titration. 

For  the  analysis  of  iron  ores  it  is  most  convenient  to  take  0*5  gram 
for  the  analysis,  and  to  use  a  solution  of  dichromate  containing 
exactly  4*390  grams  of  the  pure  crystals  per  litre.  1  c.c.  =0*005 
gram  Fe,  and  the  number  of  c.c.  used  in  each  determination  gives 
the  percentage  of  iron  present  without  any  calculation.  *f 

1.    Preparation  of  the  Decinormal  Solution  of  Potassium  Dichromate. 

4*903  grm.  per  litre. 
From  the  equation 

6FeO  +K2Cr2O7  -  3Fe2O3 +O2O3  +K20 

we  see  that  each  molecule  of  potassium  dichromate  gives  up  3  atoms 
of  oxygen,  equivalent  to  6  atoms  of  hydrogen.  Hence,  as  the 
molecular  weight  is  294*2,  one-sixth  of  this  in  grams  is  equivalent  to 
one  gram  of  hydrogen.  In  other  words,  a  normal  solution  contains 
49*033  grams  per  litre.  For  most  purposes,  howrever.  a  decinormal 
solution,  containing  4*903  grams  per  litre,  is  more  useful  and  is 
the  one  generally  employed. 

1  c.c.  =0*0008  gram  Oxygen. 

*  It  is  best  to  keep  one  for  the  pirrpose,  as  the  solution  gradually  marks  the  glass. 

t  The  weighed  portion  of  an  iron-ore  should  be  ignited,  gently  at  first,  in  a  platinum 
crucible  previous  to  being  dissolved  in  hydrochloric  acid,  in  order  to  destroy  all  organic 
matter  present. 


128  TITBATION    OF   IRON   BY    BICHROMATE. 

2.    Solution  of  Stannous  Chloride. 

About  10  gm.  of  pure  tin  in  thin  pieces  (or  granulated)  are  put 
into  a  large  platinum  capsule,  about  200  c.c.  strong  pure  hydro- 
chloric acid  poured  over  it,  and  heated  till  it  is  dissolved  ;  or  it  may 
be  dissolved  in  a  porcelain  capsule  or  glass  flask,  adding  pieces  of 
platinum  foil  to  produce  a  galvanic  current.  The  solution  so 
obtained  is  diluted  to  about  a  litre  with  distilled  water,  and  preserved 
in  the  bottle  (fig.  24)  to  which  the  air  can  only  gain  access  through 
a  strongly  alkaline  solution  of  pyrogallic  acid.  When  kept  in  this 
manner,  the  strength  will  not  alter  materially  in  a  month.  If  not 
so  preserved,  the  solution  varies  considerably  from  day  to  day, 
and  therefore  should  always  be  titrated  before  use  if  required 
for  quantitative  analysis. 

IODINE    AND    SODIUM    THIOSULPHATE. 

THE  principle  of  this  now  beautiful  and  exact  method  of  analysis 
was  first  discovered  by  Dupasquier,  who  used  a  solution  of 
sulphurous  acid  instead  of  sodium  thiosulphate.  Bunsen 
improved  his  method  considerably  by  ascertaining  the  sources  of 
failure  to  which  it  was  liable,  which  consisted  in  the  use  of  a  too 
concentrated  solution  of  sulphurous  acid.  The  reaction  between 
iodine  and  very  dilute  sulphurous  acid  may  be  represented  by  the 
equation — 

S02  +Ia +2H20 =2HI  +H2S04. 

If  the  sulphurous  acid  is  more  concentrated,  i.e.,  above  0'04  per 
cent.,  in  a  short  time  the  action  is  reversed,  the  irregularity  of 
decomposition  varying  with  the  quantity  of  water  present,  and  the 
rapidity  with  which  the  iodine  is  added.  This  irregularity  is, 
however,  now  obviatejl  by  the  method  of  Giles  and  Shearer 
in  which  solutions  of  S02  or  sulphites  of  any  strength  may 
be  accurately  titrated  with  iodine,  by  adding  the  latter  to  the 
former  in  excess,  and  when  the  reaction  is  complete  titrating  the 
excess  of  iodine  with  thiosulphate. 

Sulphurous  acid,  however,  very  rapidly  changes  by  keeping  even 
in  the  most  careful  manner,  and  cannot  therefore  be  used  for 
a  standard  solution.  The  substitution  of  sodium  thiosulphate  is 
a  great  advantage,  inasmuch  as  the  salt  is  easily  obtained  in  a  pure 
state,  and  may  be  directly  weighed  for  the  standard  solution.  The 
reaction  is  as  follows  :— 

2Na2S203 +I2  =  2NaI  +Na2S406, 

the  result  being  the  formation  of  sodium  iodide  and  tetrathionate. 
In  order  to  ascertain  the  end  of  the  reaction  in  analysis  by  this 
method  an  indicator  is  necessary,  and  the  most  delicate  and  sensitive 
for  the  purpose  is  starch,  which  produces  with  the  slightest  trace  of 
free  iodine  in  cold  solution  the  well-known  blue  iodide  of  starch. 
Hydriodic  or  mineral  acids  and  iodides  have  no  influence  upon  the 
colour.  Caustic  alkalies  destroy  it. 


IODIMETBY.  129 

The  principle  of  this  method,  namely,  the  use  of  iodine  as  an 
indirect  oxidizing  body  by  its  action  upon  the  elements  of  water, 
forming  hydriodic  acid  with  the  hydrogen  and  liberating  the 
oxygen  in  an  active  state,  can  be  applied  to  the  determination  of 
a  great  variety  of  substances  with  extreme  accuracy. 

Bodies  which  take  up  oxygen,  and  decolorize  the  iodine  solution, 
such  as  sulphurous  acid,  sulphites,  sulphuretted  hydrogen,  alkali 
thiosulphates,  and  arsenites,  stannous  chloride,  etc.,  are  brought 
into  dilute  solution,  starch  added,  and  the  iodine  delivered  in  with 
constant  shaking  or  stirring  until  a  point  occurs  at  which  a  final 
drop  of  iodine  colours  the  whole  blue, — a  sign  that  the  substance 
can  take  up  no  more  iodine,  and  that  the  drop  in  excess  has  shown 
its  characteristic  effect  upon  the  starch. 

Free  chlorine,  or  its  active  compounds,  cannot,  however,  be 
titrated  with  thiosulphate  directly,  owing  to  the  fact  that,  instead  of 
tetrathionic  acid  being  produced  as  with  iodine,  sulphuric  acid  is 
formed,  as  may  be  readily  seen  by  testing  with  barium  chloride. 
In  such  cases,  therefore,  the  chlorine  must  be  evolved  from  its 
compound  and  passed  into  an  excess  of  solution  of  pure  potassium 
iodide,  where  it  at  once  liberates  its  equivalent  of  iodine,  which  can 
then,  of  course,  be  determined  with  thiosulphate. 

All  bodies  which  contain  available  oxygen,  and  which  evolve 
chlorine  when  boiled  with  strong  hydrochloric  acid,  such  as  the 
chromates,  manganates,  and  all  metallic  peroxides,  can  be  readily 
and  most  accurately  determined  by  this  method. 

1.    Preparation  of  the  Decinormal  Solution  of  Iodine. 
12'6p2  gm.  Iodine  per  litre 

Chemically  pure  iodine  may  best  be  obtained  by  the  S  tas  method. 
Commercial  resublimed  iodine  is  mixed  with  about  one-half  of  its 
weight  of  potassium  iodide,  and  dissolved  in  half  its  weight  of  water, 
the  iodine  is  then  precipitated  by  water,  transferred  to  a  funnel 
whose  neck  is  filled  with  freshly  ignited  asbestos,  then  well  washed 
to  remove  the  potassium  iodide,  and  dried  at  a  moderate  heat,  and 
finally  over  sulphuric  acid.  It  is  then  sublimed  by  gently  heating 
the  iodine  between  two  large  watch-glasses  or  porcelain  capsules  ; 
the  lower  one  being  placed  upon  a  heated  iron  plate,  the  iodine 
sublimes  in  brilliant  plates.  It  is  then  sublimed  again  twice,  and 
finally  dried  over  sulphuric  acid. 

The  watch-glass  or  capsule  containing  the  iodine  is  placed  under 
the  exsiccator  to  cool,  then  12'692  gm.  are  accurately  weighed,  and 
together  with  about  18  gm.  of  pure  potassium  iodide  (free  from 
iodate)*  dissolved  in  about  250  c.c.  of  water  and  diluted  to  a  litre. 

. .  *Morse  and  Burton  (Amer.  Chem.  Jour.,  1888)  state  that  potassium  iodide  may 
be  completely  freed  from  iodate  by  boiling  a  solution  of  it  with  zinc  amalgam, 
prepared  by  shaking  zinc  dust  in  good  proportion  with  mercury  in  presence  of  tartaric 
acid  and  washing  with  water.  The  iodate  is  completely  reduced  with  formation  of 
zinc  hydroxide.  The  pure  solution  of  iodide  is  filtered  for  use  through  a  paper  filter 
saturated^with  hot  water. 

K 


130  IODIMETBY. 

The  flask  must  not  be  heated  in  order  to  promote  solution  lest 
iodine  vapours  be  lost  in  the  operation. 

The  iodine  solution  is  best  preserved  in  small  stoppered  bottles, 
which  should  be  completely  filled,  and  kept  in  a  cool  and  dark 
place.  It  should  be  used  with  a  tap  burette,  as  it  makes  rubber 
tubing  hard  and  useless. 

The  standardization  of  the  iodine  solution  may  be  done  in  many 
ways,  e.g.  by  pure  sodium  thiosulphate  prepared  as  described  below, 
or  a  strictly  N/10  solution  of  it,  or  again  pure  arsenious  acid  or  its  N/10 
solution,  with  the  addition  of  a  little  sodium  bicarbonate.  It  may 
be  titrated  with  barium  thiosulphate  (BaS2O3,  H20)  as  proposed  by 
Plimpton  and  Chorley;  this  latter  salt  possesses  a  high  molecular 
weight,  267*5  parts  being  equivalent  to  126*92  of  iodine,  but  being 
sparingly  soluble  in  water  the  titration  must  be  carefully  done, 
inasmuch  as  the  crystalline  powder  has  to  be  gradually  decomposed 
by  the  iodine,  and  the  end-point  may  easily  be  overstepped.  A 
weighed  quantity  of  the  finely  powdered  salt  is  put  into  a  stoppered 
bottle  with  water,  and  the  iodine  solution  run  in  from  a  burette 
with  continuous  shaking,  until  the  salt  is  nearly  dissolved  ;  starch 
indicator  is  then  added,  and  the  addition  of  iodine  continued  with 
shaking  until  the  liquid  assumes  a  blue  colour  that  does  not  dis- 
appear on  agitation. 

Pure  barium  thiosulphate  is  easily  prepared  by  mixing  together 
a  warm  solution  of  50  gm.  of  sodium  thiosulphate  in  300  c.c.  of 
water,  and  40  gm.  of  barium  chloride  in  a  like  volume  of  warm  water  ; 
after  stirring  well,  the  salt  soon  separates  in  fine  powdery  crystals. 
These  are  collected  in  a  funnel  stopped  with  glass  or  cotton  wool, 
repeatedly  washed  with  cold  water  till  all  chlorine  is  removed,  then 
dried  at  below  30°  C.  on  a  glass  or  porcelain  plate  until  all  extraneous 
moisture  is  removed  ;  or  the  crystals  may  be  treated,  after  thorough 
washing  with  alcohol  and  ether,  as  described  below  for  sodium 
thiosulphate. 

2.    Decinormal  Sodium  Thiosulphate. 

24*822  gm.  per  litre. 
Na2S2O3,  5H2O= 248*22. 

It  is  not  difficult  either  to  manufacture  or  procure  pure  sodium 
thiosulphate,  but  there  may  be  uncertainty  as  to  extraneous  water 
held  within  the  crystals.  In  order  to  avoid  this  Mei nek e*  recom- 
mends that  the  otherwise  pure  crystals  be  broken  to  coarse  powder, 
washed  first  with  pure  alcohol,  then  with  ether,  and  lastly  dried  in 
a  current  of  dry  air  at  ordinary  temperature.  The  salt  so  prepared 
may  be  weighed  directly,  dissolved  in  a  litre  of  distilled  water, 
and  then  titrated  with  the  iodine  solution  and  starch.  It  is  advis- 
able to  preserve  the  solution  in  the  dark.  After  a  time  all  solutions 
of  thiosulphate  undergo  a  slight  amount  of  oxidation,  and  sulphur 
deposits  upon  the  bottle  ;  it  is  therefore  always  advisable  to  titrate 
it  previous  to  use. 

*  Chem.  Zeit.  18,' 33. 


1ODIMBTRY.  131 

3.    Starch  Indicator. 

One  part  of  clean  potato  starch,  or  arrowroot,  is  first  mixed 
with  cold  water  into  a  smooth  emulsion,  then  gradually  poured 
into  about  150  or  200  times  its  weight  of  boiling  water,  the  boiling 
continued  for  a  few  minutes,  then  allowed  to  stand  and  settle 
thoroughly.  The  clear  solution  only  is  to  be  used  as  the  indicator, 
of  which  a  few  drops  only  are  necessary.  The  solution  may  be 
preserved  for  some  time  by  adding  to  it  a  few  drops  of  chloroform, 
and  shaking  well  in  a  stoppered  bottle,  but  it  is  preferable  to  use 
a  fresh  solution  in  all  cases. 

Lintner's  soluble  starch  acts  well  as  an  indicator,  as  it  gives  at 
once  a  clear  solution  in  boiling  water.  The  colour  which  is  produced 
with  this  form  of  starch  is  not  quite  so  pure  a  blue  as  that  given 
by  a  freshly  made  solution  of  ordinary  starch,  owing  to  the  presence 
of  some  dextrin  unavoidably  produced  in  the  preparation,  but  it 
is  no  hindrance  to  the  end-point  in  practice.  In  iodimetric  analyses 
it  is  always  advisable  in  titrating  the  free  iodine  with  thiosulphate 
or  arsenious  solution  to  delay  adding  the  starch  until  the  iodine 
colour  is  nearly  removed  ;  a  much  more  delicate  ending  may  be 
obtained  and  with  very  little  starch. 

Methylene  Blue  Indicator. — S  i  n  n  a  1 1  *  points  out  that  methylene  blue  forms 
a  convenient  indicator  in  iodimetric  titrations  in  lieu  of  starch.  In  dilute 
solutions,  when  a  solution  of  methylene  blue  is  added  to  a  solution  of  iodine  in 
potassium  iodide,  the  formation  of  an  iodo-compound  of  the  colouring  matter  is 
accompanied  by  a  colour  change  from  blue  to  yellowish-green,  and  finally  to  a  clear 
yellowish-brown  colour.  On  the  completion  of  the  iodine  titration  the  blue 
colour  recurs.  With  most  reducing  agents  methylene  blue  is  not  reduced,  but 
titanous  chloride  decolorizes  it.  1  c.c.  of  a  0*005  per  cent,  solution  of  the 
indicator  to  50  c.c.  of  liquid  gives  a  convenient  depth  of  colour  for  titrations. 


Extension  of  the  Iodimetric  System. 

The  verification  and  extension  of  iodimetric  methods  have  received  considerable 
attention  from  a  great  number  of  chemists,  among  whom  may  be  mentioned 
J.  Wagner,f  who  has  studied  the  accuracy  of  the  determination,  by  means  of 
thiosulphate  solutions,  of  the  iodine  liberated  from  acidified  potassium  iodide 
solutions  when  the  oxidizing  agents  employed  are  potassium  or  sodium  bromate, 
potassium  dichromate,  chromate,  and  iodate.  The  titrations  should  be  carried 
out  in  flasks  and  not  in  beakers ;  a  titration  with  potassium  dichromate  and 
iodide  required  25 '67  c.c.  of  thiosulphate  when  carried  out  in  a  flask,  and  three 
titrations  varied  by  only  O'Ol  c.c.  ;  a  similar  titration  in  a  beaker  required  25'52  c.c, 
.of  thiosulphate,  and  three  titrations  varied  as  much  as  0*07  c.c. 

With  reference  to  the  application  of  iodimetry'to  the  determination  of  acids 
and  alkalies  Walker  and  GillespieJ  have  shown  that  when  iodine  acts  upon 
a  solution  of  a  metallic  hydroxide  at  a  temperature  high  enough  to  destroy  any 
trace  of  hypoiodite  a  perfectly  neutral  liquid  is  produced  which  contains  1  molecule 
of  iodate  to  5  of  iodide.  On  adding  dilute  acid,  these  two  salts  interact  in  the 
well-known  way,  liberating  6  atoms  of  iodine  ;  and  by  titration  with  thiosulphate 
or  arsenious  acid,  the  iodine — that  is  to  say,  the  original  hydroxide — may  be 
determined.  Similarly,  an  acid  may  be  neutralized  by  a  known  excess  of  alkali 
standardized  in  this  way,  when  determination  of  the  surplus  will  give  the  strength 

*  Analyst  1910,  309. 
.  a.  C.,  1899,  427-453. -^~^Z.  a.  C.,  1899,  194, 


132  TITRATION   WITH   POTASSIUM   IODATE. 

of  the  acid.  The  process  has  been  tested  on  the  hydroxides  of  the  alkalies  and 
alkaline  earths,  on  sulphuric  and  hydrochloric  acids  ;  and  although  the  precautions 
necessary  to  avoid  loss  of  iodine  and  carbonation  of  the  liquid  perhaps  render 
it  somewhat  complicated,  the  reaction  proceeds  so  smoothly  that  it  should  be 
serviceable  for  the  indirect  analysis  of  acids  and  probably  for  other  suitable 
compounds.  It  cannot,  however,  be  employed  on  alkali-metal  carbonates. 
The  method  outlined  by  Phelps*  may  with  advantage  be  slightly  modified. 
A  moderate  excess  of  decinormal  iodine  is  placed  in  a  lightly-covered  conical 
flask,  the  alkali  ts  added  (or,  in  determining  acid,  the  acid  is  added,  followed  by 
a  measured  excess  of  standard  alkali),  and  the  whole  is  boiled  till  all  free  iodine 
is  volatilized.  The  bulk  of  the  liquid  in  all  tests  should  be  uniform  and  as  small 
as  possible,  starting  with  about  100  c.c.  and  boiling  down  to  about  35  c.c.  The 
vessel  is  cooled  in  a  stream  of  water,  10  c.c.  of  dilute  sulphuric  or  hydrochloric 
acid  added,  and  the  liquid  titrated  with  thiosulphate  and  starch  in  the  usual  way. 
L.  W.  Andre wsf  states  that,  as  is  well  known,  when  potassium  iodide  is 
titrated  with  chlorine  water  in  a  neutral  solution,  the  reaction  which  takes 
place  is  expressed  by  the  equation — 

KI+3C12+3H20=KC1+HI03+5HC1-.      .      .      .   (1). 

On  the  other  hand,  it  may  not  be  so  well  known  that  if  a  large  excess  of  free 
hydrochloric  acid  is  present  during  the  titration,  chloroform  or  carbon  tetra- 
chloride  being  used  as  before  for  an  indicator,  the  reaction  will  be — 

KI+C12=KC1+IC1  .  . (2). 

In  both  cases  the  end  of  the  reaction  is  shown  by  the  immiscible  solvent 
becoming  colourless.  If  instead  of  chlorine  water  we  titrate  with  a  solution  of 
potassium  iodate,  the  stage  at  which  the  reaction  stops  is  likewise  dependent 
upon  the  concentration  of  the  acid.  If  this  be  low,  the  reaction  goes  no  further 
than  to  set  the  iodine  free  in  accordance  with  the  equation — 

5KI+KIO3+6HC1=6KC1+3I2+3H2O  .  .  .  (3), 
while  if  a  great  excess  of  hydrochloric  acid  is  present  the  reaction  runs — 

2KI+KI03+6HC1=3KC1+3IC1+3H20    .      .      .    (4), 

the  immiscible  solvent  remaining  violet  in  the  former  case  (No.  3),  but  in  the 
latter  becoming  colourless,  while  the  supernatant  solution  turns  bright  yellow 
from  the  iodine  chloride.  The  probable  explanation  of  this  behaviour  is  that 
iodine  chloride,  as  the  salt  of  a  very  weak  base,  undergoes  hydrolysis  in  a  neutral 
or  feebly  acid  solution,  with  the  production  of  the  corresponding  hydroxide  and 
acid  ;  thus — 

IC1+H20=IOH+HC1 (5), 

the  iodous  hydroxide  ("  hypoiodous  acid "),  which  is  formed,  undergoing 
spontaneous  conversion  into  iodic  acid,  &c.,  whereas  the  hydrolysis  is  prevented 
by  a  great  excess  of  hydrochloric  acid. 

The  reaction  of  equation  (1)  was  used  long  ago  by  A.  and  F.  Dupre  (Liebig's 
Ann.  Chim.,  1855,  xciv.  365)  for  the  titration  of  'iodides.  In  order  to  compare 
the  reactions  of  the  first  two  equations,  5  c.c.  of  a  decinormal  potassium  iodide 
solution  were  titrated  with  chlorine  water  in  presence  of  5  c.c.  of  chloroform. 
After  the  addition  of  75 '4  c.c.  of  the  latter  the  chloroform  became  colourless. 
The  titration  was  now  repeated  with  the  further  addition  of  respectively  15,  20, 
and  30  c.c.  of  strongest  hydrochloric  acid,  and  the  amounts  of  chlorine  water 
required  were  25'4,  25'2,  and  25'25  c.c.,  the  end-reaction  being  of  extraordinary 
sharpness.  Nearly  three  times  as  much  chlorine  was  therefore  required  in  the 
absence  of  hydrochloric  acid  as  in  its  presence,  as  the  theory  demands.  Probably, 
if  the  small  amount  of  acid  produced  by  the  reaction  itself  (Equation  1)  had 
been  neutralized  by  the  addition  of  calcium  carbonate  the  theoretical  amount  of 
75'75  c.c.  of  chlorine  solution  would  have  been  required.  In  order  to  judge  the 
influence  of  small  quantities  of  acid,  the  titration  was  repeated  with  addition 
of  1,  2,  5,  and  10  c.c.  of  concentrated  hydrochloric  acid,  when  respectively  34'1, 
26'9,  26*0,  and  25'6  c.c.  of  chlorine  water  were  required. 

*  Analyst,  1897i  55. 
t  Zeit.  anorg.  Chem.,  1903,  76,  and,/.  Am.  Ghent.  Soc.,  25,  756. 

M  ^ 


TITRATION   WITH   POTASSIUM   IODATE.  133 

From  these  preliminary  experiments,  it  appeared  that  the  hydrolysis  of  the 
iodine  chloride  might  be  wholly  inhibited  by  addition  of  a  sufficiency  of  acid, 
and  that  a  solution  of  potassium  iodate  might  be  successfully  substituted  for  the 
chlorine  water,  thus  realizing  the  reaction  of  Equation  4.  9 '7465  gm.  of  acid 
potassium  iodate  were  dissolved  in  water  and  made  up  to  1  litre.  According  to 
the  theory,  each  c.c.  of  this  solution  should  be  equivalent  to  16*6  mgm.  of  I/ 
potassium  iodide.  To  10  c.c.  of  a  solution  of  pure  potassium  iodide  (20*6  gm.  to  ' 
the  litre),  5  c.c.  of  chloroform,  20  c.c.  of  water,  and  30  c.c.  of  concentrated 
hydrochloric  acid  (sp.  gr.  T21)  were  added,  and  the  mixture  was  titrated  in 
a  glass-stoppered  bottle  of  250  c.c.  capacity  with  the  iodate  solution,  shaking 
briskly,  until  the  chloroform  lost  its  colour,  the  end-point  being  exceedingly 
sharp.  12-45  c.c.  of  the  iodate  solution  were  required.  Hence,  0'20634  gm. 
potassium  iodide  was  found  against  0 '20600  taken,  or  100' 17  per  cent.  In 
a  second  experiment,  15  c.c.  of  the  iodide  solution,  titrated  in  the  same  way 
with  33  c.c.  of  hydrochloric  acid  and  no  additional  water,  required  18'6  c.c.  of 
the  iodate  solution,  corresponding  to  0'30900  gm.  found,  against  0'30900  gm. 
taken,  or  100 '00  per  cent,  taken. 

The  process  as  described  can  be  applied  to  the  titration  of  chromates.  For 
this  purpose  the  chromate  is  added  to  an  excess  of  a  titrated  potassium  iodide 
solution,  with  5  c.c.  of  chloroform  and  sufficient  concentrated  hydrochloric  acid 
to  be  at  least  half  the  volume  of  the  entire  mixture  at  the  close  of  the  titration. 
The  titration  is  then  carried  out  precisely  as  described  above.  In  one  experiment 
of  this  sort,  36'3  mgm.  of  potassium  pyrochromate  were  taken,  and  36'8  mgm. 
found. 

The  following  experiment  shows  the  applicability  of  the  process  to  the  titration 
of  free  iodine : — 0'3447  gm.  of  pure  iodine  was  weighed  and  placed  in  the 
stoppered  bottle  previously  used,  with  5  c.c.  of  a  potassium  iodide  solution 
containing  20'6  gm.  per  litre  ;  10  c.c.  of  fuming  hydrochloric  acid,  and  5  c.c.  of 
chloroform  were  added,  and  the  titration  was  carried  out  in  the  usual  way. 
Required,  19'85  c.c.  of  standard  iodate.  Since  6'20  c.c.  are  required  for  the 
iodide,  13'65  c.c.  remain  as  corresponding  to  the  free  iodine,  or  0'3467  gm.  iodine 
found  ;  100 '46  per  cent. 

To  determine  whether  the  method  can  be  used  for  determination  of  chlorates, 
and  under  what  conditions,  the  succeeding  experiments  were  tried.  Five  c.c.  of 
a  solution  of  potassium  chlorate  containing  70'3  mgm.  of  the  pure  salt  was 
added  to  25  c.c.  of  the  potassium  iodide  solution  mentioned  above,  and  50  c.c.  of 
fuming  hydrochloric  acid.  After  standing  fifteen  minutes  in  the  stoppered 
bottle,  5  c.c.  of  chloroform  were  added,  and  the  titration  completed.  Required, 
13'65  c.c.  of  the  iodate.  As  the  iodide  is  equivalent  to  31 '0  c.c.  17'35  c.c. 
correspond  to  the  chlorate,  whence  70'9  mgm.  of  potassium  chlorate  were  found. 
In  a  second  similar  experiment,  only  40  c.c.  of  hydrochloric  acid  were  used,  and 
the  mixture  was  titrated  at  once,  without  standing.  In  this  case  14'0  c.c.  of 
iodate  were  required,  hence  69 '55  mgm.  of  chlorate  were  found.  This  shows, 
as  was  expected,  that  the  chlorate  must  be  left  for  some  time  in  contact  with  the 
hydrochloric  acid  and  potassium  iodide  for  the  completion  of  the  reaction.  In 
a  third  experiment,  exactly  similar  to  the  last  except  that  the  mixture  was 
allowed  to  stand  twenty-four  hours  before  titration,  13'75  c.c.  of  iodate  were 
required,  whence  70 '4  mgm.  of  chlorate  were  found.  It  is  therefore  a  matter 
of  indifference  whether  the  time  of  digestion  is  a  quarter  of  an  hour  or  twenty- 
four  hours.  In  a  fourth  experiment,  5  c.c.  of  another  potassium  chlorate 
solution  containing  33 '46  mgm.  of  the  pure  salt  was  allowed  to  stand  for  ten 
minutes  with  10  c.c.  of  iodide  solution,  and  20  c.c.  of  fuming  hydrochloric  acid  ; 
then  5  c.c.  of  chloroform  were  added,  and  the  titration  was  performed.  Required, 
4'25  c.c.  of  iodate.  Calculated  for  the  iodide,  12'40  c.c.,  whence  33'39  mgm.  of 
chlorate  were  found.  Other  experiments,  not  necessary  to  detail,  show  that  there 
must  be  a  decided  excess  of  iodide  as  compared  with  the  chlorate  ;  otherwise  the 
results  are  likely  to  be  a  little  too  low.  The  necessary  working  conditions  for 
the  titration  of  a  chlorate  can  be  prescribed  as  follows : — 

To  the  solution  of  the  chlorate,  add  an  exactly  known  amount  of  pure  potassium 
iodide  (a  titrated  solution  may  be  used),  in  a  glass-stoppered  bottle,  and  an 
amount  of  fuming,  pure  hydrochloric  acid  at  least  one-third  greater  than  the 


134  TITBATION   WITH   POTASSIUM   IODATE. 

volume  of  the  solution.  Close  the  bottle  tightly,  and  allow  it  to  stand  fifteen 
minutes  after  shaking,  then  add  5  c.c.  of  chloroform.  On  now  shaking,  the 
chloroform  must  become  deep  violet.  If  the  colour  is  pale,  an  insufficiency  of 
iodide  has  been  added,  and  it  is  better  to  begin  again  rather  than  to  attempt  €b 
bring  the  analysis  into  order.  Now  add  the  decinormal  iodate  with  intermittent 
violent  shaking  until  the  chloroform  becomes  colourless,  which  point  can  be 
determined  with  the  utmost  precision.  Each  c.c.  of  a  decinormal  iodate  solution 
is  equivalent  to  2'782  mgm,  of  (C1O3). 

Solutions  of  arsenious  acid  or  chloride  can  be  titrated  in  the  same  way  as 
iodides,  the  reaction  being  expressed  by  the  equation — 

2AsCl3  +KIO3  +5H2O  =2H3As04  +KC1  +IC1  +4HC1. 

In  this  case,  however,  unlike  the  other,  a  too  great  concentration  of  hydro- 
chloric acid  must  be  avoided,  since  under  those  conditions  the  end-point  becomes 
obscure,  probably  a  phenomenon  connected  with  the  formation  and  dissociation 
of  arsenic  pentachloride.  The  suitable  concentration  of  the  acid  is  therefore  con- 
fined within  somewhat  narrow  limits,  but  not  so  narrow  as  to  cause  any  practical 
difficulty  in  working.  It  was  found  that  30  per  cent,  of  hydrochloric  acid, 
calculated  on  the  weight  of  the  entire  liquid  at  the  close  of  the  titration,  exceeds 
the  permissible  maximum  limit,  while  25  per  cent,  does  not.  On  the  other  hand 
the  minimum  limit  is  in  the  neighbourhood  of  12  to  15  per  cent,  of  acid.  For 
the  experiments  noted  below,  a  solution  of  sodium  arsenite  was  employed  in 
which  the  amount  of  arsenious  oxide  had  been  determined  by  titration  with 
iodine  solution  in  the  ordinary  way.  Taken,  25  c.c.  arsenious  solution 
(243*8  mgm.  As203)  and  50  c.c.  of  fuming  hydrochloric  acid  ;  required,  24*45  c.c. 
decinormal  =  242 '1  mgm.  of  arsenious  oxide.  Taken,  5  c.c.  arsenious  solution, 
5  c.c.  hydrochloric  acid,  and  10  c.c.  water  ;  required,  4*9  c.c.  of  iodate  =48*6  mgm.  ; 
found,  48 '8  mgm.  by  iodine  titration.  Taken,  20  c.c.  arsenious  solution  and 
40  c.c.  hydrochloric  acid  ;  required,  19*7  c.c.  iodate  =194*9  mgm.  arsenious  oxide  ; 
found,  194'7  mgm.  by  iodine  titration.  Taken,  15  c.c.  arsenious  solution  and 
30  c.c.  hydrochloric  acid  ;  required,  14*8  c.c.  iodate  =146*4  mgm.  arsenious  oxide  ; 
found,  146 '3  mgm.  by  iodine  titration. 

To  summarize,  add  to  the  arsenious  solution  an  amount  of  fuming  hydro- 
chloric acid  sufficient  to  make  the  hydrochloric  acid  equal  to  about  20  per  cent, 
of  the  entire  mixture  at  the  end  of  the  titration,  and  5  c.c.  of  chloroform ;  then 
run  in  from  a  burette  as  large  a  proportion  as  can  be  judged  of  the  whole 
amount  of  decinormal  iodate  requisite,  shake  well,  and  continue  titrating  with 
the  iodate  until  the  chloroform  is  colourless.  Each  c.c.  of  the  standard  solution 
corresponds  to  9*9  mgm.  arsenious  acid  or  7 '5  mgm.  arsenic. 

The  determination  of  antimony  is  precisely  like  that  of  arsenic.  A  solution 
was  prepared  of  pure  re- crystallized  antimonyl  tartrate,  containing  31*251  gm. 
per  litre.  Twenty-five  c.c.  of  this  were  mixed  with  30  c.c.  hydrochloric  acid  and 
20  c.c.  water,  and  titrated  as  usual.  23'6  c.c.  of  the  iodate  were  required, 
equivalent  to  784*6  mgm.  tartar  emetic  found  as  against  781*3  mgm.  taken. 
In  this  determination  the  amount  of  hydrochloric  acid  should  have  been  greater 
by  15  c.c.  In  the  next  experiment,  25  c.c.  of  the  antimonious  solution  with 
25  c.c.  of  hydrochloric  acid  required  23*50  c.c.  of  iodate,  equivalent  to  780'6 
mgm.  of  antimony  salt  found  (781*3  taken).  Twenty-five  c.c.  antimony  solution 
with  35  c.c.  fuming  hydrochloric  acid  required  23*55  c.c.  of  iodate,  whence  is 
calculated  781*2  mgm.  potassium  antimonyl  tartrate. 

Since  copper  does  not  interfere  in  the  least  with  the  application  of  the  method, 
it  is  possible,  for  example,  to  titrate  the  arsenic  in  Paris  green  directly  without 
preliminary  separation.  Thus,  20  c.c.  of  a  sodium  arsenite  solution  with  20  c.c. 
of  fuming  hydrochloric  acid  required  8.95  c.c.  of  iodate,  the  same,  plus  1  gm.  of 
copper  sulphate,  required  9*00  c.c.  of  iodate.  For  the  analysis  of  Paris  green, 
0*5  gm.  of  the  substance  is  dissolved  in  15  c.c.  of  water  and  25  c.c.  of  fuming 
hydrochloric  acid,  and  directly  titrated  with  5  c.c.  of  chloroform  and  the 
decinormal  solution  of  iodate. 

Ferrous  salts  can  be  titrated  in  exactly  the  same  way  as  iodides.  Taken, 
2*0874  gm.  ammonium  ferrous  sulphate  ;  required,  26*05  c.c.  iodate,  equivalent 
to  297*6  mgm.  iron  found,  or  14*26  per  cent.  ;  theory,  14.25  per  cent.  Unlike 
the  titration  with  potassium  permanganate,  oxalic  acid  does  not  interfere  with 


DISTILLATION    METHODS. 


135 


this  determination.  Taken,  2*0843  gm.  ammonium  ferrous  sulphate  and  1  gm. 
oxalic  acid  ;  required,  25 '95  c.c.  iodate,  equivalent  to  296'3  mgm.  iron,  or 
14 '22  per  cent.  Ferric  salts  do  not  interfere  with  any  of  these  titrations,  nor  do 
bromides  to  any  serious  extent,  if  the  amount  is  small.  The  end-reaction  in  the 
titration  of  ferrous  salts  is  somewhat  slow,  and,  in  spite  of  the  satisfactory 
results  of  the  test  analyses,  is  lacking  in  the  sharpness  that  distinguishes  the 
other  titrations  described  in  this  paper.  This  difficulty  appears  to  be  avoided  by 
the  addition  of  a  small  amount  of  manganous  chloride,  but  the  point  requires 
further  examination. 

The  method  which  has  been  described  is  adapted  to  the  determination  of  almost 
all  the  substances  to  which  Bun  sen's  process  of  distillation  with  potassium 
iodide  and  hydrochloric  acid  is  applicable,  with  at  least  equal  precision,  with  less 
expenditure  of  time  and  far  simpler  apparatus.  It  is  furthermore  applicable  in 
certain  cases  in  which  the  B  u  n  s  e  n  method  is  not,  as,  for  example,  the  titration 
of  arsenic  or  antimony  in  the  presence  of  copper  and  ferric  compounds. 


ANALYSIS    OF    SUBSTANCES    BY    DISTILLATION    WITH 
HYDROCHLORIC    ACID    INTO    ALKALI    IODIDE. 

THERE  is  a  great  variety  of  substances  containing  oxygen,  which 
when  boiled  with  hydrochloric  acid  yield  chlorine,  equivalent  to 


Fig.  38. 

the  whole  or  a  part  only  of  the  oxygen  they  contain  according  to 
circumstances.  Upon  this  fact  is  based  the  variety  of  analyses 
which  may  be  accomplished  by  means  of  iodine  and  sodium  thio- 
sulphate,  or  arsenite  ;  the  chlorine  so  evolved,  however,  is  not  itself 
determined,  but  is  conveyed  by  means  of  a  suitable  apparatus  into 
a  solution  of  potassium  iodide,  thereby  liberating  an  equivalent 
quantity  of  iodine.  This  latter  body  is  then  determined  by  thio- 
sulphate ;  the  quantity  so  found  is,  therefore,  a  measure  of  the  oxygen 


136 


DISTILLATION    METHODS. 


existing  in  the  original  substance,  and  consequently  a  measure 
of  the  substance  itself.  Analyses  of  this  class  may  be  made  the 
most  exact  in  the  whole  range  of  volumetric  analysis,  far  out- 
stripping any  gravimetric  process. 

The  apparatus  used  for  distilling  the  substances,  and  conveying 
the  liberated  chlorine  into  the  alkali  iodide,  may  possess  a  variety 
of  forms,  the  most  serviceable,  however,  being  the  kinds  devised 
respectively  by  Bunsen,  Fresenius,  Mohr,  and  others,  among 
which  one  of  the  best  is  so  constructed  as  to  avoid  the  use  of  corks 
or  india-rubber,  which  are  soon  destroyed  by  the  corrosive  action 
of  iodine  and  acid  (see  p.  8,  fig.  43). 


Fig.  39. 

Buns  en's  arrangement  consists  of  an  inverted  retort,  into  the 
neck  of  which  the  tube  from  the  small  distilling  flask  is  passed. 

Ou  ing  to  the  great  solubility  of  HC1  in  the  form  of  gas,  the 
apparatus  must  be  so  constructed  that  when  all  chlorine  is  liberated 
and  HC1  begins  to  distil  the  liquid  may  not  rush  back  into  the  flask 
owing  to  condensation. 

The  best  preventive  of  this  regurgitation  is,  however,  one  suggested 
by  Fresenius,  and  applicable  to  each  kind  of  apparatus;  namely, 
the  addition  of  a  few  pieces  of  pure  magnesite.  This  substance 
dissolves  but  slowly  in  the  hydrochloric  acid,  and  so  keeps  up  a 


DISTILLATION    METHODS.  137 

constant  evolution  of  CO2,  the  pressure  of  which  is  sufficient  to 
prevent  the  return  of  the  liquid. 

The  apparatus  contrived  by  Fresenius  is  shown  in  fig.  38, 
and  is  exceedingly  useful  as  an  absorption  apparatus  for  general 
purposes. 

M  o  h  r '  s  apparatus  is  shown  in  fig.  39,  and  is,  on  account  of  its 
simplicity  of  construction,  very  easy  to  use. 

The  distilling  flask  is  of  about  2  oz.  capacity,  and  is  fitted  with 
a  cork  soaked  to  saturation  in  melted  paraffin  ;  through  the  cork 
the  delivery  tube  containing  one  bulb  passes,  and  is  again  passed 
through  a  common  cork,  fitted  loosely  in  a  stout  tube  about  12  or 
13  inches  long  and  1  inch  wide,  closed  at  one  end  like  a  test  tube. 
This  tube,  containing  the  alkali  iodide,  is  placed  in  an  hydrometer 
glass,  about  12  inches  high,  and  surrounded  by  cold  water  ;  the 
delivery  tube  is  drawn  out  to  a  fine  point,  and  reaches  nearly  to 
the  bottom  of  the  condenser.  No  support  or  clamp  is  necessary, 
as  the  hydrometer  glass  keeps  everything  in  position.  The  substance 
to  be  distilled  is  put  into  the  flask,  and  covered  with  strong  hydro- 
chloric acid,  the  magnesite  added,  the  condenser  supplied  with 
a  sufficient  quantity  of  iodide  solution,  and  the  apparatus  put 
together  tightly.  Either  an  argand  or  common  spirit  lamp,  or  gas, 
may  be  used  for  heating  the  flask,  but  the  flame  must  be  manageable, 
so  that  the  boiling  can  be  regulated  at  will.  In  the  case  of  the 
common  spirit  lamp  it  may  be  held  in  the  hand,  and  applied  or 
withdrawn  according  to  the  necessities  of  the  case  ;  the  argand 
spirit  or  gas  lamp  can,  of  course,  be  regulated  by  the  usual  arrange- 
ments for  the  purpose.  If  the  iodine  liberated  by  the  chlorine 
evolved  should  be  more  than  will  remain  in  solution,  the  cork  of 
the  condensing  tube  must  be  lifted,  and  more  solution  added. 
When  the  operation  is  judged  to  be  at  an  end,  the  apparatus  is 
disconnected,  and  the  delivery  tube  washed  out  into  the  iodide 
solution,  which  is  then  emptied  into  a  beaker  or  flask  and  preserved 
for  titration,  a  little  fresh  iodide  solution  is  put  into  the  condenser, 
the  apparatus  again  put  together,  and  a  second  distillation  com- 
menced, and  continued  for  a  minute  or  so,  to  collect  every  trace  of 
free  chlorine  present.  This  second  operation  is  only  necessary 
as  a  safeguard  in  case  the  first  should  not  have  been  complete. 

The  solutions  are  then  mixed  together  and  titrated  in  the  manner 
previously  described.  In  all  cases  the  solution  must  be  cooled 
before  adding  the  thiosulphate,  otherwise  sulphuric  acid  might 
be  formed. 

Instead  of  the  large  test  tube,  some  operators  use  a  (J  tube  to 
contain  the  potassium  iodide,  having  a  bulb  in  each  limb,  but  the 
latter  is  not  necessary  if  magnesite  is  used. 

The  solution  of  potassium  iodide  may  conveniently  be  made  of 
such  a  strength  that  T%  eq.  or  33'2  gm.  are  contained  in  the  litre. 
1  c.c.  will  then  be  sufficient  to  absorb  the  quantity  of  free  iodine 
representing  1  per  cent,  of  oxygen  in  the  substance  analyzed, 
supposing  it  to  be  weighed  in  the  metric  system.  In  examining 


138  DIGESTION   WITH  KI   AND   HCL. 

peroxide  of  manganese,  for  instance,  0'4346  gm.  would  be  used,  and 
supposing  the  percentage  of  peroxide  to  be  about  sixty,  60  c.c.  of 
iodide  solution  would  be  sufficient  to  absorb  the  chlorine  and  keep 
in  solution  the  iodine  liberated  by  the  process  ;  it  is  advisable, 
however,  to  have  an  excess  of  iodide,  and,  therefore,  in  this  case, 
about  70  c.c  should  be  used.  A  solution  of  indefinite  strength 
will  answer  as  well,  so  long  as  enough  is  used  to  absorb  all 
the  iodine.  It  may  sometimes  happen  that  not  enough  iodide  is 
present  to  keep  all  the  liberated  iodine  in  solution,  in  which  case 
it  will  separate  out  in  the  solid  form  ;  more  iodide,  however,  may  be 
added  to  dissolve  the  iodine,  and  the  titration  can  then  be  made 
as  usual. 

The  process  of  distillation  above  described  may  be  avoided  in 
many  cases.  There  is  a  great  number  of  substances  which,  by 
mere  digestion  with  hydrochloric  acid  and  potassium  iodide  at  an 
elevated  temperature,  undergo  decomposition 
quite  as  completely  as  by  distillation.  For 
this  purpose  a  strong  bottle  with  a  very 
accurately  ground  stopper  is  necessary  ;  and 
as  the  ordinary  stoppered  bottles  of  com- 
merce are  not  sufficiently  tight,  it  is  better 
to  re-grind  the  stopper  with  a  little  very  fine 
emery  and  water.  It  must  then  be  tested 
by  tying  the  stopper  tightly  down  and 
immersing  in  hot  water  ;  if  any  bubbles  of 
air  find  their  way  through  the  stopper  the 

bottle   is   useless.     The   capacity   may   vary  "^         'I,'"' 

from    30    to    150    c.c.,     according    to    the 
necessities  of  the  case. 

The  stopper  may  be  secured  by  fine  copper  binding- wire,  or  a  kind 
of  clamp  contrived  by  Mohr  may  be  used,  as  shown  in  fig.  40;  by 
means  of  the  thumb-screws  the  pressure  upon  the  stopper  may  be 
increased  to  almost  any  extent. 

The  substance  to  be  examined,  if  in  powder,  is  put  into  the  bottle 
with  pure  flint  pebbles  or  small  garnets,  so  as  to  divide  it  better, 
and  a  sufficient  quantity  of  saturated  solution  of  potassium  iodide 
and  pure  hydrochloric  acid  added  ;  the  stopper  is  then  inserted, 
fastened  down,  and  the  bottle  suspended  in  a  water  bath,  and  the 
water  is  gradually  heated  to  boiling  by  a  gas  flame  or  hot  plate  as 
may  be  most  convenient.  When  the  decomposition  is  complete 
the  bottle  is  removed,  allowed  to  cool  somewhat,  then  placed  in 
cold  water,  and,  after  being  shaken,  emptied  into  a  beaker,  and  the 
liquid  diluted  by  the  washings  for  titration. 

The  salts  of  chloric,  iodic,  bromic,  and  chromic  acids,  together 
with  many  other  compounds,  may  be  as  effectually  decomposed  by 
digestion  as  by  distillation,  many  of  them  even  at  ordinary 
temperatures.  Recently  precipitated  oxides,  or  the  natural  oxides 
when  reduced  to  fine  powder,  are  readily  dissolved  and  decomposed 
by  very  weak  acid  in  the  presence  of  potassium  iodide  (Pickering). 


ARSENIOUS   ACID    AND    IODINE.  139 

The  potassium  iodide  used  in  the  various  analyses  must  be 
absolutely  free  from  iodate  and  free  iodine,  or,  if  otherwise,  the 
effect  of  the  impurity  must  be  ascertained  by  a  blank  experiment. 


ARSENIOUS    ACID    AND    IODINE. 

THE  principle  upon  which  this  method  of  analysis  is  based  is 
the  fact  that  when  arsenious  acid  is  brought  in  contact  with  iodine 
in  the  presence  of  water  and  free  alkali,  it  is  converted  into  arsenic 
acid,  the  reaction  being — 

*As203+2I2+2K^O=As205+4KI. 

The  alkali  must  be  in  sufficient  quantity  to  combine  with  the 
hydriodic  acid  set  free,  and  it  is  necessary  that  it  should  exist  in  the 
state  of  bicarbonate,  as  caustic  or  monocarbonated  alkalies  interfere 
with'  the  colour  of  the  blue  iodide  of  starch  used  as  indicator. 

If,  therefore,  a  solution  of  arsenious  acid  containing  starch  is 
titrated  with  a  solution  of  iodine  in  the  presence  of  an  alkali 
bicarbonate,  the  blue  colour  does  not  occur  until  all  the  arsenious 
acid  is  oxidized  into  arsenic  acid.  In  like  manner,  a  standard 
solution  of  arsenious  acid  may  be  used  for  the  determination  of 
iodine  or  other  bodies  which  possess  the  power  of  oxidizing  it. 

The  chief  value,  however,  of  this  method  is  found  in  the  deter- 
mination of  free  chlorine  existing  in  the  so-called  chloride  of  lime, 
chlorine  water,  hypochlorites  of  lime,  soda,  etc.,  in  solution  ;  gener- 
ally included  under  the  term  of  chlorimetry. 

Preparation  of  the  N/10  Solution  of  Alkali  Arsenite. 

4-948  gm.  As2O3  per  litre. 

tAs203= 197-92. 

The  iodine  solution  used  is  the  same  as  described  on  p.  129. 

The  corresponding  solution  of  alkali  arsenite  is  prepared  by 
dissolving  4 '948  gm.  of  the  purest  sublimed  arsenious  oxide  reduced 
to  powder  in  about  250  c.c.  of  distilled  water  in  a  flask,  with  about 
20  gm.  of  pure  sodium  carbonate.  J  See  note  p.  155. 

The  mixture  needs  warming  and  shaking  for  some  time  in  order 
to  complete  the  solution  ;  when  this  is  accomplished  the  mixture 
is  diluted  somewhat,  cooled,  then  made  up  to  the  litre. 

In  order  to  test  this  solution,  20  c.c.  are  put  into  a  beaker  with 
a  little  starch  indicator,  and  the  iodine  solution  allowed  to  flow  in 
from  a  burette,  graduated  in  T^  c.c.,  until  the  blue  colour  appears. 
If  exactly  20  c.c.  are  required,  the  solution  is  strictly  decinormal ; 

*  Properly  As-jOe-  t  Properly  As4Oe. 

J  In  a  former  edition  of  this  book,  the  arsenious  solution  was  recommended  to  be 
made  with  alkali  bicarbonate,  but  this  has,  after  keeping,  been  found  to  give 
defective  results  with  bleach  analyses  from  some  cause  not  yet  understood. 


140  ARSENIOUS    ACID    AND    IODINE. 

if  otherwise,  the  necessary  factor  must  be  found  for  converting  it 
to  that  strength. 

Iodized  Starch-paper.— Starch  solution  cannot  be  used  for  the 
direct  determination  of  free  chlorine,  consequently  resort  must  be 
had  to  an  external  indicator  ;  and  this  is  very  conveniently  found 
in  starch-iodide  paper,  which  is  best  prepared  by  mixing  a  portion 
of  starch  solution  with  a  few  drops  of  solution  of  potassium  iodide 
on  a  plate,  and  soaking  strips  of  pure  filtering  paper  therein.  The 
paper  so  prepared  is  used  in  the  damp  state,  and  is  far  more  sensitive 
than  when  dried. 


PRECIPITATION    ANALYSES.  141 


PART    IV. 

ANALYSIS    BY    PRECIPITATION. 

THE  general  principle  of  this  method  of  determining  the  quantity 
of  any  given  substance  is  alluded  to  on  p.  3,  and  in  all  instances 
is  such  that  the  body  to  be  determined  forms  an  insoluble  precipitate 
with  a  titrated  reagent.  The  end  of  the  reaction  is,  however, 
determined  in  three  ways. 

1 .  By  adding  the  reagent  until  no  further  precipitate  is  produced, 
as  in  the  determination  of  chlorine  by  silver. 

2.  By  adding  the  reagent  in  the  presence  of  an  indicator  con- 
tained either  in  the  liquid  itself,  or  brought  externally  in  contact 
with  it,  so  that  the  slightest  excess  of  the  reagent  shall  produce 
a  characteristic  reaction  with  the  indicator  ;  as  in  the  determination 
of  silver  with  sodium  chloride  by  the  aid  of  potassium  chromate, 
or  with  thiocyanate  and  ferric  sulphate,  or  that  of  phosphoric  acid 
with  uranium  by  the  aid  of  potassium  ferrocyanide  as  indicator. 

3.  By  adding  the  reagent  to  a  clear  solution  until  a  precipitate 
is  formed,  as  in  the  determination  of  cyanogen  by  silver. 

The  first  of  these  endings  can  only  be  applied  with  great  accuracy 
to  silver  and  chlorine  determinations.  Very  few  precipitates  have 
the  peculiar  quality  of  chloride  of  silver  ;  namely,  almost  perfect 
insolubility,  and  the  tendency  to  curdle  closely  by  shaking,  so  as 
to  leave  the  menstruum  clear.  Some  of  the  most  insoluble  pre- 
cipitates, such  as  barium  sulphate  and  calcium  oxalate,  are 
unfortunately  excluded  from  this  class,  because  their  finely  divided 
or  powdery  nature  prevents  their  ready  and  perfect  subsidence. 

In  all  these  cases,  therefore,  it  is  necessary  to  find  an  indicator, 
which  brings  them  into  class  2. 

The  third  class  comprises  only  two  processes  ;  viz.,  the  deter- 
mination of  cyanogen  by  silver,  and  that  of  chlorine  by  mercuric 
nitrate. 

Since  the  determination  of  chlorine  by  precipitation  with  silver, 
and  that  of  silver  by  thiocyanic  acid,  can  be  used  in  many  cases 
for  the  indirect  determination  of  many  other  substances  with  great 
exactness,  the  preparation  of  the  necessary  standard  solutions  will 
now  be  described. 


SILVER    AND    CHLORINE. 
1.     Decinormal  Solution  of  Silver. 

10*788  gm.  Ag  or  16'989  gm.  AgNO3  per  litre. 

10*788  gm.  of  pure  silver  are  dissolved  in  pure  dilute  nitric  acid 
with  gentle  heat  in  a  flask,  into  the  neck  of  which  a  small  funnel  is 


142  SILVER   AND    CHLORINE. 

dropped  to  prevent  loss  of  liquid  by  spirting.  When  solution  is 
complete,  the  funnel  must  be  washed  inside  and  out  with  distilled 
water  into  the  flask,  and  the  liquid  diluted  to  a  litre  ;  but  if  it  be 
desired  to  use  chromate  as  indicator  in  any  analysis,  the  solution 
must  be  neutral.  In  the  latter  case  the  solution  of  silver  in  nitric 
acid  is  evaporated  to  dryness,  and  the  residue  dissolved  in  a  litre  ; 
or,  what  is  preferable,  16*989  gm.  of  pure  crystallized  silver  nitrate, 
previously  heated  to  120°  C.  for  ten  minutes,  are  dissolved  in  a  litre 
of  distilled  water.  Fused  nitrate  of  silver  is,  however,  best  of  all 
for  this  purpose.  17' 1  grams  of  the  fused  salt  are  dissolved  in  a  litre 
of  distilled  water,  so  as  to  make  a  solution  rather  stronger  than  is 
required.  A  burette  is  then  filled  with  the  solution  and  it  is  titrated 
with  25  c.c.  of  Decinormal  Sodium  Chloride  Solution  in  a  white 
porcelain  dish,  using  potassium  chromate  as  indicator.  It  is  then 
diluted  with  water  to  exact  strength,  and  finally  tested  as  before. 

2.    Decinormal  Solution  of  Sodium  Chloride. 
5-846  gm.  NaCl  per  litre. 

5-846  gm.  of  pure  sodium  chloride  are  dissolved  in  distilled  water, 
and  the  solution  made  up  to  a  litre. 

There  are  two  methods  by  which  the  analysis  may  be  ended  : 

(a)  By  adding  silver  cautiously,  and  well  shaking  after  each 
addition  till  no  further  precipitate  is  produced.  For  details  see 
under  Silver  4. 

(6)  By  using  a  few  drops  of  solution  of  pure  potassium  chromate 
as  indicator,  as  devised  by  Mohr.  If  the  pure  salt  is  not  at  hand, 
some  drops  of  silver  nitrate  solution  should  be  added  to  the  solution 
of  the  ordinary  salt,  to  remove  chlorine,  and  the  clear  liquid  used. 

The  method  b  is  exceedingly  serviceable,  on  the  score  of  saving 
both  time  and  trouble.  The  solutions  must  be  neutral,  and  cold. 
When,  therefore,  acid  is  present  in  any  solution  to  be  examined,  it 
should  be  neutralized  with  pure  sodium  or  calcium  carbonate,  or 
the  latter  may  be  added  in  very  slight  excess.* 

METHOD  OF  PROCEDURE  :  To  the  neutral  or  faintly  alkaline  solution  two  or 
three  drops  of  a  cold  saturated  solution  of  chromate  are  added,  and  the  silver 
solution  delivered  from  the  burette  until  the  last  drop  or  two  produce  a  faint 
blood-red  tinge,  an  evidence  that  all  the  chlorine  has  combined  with  the  silver, 
and  the  slight  excess  has  formed  a  precipitate  of  silver  chromate ;  the  reaction  is 
very  delicate  and  easily  distinguished.  The  colour  reaction  is  even  more  easily 
seen  by  gas-light  than  by  daylight.  It  may  be  rendered  more  delicate  by  adopting 
the  plan  suggested  by  Dupre.f  A  glass  cell,  about  1  centimetre  in  depth,  is 
filled  with  water  tinted  with  chromate  to  the  same  colour  as  the  solution  to  be 
titrated.  The  operation  is  performed  in  a  white  porcelain  basin.  The  faintest 
appearance  of  the  red  change  is  at  once  detected  on  looking  through  the  coloured 
cell.  For  the  analysis  of  waters  weak  in  chlorine  this  method  is  very  serviceable, 
but,  contrary  to  what  has  been  generally  accepted,  the  accuracy  of  the  results 

*  Silver  chromate  is  sensibly  soluble  in  the  presence  of  alkali  or  alkaline  earthy 
nitrates,  especially  at  a  high  temperature ;  sodixim  and  calcium  nitrates  have  the  least 
effect ;  ammonium,  potassium,  and  magnesium  nitrates  the  greatest.  See  also  Forbes 
Carpenter  (J.S.C.I.  5,  286). 

^Analyst  5,  123. 


INDIRECT   DETERMINATIONS.  143 

is  seriously  interfered  with  by  great  dilution  or  high  temperature.*  It  is,  therefore, 
necessary,  as  is  the  case  with  most  volumetric  processes  in  order  to  secure  a 
high  degree  of  accuracy,  to  titrate  under  the  same  conditions  as  those  observed 
when  the  standard  was  fixed. 


INDIRECT  DETERMINATION  OF  AMMONIA,  SODA,  POTASH, 
LIME,  AND  OTHER  ALKALIES  AND  ALKALINE  EARTHS, 
WITH  THEIR  CARBONATES,  NITRATES,  AND  CHLORATES, 
ALSO  NITROGEN,  BY  MEANS  OF  DECINORMAL  SILVER 
SOLUTION  AND  POTASSIUM  CHROMATE  AS  INDICATOR. 

.     1  c.c.  N/10  silver  solution  =  TOUUO  H.  eq.  of  each  substance. 


MOHR  with  his  characteristic  ingenuity  has  made  use  of  the 
delicate  reaction  between  chlorine  and  silver,  with  potassium 
cbromate  as  indicator,  for  the  determination  of  the  bodies  mentioned 
above.  All  compounds  capable  of  being  converted  into  neutral 
chlorides  by  evaporation  to  dryness  with  hydrochloric  acid  may  be 
determined  with  great  accuracy.  The  chlorine  in  a  combined  state 
is,  of  course,  the  only  substance  actually  determined  ;  but  as  the 
laws  of  chemical  combination  are  exact  and  well  known,  the 
measure  of  chlorine  is  also  the  measure  of  the  base  with  which  it  is 
combined. 

In  most  cases  it  is  only  necessary  to  slightly  supersaturate  the 
alkali,  or  its  carbonate,  with  pure  hydrochloric  acid  ;  evaporate  on 
the  water  bath  to  dryness,  and  heat  for  a  time  to  120°  C.  in  the  air 
bath,  then  dissolve  to  a  given  measure,  and  take  a  portion  for 
titration  ;  too  great  dilution  must  be  avoided. 

Alkalies  and  Alkaline  Earths  combined  with  organic  acids  are 
ignited  to  convert  them  into  carbonates,  then  treated  with 
hydrochloric  acid,  and  evaporated  as  before  described. 

Carbonic  Acid  in  combination  may  be  determined  by  precipitation 
with  barium  chloride,  as  on  p.  96  et  seq.  The  washed  precipitate  is 
dissolved  on  the  filter  with  hydrochloric  acid  (covering  it  with  a 
watch-glass  to  prevent  loss),  and  then  evaporated  to  dryness 
repeatedly  till  all  HC1  is  driven  off.  In  order  to  titrate  with  accuracy 
by  the  help  of  chromate,  the  baryta  must  be  precipitated  by  means 
of  a  solution  of  pure  sodium  or  potassium  sulphate  in  slight  excess  ; 
the  precipitated  barium  sulphate  does  not  interfere  with  the  delicacy 
of  the  reaction.  If  this  precaution  were  not  taken,  the  yellow 
barium  chromate  would  mislead. 

Free  Carbonic  Acid  is  collected  by  means  of  ammonia  and  barium 
chloride  (as  on  p.  97),  and  the  determination  completed  as  in  the 
case  of  combined  CO2. 

Chlorates  are  converted  into  chlorides  by  ignition  before  titration. 

*  W.  G.  Young,  Analyst  18,  125. 


144  INDIRECT   DETERMINATIONS. 

Nitrates  are  evaporated  with  concentrated  hydrochloric  acid  and 
the  resulting  chlorides  titrated,  as  in  the  previous  case. 

Nitrogen. — The  ammonia  evolved  from  guano,  manures,  oilcakes, 
and  sundry  other  substances,  when  burned  with  soda  lime  or 
obtained  by  the  Kjeldahl  method,  is  conducted  into  dilute 
hydrochloric  acid  ;  the  liquid  is  carefully  evaporated  to  dry  ness 
before  titration. 

In  all  cases  the  operator  will,  of  course,  take  care  that  no  chlorine 
from  extraneous  sources  other  than  the  hydrochloric  acid  is  present ; 
or  if  it  exists  in  the  bodies  themselves  as  an  impurity,  its  quantity 
must  be  first  determined. 

EXAMPLE  :  0-25  gm.  pure  sodium  carbonate  was  dissolved  in  water,  and 
hydrochloric  acid  added  till  in  excess ;  it  was  then  dried  on  the  water  bath  till  no 
further  vapours  of  acid  were  evolved  ;  the  resulting  white  mass  was  heated  for 
a  few  minutes  to  about  150°  C.,  dissolved  and  made  up  to  300  c.c.  100  c.c. 
required  15'7  c.c  N/io  silver,  this  multiplied  by  3  gave  47'1  c.c.  which  multiplied 
by  the  N/io  factor  for  sodium  carbonate  (  =0-0053)  gave  0*2496  gm.  instead  of 
0-25  gm. 

Indirect  Determination  of  Potassium  and  Sodium  existing  as 
Mixed  Chlorides. — It  is  a  problem  of  frequent  occurrence  to  deter- 
mine the  relative  quantities  of  potassium  and  sodium  existing  in 
mixtures  of  the  two  chlorides,  such  as  occur,  for  instance,  in  urine, 
manures,  soils,  waters,  etc.  The  actual  separation  of  potash  from 
soda  by  means  of  platinum  is  tedious,  and  not  always  satisfactory. 

The  following  method  of  calculation  is  frequently  convenient, 
since  a  careful  determination  of  the  chlorine  present  in  the  mixture 
is  the  only  labour  required  ;  and  this  can  most  readily  be  accom- 
plished by  N/10  silver  solution  and  chromate,  als  previously 
described. 

(1)  The  weight  of  the  mixed  pure  chlorides  is  accurately  found  and  noted. 

(2)  The  chlorides  are  then  dissolved  in  water,  and  very  carefully  titrated 
with  N/io  silver  and  chromate  for  the  amount  of  chlorine  present,  which    is   also 
recorded  ;  the  calculation  is  then  as  follows  : — 

The  weight  of  chlorine  is  multiplied  by  the  factor  2-103  ;  from  the  product 
so  obtained  is  deducted  the  weight  of  the  mixed  chlorides  found  in  (1).  The 
remainder  multiplied  by  3*6305  will  give  the  weight  of  sodium  chloride  present  in 
the  mixture. 

The  weight  of 'sodium  chloride  deducted  from  the  total  as  found  in  (1)  will  give 
the  weight  of  potassium  chloride. 

Sodium  chloride         x     0'5303  =Soda  (Na20). 

Potassium  chloride     x     0-6317  =  Potash  (K20). 

The  principle  of  the  calculation,  which  is  based  on  the  atomic  constitution  of  the 
individual  chlorides,  is  explained  in  most  of  the  standard  works  on  general 
analysis.  Indirect  methods  like  this  can  only  give  useful  results  when  the 
atomic  weights  of  the  two  substances  differ  considerably,  and  when  the  proportions 
are  approximately  equal. 

Another  method  of  calculation  in  the  case  of  mixed  potassium 
and  sodium  chlorides  is  as  follows  : — 

The  weight  of  the  mixture  is  first  ascertained  and  noted  ;  the  chlorine  is  then 
found  by  titration  with  N/io  silver,  and  calculated  to  NaCl ;  the  weight  so  obtained 
is  deducted  from  the  original  weight  of  the  mixture,  and  the  remainder  multiplied 
by  2*42857  will  give  the  potassium. 


METHOD.  145 

SILVER    AND    THIOCYANIC    ACID. 

THIS  excellent  and  most  accurate  method  has  been  devised  by 
V  o  1  h  a  r  d*  and  fully  described  by  the  author,  and  has  been  favourably 
noticed  by  many  other  well  known  chemists.  It  differs  from 
Mohr's  chromate  method  in  that  the  silver  solutions  may  contain 
free  nitric  acid,  which  renders  it  of  great  service  in  indirect  analyses. 

This  method  is  based  on  the  fact  that  when  solutions  of  silver  and 
an  alkali  thiocyanate  are  mixed  in  the  presence  of  a  ferric  salt,  so 
long  as  silver  is  in  excess  the  thiocyanate  of  that  metal  is  precipitated, 
and  any  brown  ferric  thiocyanate  which  may  form  is  at  once  decom- 
posed. When,  however,  the  thiocyanate  is  added  in  the  slightest 
excess,  brown  ferric  thiocyanate  is  formed,  and  asserts  its  colour 
even  in  the  presence  of  much  free  acid.  The  method  may,  of  course, 
be  used  for  the  determination  of  silver,  and,  by  the  residual  process, 
for  the  determination  of  substances  which  are  completely  pre- 
cipitated by  silver.  In  cases  where  chlorine  is  precipitated  by 
excess  of  silver,  and  the  excess  has  to  be  found  by  thiocyanate, 
experience  has  proved  that  it  is  absolutely  necessary  to  filter  off 
the  chloride  and  titrate  the  filtrate  and  washings.  If  this  be  not 
done  the  solvent  effect  of  the  thiocyanate  upon  the  AgCl  will  give 
inaccurate  results.  This  fact  seems  to  have  been  overlooked  at 
the  time  the  method  was  first  introduced. 

It  may  be  used  for  the  determination  of  silver  in  the  presence  of 
copper  up  to  70  per  cent.  ;  also  in  presence  of  antimony,  arsenic, 
iron,  zinc,  manganese,  lead,  cadmium,  bismuth,  and  also  cobalt  and 
nickel,  unless  the  proportion  of  these  latter  metals  is  such  as  to 
interfere  by  intensity  of  colour. 

It  may  further  be  used  for  the  indirect  determination  of  chlorine, 
bromine,  and  iodine,  in  presence  of  each  other,  existing  either  in 
minerals  or  inorganic  compounds,  and  for  copper,  manganese,  and 
zinc  ;  these  will  be  noticed  under  their  respective  heads. 

1.     Decinormal  Ammonium  or  Potassium  Thiocyanate. 

This  solution  cannot  be  prepared  by  weighing  the  thiocyanate 
direct,  owing  to  the  deliquescent  nature  of  the  salts  ;  therefore 
about  8  gm.  of  the  ammonium,  or  10  gm.  of  the  potassium,  salt  may 
be  dissolved  in  about  a  litre  of  water  as  a  basis  for  getting  an  exact 
solution,  which  must  be  finally  adjusted  by  a  correct  decinormal 
silver  solution. 

The  standard  solution  so  prepared  remains  of  the  same  strength 
for  a  very  long  period  if  preserved  from  evaporation. 

2.    Decinormal  Silver  Solution. 

This  is  the  same  as  described  in  a  preceding  section  (p.  141),  and 
may  contain  free  nitric  acid  if  made  direct  from  metallic  silver. 

*  Liebig's  Ann.  d.  Chem.  190,  1. 


146  VOLHARD'S  METHOD. 

3.    Ferric  Indicator. 

This  may  consist  simply  of  a  saturated  solution  of  iron  alum ; 
or  may  be  made  by  oxidizing  ferrous  sulphate  with  nitric  acid, 
evaporating  with  excess  of  sulphuric  acid  to  dissipate  nitrous  fumes, 
and  dissolving  the  residue  in  water  so  that  the  strength  is  about 
10  per  cent. 

5  c.c.  of  either  of  these  solutions  are  used  for  each  titration, 
which  must  always  take  place  at  ordinary  temperatures. 


4.    Pure  Nitric  Acid. 

This  must  be  free  from  the  lower  oxides  of  nitrogen,  secured  by 
diluting  the  usual  pure  acid  with  about  a  fourth  part  of  water,  and 
boiling  till  perfectly  colourless,  It  should  then  be  preserved  in  the 
dark. 

The  quantity  of  nitric  acid  used  in  the  titration  may  vary  within 
wide  limits,  and  seems  to  have  no  effect  upon  the  precision  of  the 
method. 

EXAMPLE  :  50  c.c.  of  N/1O  silver  solution  are  measured  into  a  flask,  diluted 
somewhat  with  water,  and  5  c.c.  of  ferric  indicator  added,  together  with  about 
10  c.c.  of  nitric  acid.  If  the  iron  solution  should  cause  a  yellow  colour,  the 
nitric  acid  will  remove  it.  The  thiocyanate  is  then  delivered  in  from  a  burette  ; 
at  first  a  white  precipitate  is  produced  rendering  the  fluid  of  a  milky  appearance, 
and  as  each  drop  of  thiocyanate  falls  in,  it  produces  a  reddish-brown  cloud  which 
quickly  disappears  on  shaking.  As  the  point  of  saturation  approaches,  the 
precipitate  becomes  flocculent  and  settles  easily ;  finally,  a  drop  or  two  of 
thiocyanate  produces  a  faint  brown  colour  which  no  longer  disappears  on  shaking. 
If  the  solutions  are  correctly  balanced,  exactly  50  c.c.  of  thiocyanate  should  be 
required  to  produce  this  effect. 

The  colour  is  best  seen  by  holding  the  flask  so  as  to  catch  the  reflected  light 
of  a  white  wall  or  a  suspended  sheet  of  white  paper. 


Precision  in  Colour  Reactions 

DUPR£*  adopts  the  following  ingenious  method  for  colour  titrations : — 
As  is  well  known,  the  change  from  pale  yellow  to  red,  in  the  titration  of  chlorides 
by  means  of  silver  nitrate  with  neutral  chromate  as  indicator,  is  more  distinctly 
perceived  by  gas-light  than  by  daylight;  and  in  the  case  of  potable  waters, 
containing  from  one  to  two  grains  of  chlorine  per  gallon,  it  is  absolutely  necessary 
to  concentrate  by  evaporation  previous  to  titration,  or  else  to  perform  the  titration 
by  gas  or  electric  light.  The  adoption  of  the  following  simple  plan  enables  the 
operator  to  perceive  the  change  of  colour  as  sharply,  and  with  as  great  a  certainty, 
by  daylight  as  by  artificial  light.  Nevertheless,  as  has  been  before  mentioned, 
it  is  impossible  to  get  accurate  results  with  very  weak  solutions  of  chlorine  unless 
the  silver  solution  is  standardized  upon  similar  solutions. 

The  water  is  measured  into  a  white  porcelain  dish  (100  c.c.  are  a  useful  quantity), 
a  moderate  amount  of  neutral  chromate  is  added  (sufficient  to  impart  a  marked 
yellow  colour  to  the  water),  but,  instead  of  looking  at  the  water  directly,  a  flat 
glass  cell  containing  some  of  the  neutral  chromate  solution  is  interposed  between 
the  eye  and  the  dish.  The  effect  of  this  is  to  neutralize  the  yellow  tint  of  the 
water ;  or,  in  other  words,  if  the  concentration  of  the  solution  "in  the  cell  is  even 
moderately  fairly  adjusted  to  the  depth  of  tint  imparted  to  the-  water,  the 

*  Analyst  5,  123. 


COLOUR  REACTIONS.  147 

appearance  of  the  latter,  looked  at  through  the  cell,  is  the  same  as  if  the  dish 
were  filled  with  pure  water.  If  now  the  standard  silver  solution  is  run  in,  still 
looking  through  the  cell,  the  first  faint  appearance  of  a  red  coloration  becomes 
strikingly  manifest ;  and  what  is  more,  when  once  the  correct  point  has  been 
reached  the  eye  is  never  left  in  doubt,  however  long  we  may  be  looking  at  the 
water.  A  check  experiment  in  which  the  water,  with  just  a  slight  deficiency  of 
silver,  or  excess  of  chloride,  is  used  for  comparison  is  therefore  unnecessary. 

A  similar  plan  will  be  found  useful  in  other  titrations.  Thus,  in  the  case  of 
turmeric,  the  change  from  yellow  to  brown  is  perceived  more  sharply  and  with 
greater  certainty  when  looking  through  a  flat  cell  containing  tincture  of  turmeric 
of  suitable  concentration  than  with  the  naked  eye.  The  liquid  to  be  titrated 
should,  as  in  the  former  case,  be  placed  in  a  white  porcelain  dish.  Again,  in 
determining  the  amount  of  carbonate  of  lime  in  a  water  by  means  of  decinormal 
acid  and  cochineal,  the  exact  point  of  neutrality  can  be  more  sharply  fixed  by 
looking  through  the  cell  filled  with  a  cochineal  solution.  In  this  case  the 
following  plan  is  found  to  answer  best.  The  water  to  be  tested — about  250  c.c. — 
is  measured  into  a  flat  porcelain  evaporating  dish,  part  of  which  is  covered 
over  with  a  white  porcelain  plate.  The  water  is  now  tinted  with  cochineal  as 
usual,  and  the  sulphuric  acid  run  in,  the  operator  looking  at  the  dish  through 
the  cell  containing  the  neutral  cochineal  solution.  At  first  the  tint  of  the  water 
arid  the  tint  in  which  the  porcelain  plate  is  seen  are  widely  different ;  as,  however, 
the  carbonate  becomes  gradually  neutralized,  the  two  tints  approach  each  other 
more  and  more,  and  when  neutrality  is  reached  they  appear  identical ;  assuming 
that  the  strength  of  the  cochineal  solution  in  the  cell,  and  the  amount  of  this 
solution  added  to  the  water,  have  been  fairly  well  matched.  Working  in  this 
manner  it  is  not  difficult  (taking  £  litre  of  water)  to  come  within  O'l  c.c.  of 
decinormal  acid  in  two  successive  experiments,  and  the  difference  need  never 
exceed  O2  c.c.  In  the  cell  employed  the  two  glass  plates  are  a  little  less  than 
half  an  inch  apart. 

A  somewhat  similar  plan  may  be  found  useful  in  other  titrations,  or,  in  fact, 
in  many  operations  depending  on  the  perception  of  colour  change. 


L    2 


148  ALUMINIUM. 


PART    V. 

APPLICATION    OF    THE    FOREGOING    PRINCIPLES    OF 
ANALYSIS    TO    SPECIAL    SUBSTANCES. 

ALUMINIUM. 
Al  =  27'l. 

ALUMINIUM  salts  (the  alums  and  aluminium  sulphates  used  in 
dyeing  and  paper-making)  may  be  titrated  for  alumina  in  the 
absence  of  iron  (except  for  mere  traces)  by  mixing  the  acid  solutions 
with  a  tolerable  quantity  of  sodium  acetate,  then  a  known  volume 
in  excess  of  N/10  phosphate  solution  (20'9  gm.  of  ammonio-sodium 
phosphate  per  litre),  heating  to  boiling,  without  filtration  ;  the 
excess  of  phosphate  is  found  at  once  by  titration  with  standard 
uranium.  If  iron  in  any  quantity  is  present,  it  may  be  determined 
in  a  separate  portion  of  the  substance,  and  its  amount  deducted 
before  calculating  the  alumina.  The  latter  is  precipitated  as 
A1PO4,  and  any  iron  in  like  manner  as  FePO4.  Each  c.c.  of  N/10 
phosphate =0-00513  gm.  A12O3.  This  method  is  only  available 
for  rough  purposes. 

Baeyer's  Method. — As  originally  proposed,  this  process  for 
determining  alumina  in  alums  and  aluminic  sulphates  was  carried 
out  by  two  titrations,  a  measured  portion  of  the  solution  being 
first  treated  with  an  excess  of  normal  soda  sufficient  to  dissolve  the 
precipitate  of  hydrate  of  alumina  first  formed.  It  was  then  diluted 
to  a  definite  volume,  one  half  being  titrated  with  normal  acid  and 
litmus,  the  other  half  with  tropoeolin  OO,  the  difference  being 
calculated  to  alumina. 

A  considerable  improvement,  however,  has  been  made  by  using 
phenolphthalein  as  the  indicator,  one  titration  only  being 
necessary.  The  method  is  based  on  the  fact  that  if  to  a  solution 
of  alumina,  containing  the  indicator,  normal  soda  is  added  in 
excess,  or  until  the  red  colour  is  produced,  and  normal  acid  be  then 
added  until  the  colour  disappears,  the  volume  of  acid  so  required 
is  less  than  the  soda  originally  added  in  proportion  to  the  quantity 
of  alumina  present. 

The  volume  of  acid  which  so  disappears  is  in  reality  the  quantity 
necessary  to  combine  with  the  alumina  set  free  by  the  alkali  ;  and 


ALUMINIUM.  149 

if  this  deficient  measure  of  acid  be  multiplied  by  the  factor  0'01716 
(J  mol.  wt.  of  A1203),  the  weight  of  alumina  will  be  obtained.  This 
factor  is  given  on  the  assumption  that  the  normal  sulphate  A123SO4, 
is  formed. 

The  titration  must  take  place  in  the  cold  and  in  dilute  solutions. 
Very  fair  technical  results  have  been  obtained  by  me  with  potash 
and  ammonia  alums  and  the  commercial  sulphates  of  alumina. 

Alumina  existing  as  aluminate  of  alkali  in  caustic  soda,  for 
instance,  may  be  very  well  determined  by  taking  advantage  of  the 
fact  that  such  alumina  is  quite  indifferent  to  methyl  orange,  but 
reacts  acid  with  phenolphthalein.  This  fact  has  been  recorded  by 
Thomson  and  others,  but  the  priority  of  discovery  appears  to  be 
due  to  Baeyer,*  who,  however,  used  litmus  in  the  place  of 
phenolphthalein  and  tropoeolin  00  instead  of  methyl  orange. 

Cross  and  Bevanf  in  their  examination  of  caustic  soda  for 
alumina  found  by  experiment  that  the  mean  of  the  difference 
between  the  titration  with  methyl  orange  and  that  with  phenolph- 
thalein required  the  factor  O0205  per  c.c.  of  normal  acid  for  the 
alumina,  pointing  to  the  salt  as  2A12O3  :  5S03. 

The  determination  of  the  alumina  in  caustic  soda  has  given  rise 
to  much  discussion  between  even  very  experienced  operators, 
notably  Cross  and  Be  van  and  Lunge,  but  the  former  chemists 
have  proved,  as  far  as  possible  by  various  methods,  the  accuracy 
of  their  views  that  the  above-named  equation  is  correct.  The 
method  adopted  by  them  consists  in  boiling  the  weighed  sample 
with  a  slight  excess  of  standard  acid,  allowing  to  cool  and  titrating 
back  with  standard  soda  and  phenolphthalein.  The  acid  so  con- 
sumed represents  the  total  alkali  present.  To  a  similar  portion 
a  slight  excess  of  acid  is  added  and  titrated  back  with  soda  and 
methyl  orange. 

Determination  of  free  Acid. — Alum  cakes  or  aluminic  sulphates  of 
various  kinds  often  contain  free  H2SO4,  and  many  methods  have 
been  proposed  for  its  determination.  Baeyer  titrates  a  10  per 
cent,  solution  of  the  substance  in  water  with  normal  soda  and 
tropceolin  OO  or  methyl  orange. 

R,.  William sj  adopts  the  alcohol  method,  digesting  the  substance 
for  at  least  twelve  hours  with  strong  alcohol,  filtering  off  and 
washing  with  the  same,  and  titrating  the  solution  without  dilution 
or  evaporation  with  N/10  acid  and  phenolphthalein. 

Beilstein  and  Gross et||  have  examined  with  great  care  all  the 
methods  proposed  for  this  purpose,  and  have  devised  one  which 
gives  very  good  technical  results. 

METHOD  OF  PROCEDURE  :  1  to  2  gm.  of  substance  is  dissolved  in  5  c.c.  of  water, 
5  c.c.  of  a  cold  saturated  neutral  solution  of  ammonium  sulphate  added,  and 
stirred  for  a  quarter  of  an  hour.  50  c.c.  of  95  per  cent,  alcohol  are  then  added, 

*  Z.  a.  C.  24,  542.  f  J-  8.  C.  I.  8,  252.  J  C.  N.  56,  194. 

\\Butt.  de  V Academic  Imp-  des  Sciences  de  St.  Petersburg,  13,  41. 


150  ALUMINIUM. 

the  mixture  thrown  on  a  small  filter,  and  washed  with  50  c.c.  of  the  same  alcohol. 
The  filtrate  is  evaporated  on  the  water  bath,  the  residue  dissolved  in  water,  and 
titrated  with  N/iO  alkali  and  litmus.  The  whole  of  the  neutral  aluminic 
sulphate  is  precipitated  as  ammonia  alum,  the  alcohol  contains  all  the  free  acid. 

A.  H.  White*  has  proposed  another  method  of  determining 
aluminium  sulphates  :  the  summary  is  as  follows  : — 

If  a  solution  of  an  alum  to  which  has  been  added  neutral  potassium  sodium 
tartrate  (Rochelle  salt)  is  titrated  with  barium  hydroxide,  the  barium  hydroxide 
used  will  correspond  to  the  sulphuric  acid  combined  with  the  alumina  plus  the 
free  acid.  The  sulphuric  acid  combined  with  sodium  or  potassium  is  not 
determined.  If  a  duplicate  solution  of  alum  is  evaporated  to  dryness,  re-dissolved 
in  neutral  sodium  citrate,  and  titrated  with  barium  hydroxide,  a  smaller  quantity 
of  barium  hydroxide  is  required,  and  the  difference  between  the  amounts  of 
barium  hydroxide  used  in  the  two  titrations  is  equivalent  to  one-third  of  the 
alumina.  From  these  two  titrations  can  be  calculated  the  alumina  and  the 
sulphuric  acid  combined  with  it,  whether  the  alum  be  basic  or  acid,  and  if  the 
alum  is  acid  the  excess  of  acid  over  that  necessary  to  form  the  normal  sulphate. 
Commercial  aluminium  sulphate  may,  in  its  solid  state,  carry  free  acid,  although 
in  the  solution  such  uncombined  acid  may  disappear,  combining  with  what  had 
been  basic  portions  of  the  solid  salt.  Such  free  acid  may  be  determined  by  dis- 
solving the  solid  salt  directly  in  citrate,  and  titrating  with  barium  hydroxide  at 
once.  This  method  gives  results  closely  'concordant  with  Beilstein  and 
Grosset's  method,  but  it  does  not  show  that  the  alum  contains  more  acid  than 
is  sufficient  to  form  with  the  alumina  the  normal  salt. 

lodimetric  Method  of  A.  Stock  .f — Reagents  required  :  A 
mixture  of  equal  parts  of  a  25  %  solution  of  potassium  iodide  and 
a  saturated  solution  of  iodate  (containing  6-7  %  of  the  salt). 
Standard  sodium  thiosulphate  (20  %  solution). 

When  a  mixture  of  potassium  iodide  and  iodate  is  added  to 
a  solution  of  an  aluminium  salt,  a  precipitate  of  aluminium  hydrate 
is  formed  and  a  quantity  of  iodine  set  it ee  according  to  the  following 
equation  : — 

A12  (S04)3+5  KI+KI03  +  3  H20  =  2  Al(OH)3+3  K2S04  +  3  I2. 

The  reaction,  although  commencing  rapidly  in  the  cold,  is  not 
complete  for  some  days,  especially  in  dilute  solutions.  The  rapidity 
is  increased  if  the  liberated  iodine  be  removed  by  means  of  standard 
thiosulphate,  especially  when  warmed.  By  heating  the  solution 
on  a  water-bath  the  reaction  is  complete  in  a  few  minutes,  even  in 
the  case  of  very  dilute  solutions..  The  process  is  not  available  in 
a  solution  containing  tartaric,  oxalic,  or  phosphoric  acid,  but  boric 
acid  does  not  appear  to  interfere. 

METHOD  OF  PROCERURE  :  The  solution  is  first  neutralized  with  sodium  hydrate, 
as  it  must  be  neither  too  acid  nor  alkaline,  then  some  of  the  iodide  and  iodate 
reagent  added.  After  five  minutes  the  sodium  thiosulphate  solution  is  run  in 
from  a  burette  until  the  solution  becomes  decolourized,  then  a  further  quantity 
of  the,  iodide  reagent  added  to  make  sure  of  complete  precipitation.  After 
heating  on  a  water-bath  for  half  an  hour,  the  flocculent  precipitate  is  filtered  off 
and  the  titration  of  the  filtrate  with  standard  thiosulphate  completed. 

*  J.  Am.  C.  S.  24,  457. 
t  Compt.  rend.  130  [4]  175,  and  J.  S.  C.  I,,  1900, 19,  276. 


ANTIMONY.  151 

ANTIMONY. 

Sb  =  120-2. 

1.     Conversion  of  Antimonious  Acid  in  Alkaline  Solution  into 
Antimonic  Acid  by  Iodine  (Mohr). 

ANTIMONIOUS  oxide,  or  any  of  its  compounds,  is  brought  into 
solution  as  tartrate  by  tartaric  acid  and  water ;  the  excess  of  acid 
neutralized  by  sodium  carbonate  ;  then  a  cold  saturated  solution  of 
sodium  bicarbonate  added,  in  the  proportion  of  10  c.c.  to  about 
O'l  gm.  SbjjOg ;  to  the  clear  solution  starch  is  added  and  N/10  iodine 
until  the  blue  colour  appears.  No  delay  must  occur  in  the  titration 
when  the  bicarbonate  is  added,  otherwise  a  portion  of  the  metal  is 
precipitated  as  antimonious  hydrate,  upon  which  the  iodine  has 
little  effect.  Duns  tan  and  Boole*  have  proved  that  the  accurate 
determination  of  the  antimony  in  tartar  emetic  may  be  secured  by 
this  method,  using  the  precautions  mentioned. 

For  the  determination  of  antimonic  acid  and  its  salts,  G.  von 
Knorref  gives  the  following  method  as  accurate  : — 

METHOD  OF  PROCEDURE  :  The  solution  of  the  salt,  strongly  acidified  with 
hydrochloric  acid,  is  treated  in  a  roomy  flask  with  strong  solution  of  sodium 
sulphite,  added  gradually  in  small  portions.  It  is  then  vigorously  boiled  until 
all  S02  is  expelled,  a  drop  of  phenolphthalein  is  added,  then  caustic  potash  until 
red ;  this  is  in  turn  removed  by  a  small  excess  of  tartaric  acid.  Sodium  bicar- 
bonate is  then  added,  and  the  titration  with  iodine  carried  out  in  the  usual  way. 

The  colour  disappears  after  a  little  time,  therefore  the  first 
appearance  of  a  reddish  tint  with  starch  is  accepted  as  the  true 
measure  of  iodine  required.  Greater  accuracy  is  attained  by 
adding  an  excess  of  1  c.c.  of  .N/100  iodine  and  titrating  back  with 
thiosulphate. 

1  c.c.  N/10  iodine =0*006  gm.  Sb. 

In  the  case  of  commercial  oxides  of  antimony,  1  gm.  of  material  is  dissolved 
iii  10  c.c.  of  strong  HC1  and  gaseous  H2S  passed  through  it  to  remove  As.  The 
solution  is  put  into  a  250  c.c.  flask,  the  beaker  being  rinsed  with  strong  .HC1  and 
an  equal  volume  of  water.  All  H2S  is  removed  by  a  current  of  air.  5  gm.  of 
tartaric  acid  are  then  added,  the  liquid  diluted  to  the  mark  with  water,  and 
a  portion  filtered  through  a  dry  filter.  25  c.c.  are  taken  and  neutralized  with 
sodium  bicarbonate,  a  pinch  of  the  latter  together  with  starch  is  then  added,  and 
the  mixture  titrated  with  N/iO  iodine. 

In  the  case  of  sulphides  1-6  gm.  is  dissolved  in  hot,  strong  HC1,  and  when 
perfectly  cold  treated  with  H2S,  and  the  titration  carried  out  as  before. 

Determination  of  Antimony  in  presence  of  Tin  (Type  and 
Britannia  metal,  etc.).J — The  finely  divided  alloy  is  dissolved  in 
strong  hydrochloric  acid  by  heat,  adding  frequently  small  quantities 
of  potassium  chlorate.  The  liquid  is  boiled  to  remove  free  chlorine, 
cooled,  a  slight  excess  of  strong  solution  of  potassium  iodide  added, 
and  the  liberated  iodine  determined  by  standard  thiosulphate. 

*  Pharm.  Jour.,  Nov.,  1888.  \Zeii.  angew.  Chem.,  1888,  155 

JSeealsoH.  Yockey,./.^.  C.  8.  1906,  1435. 


152  ANTIMONY. 

Mohr's  process  for  the  determination  of  antimonious  oxide  is 
both  convenient  and  exact.  It  depends  on  the  reaction 

Sb2034-2I,+2H20=Sb205+4HI. 

which  takes  place  in  a  solution  containing  an  excess  of  alkaH 
bicarbonate.  The  above  equation  shows  that  120*2  parts  of 
antimony  (as  Sb2O3)  are  equivalent  to  253*84  parts  Iodine,  and  the 
weight  of  Iodine  found  multiplied  by  0*4735  =  Sb. 

Clark*  has  made  experiments  as  to  the  value  of  this  process 
in  antimony  ores  and  alloys,  and  makes  the  following  remarks  with 
respect  to  them. 

Mohr's  process  leaves  nothing  to  be  desired  in  point  of  sharpness  and 
accuracy  v  and  the  chief  object  of  my  experiments  has  been  to  ascertain  the  best 
condition  for  the  application  of  this  method  to  the  determination  of  antimony  in 
ores  and  metals.  I  have  proved  by  experiments  that  the  presence  of  lead,  even 
in  large  quantity,  has  no  influence  on  the  result,  but  the  process  is  affected  by 
iron,  and  by  copper,  arsenic;  and  tin  in  the  lower  state  of  oxidation.  The 
following  is  a  short  summary  of  my  results  : — 

I.  In  the  case  of  pure  antimony  ores  practically  free  from  arsenic  and  iron, 
the  ore  may  be  dissolved  in  HC1,  heated  till  all  the  H2S  has  been  driven  off,  then 
mixed  with  tartaric  acid  or  Rochelle  salt,  rendered  alkaline  by  bicarbonate  of 
soda,  and  titrated  with  N/io  iodine  solution,  as  recommended  by  Mohr.     1  gm. 
antimony  ore  containing  traces  of  Fe  gave  antimony  46*77  per  cent.     Another 
1  gm.  antimony  ore  containing  traces  of  Fe  gave  antimony  46 '80  per  cent. 

II.  When  the  ore  contains  more  than  traces  of  iron,  it  is  dissolved  in  HC1, 
precipitated  with  H2S,  nitrated,  washed,  re-dissolved  in  HC1,  and  the  antimony 
titrated  in  an  alkaline  tartrate  solution  as  before. 

III.  When  the  ore  contains  arsenic,  which  is  by  no  means  a  rare  occurrence, 
it  is  dissolved  in  strong  HC1  containing  sufficient  feme  chloride  to  decompose  the 
sulphides,  and  the  arsenic  is  distilled  off ;  the  antimony  is  then  precipitated  with 
H2S,  filtered,  washed,  re-dissolved  in  HC1,  and  titrated  with  N/io  iodine  in  an 
alkaline  tartrate  solution.     The  arsenic  in  the  distillate  can  also  be  titrated  with 
iodine  in  presence  of  excess  of  bicarbonate  of  soda. 

IV.  When  an  alloy  or  sulphide  contains  tin  as  well  as  arsenic  and  antimony, 
it  may  be  dissolved  in  HC1  and  FegClg,  the  arsenic  distilled  off  as  before,  and  the 
antimony  precipitated  with  metallic  iron  (which  should  be  as  pure  as  possible, 
steel  filings  answer  well.)     The  precipitated  antimony,  after  being  filtered  and 
washed,  is  then  dissolved  in  HC1  with  the  assistance  of  a  little  chlorate  of  potash, 
filtered  from  any  insoluble  impurity  derived  from  the  iron,  precipitated  with  H2S, 
filtered,  washed,  dissolved  in  HC1,  boiled  to  expel  H2S,  and  titrated  with  N/io 
iodine  in  an  alkaline  tartrate  solution. 

The  antimony  ore  referred  to  above,  when  treated  in  this  way,  gave  the 
following  results : — 

1  gm.  gave  antimony  46'62  per  cent.  Another  1  gm.  gave  antimony  46-68  per 
cent. 

Clark  has  also  discovered  a  modified  iodimetric  process  which 
may  be  used  where  the  original  process  is  inadmissible. 

When  antimony  is  dissolved  in  HC1  with  the  assistance  of  chlorate  of  potash, 
nitric  acid,  or  bromine,  the  oxidizing  agent  converts  the  antimony  into  the 
highest  state  of  oxidation,  on  which  account  it  is  necessary  to  reduce  it  again 
to  render  it  suitable  for  the  application  of  M  o  h  r '  s  process.  It  has  been  discovered, 
however,  that  when  antimony  is  dissolved  in  HC1  with  the  assistance  of  iodine, 
no  reducing  agent  is  required,  as  iodine  in  an  acid  solution  does  not  oxidize 
antimony  beyond  Sb»O3,  so  that  after  boiling  off  the  excess  of  iodine  Mohr's 
process  can  be  at  once  applied  to  the  solution. 

*  J.  S.  C.  I.  15,  255. 


ANTIMONY.  153 

This  action  of  iodine  is  of  very  great  importance,  as  it  simplifies  very  much 
the  determination  of  antimony  in  alloys  containing  lead  and  tin,  as  the  tin  is  oxidized 
by  the  iodine  to  the  stannic  state,  and  the  lead  has  no  influence  on  the  result.  In 
applying  the  process,  a  weighed  quantity  of  the  alloy  is  treated  with  HC1  so  long 
as  there  is  any  action,  then  solid  iodine  is  added  in  small  quantities  at  a  time,  and 
heat  applied  till  complete  solution  has  taken  place.  The  excess  of  iodine  is 
removed  by  boiling,  and  the  solution  cooled,  diluted,  and  mixed  with  a  little 
starch.  Should  the  addition  of  starch  produce  a  blue  colour  in  the  acid  solution 
owing  to  the  presence  of  a  trace  of  free  iodine,  a  very  weak  solution  of  sulphite  of 
sodium  is  added  drop  by  drop  till  the  blue  colour  disappears.  It  is  then  mixed 
with  Rochelle  salt,  rendered  alkaline  with  sodium  carbonate  and  titrated  with  N/io 
iodine  in  the  presence  of  a  considerable  excess  of  sodium  bicarbonate. 

In  this  way  good  results  were  obtained  with  white  metal  and 
alloys  containing  large  proportions  of  lead  ;  but  if  copper  is  present 
the  result  is  too  low.  and  the  copper  should  be  removed  by  con- 
verting the  metals  into  sulphides,  and  dissolving  out  the  antimony 
by  caustic  soda  or  potash. 

2.    Oxidation  by  Potassium  Bichromate  or 
Permanganate  (Kessler). 

5SbCl3  + 1 6HC1 + 2KMnO4  =  5SbCl5  +  2KC1 + 2MnCl2  +  8H2O. 

Bichromate  or  permanganate  added  to  a  solution  of  antimonious 
chloride,  containing  not  less  than  J  of  its  volume  of  hydrochloric 
acid  (sp.  gr.  1'12),  converts  it  into  antimonic  chloride. 

The  reaction  is  uniform  only  when  the  minimum  quantity  of 
acid  indicated  above  is  present,  but  it  ought  not  to  exceed  J  the 
volume,  and  the  precautions  before  given  as  to  the  action  of 
hydrochloric  acid  on  permanganate  must  be  taken  into  account, 
hence  it  is  preferable  to  use  dichromate. 

Kessler*  has  carefully  experimented  upon  this  method  and 
adopts  the  following  processes. 

A  standard  solution  of  arsenious  oxide  is  prepared  containing 
5  gm.  of  the  pure  oxide,  dissolved  by  the  aid  of  sodium  hydrate, 
neutralized  with  hydrochloric  acid,  100  c.c.  concentrated  hydro- 
chloric acid  added,  then  diluted  with  water  to  1  litre  ;  each  c.c.  of 
this  solution  contains  0'005  gm.  As2O3,  and  represents  exactly 
0-007283  gm.  Sb2O3. 

Solutions  of  potassium  dichromate  and  ferrous  sulphate,  of 
known  strength  in  relation  to  each  other,  are  prepared  in  the  usual 
way  ;  and  a  freshly  prepared  solution  of  potassium  ferricyanide  is 
used  as  indicator. 

The  relation  between  the  dichromate  and  arsenious  solution  is 
found  by  measuring  10  c.c.  of  the  latter  into  a  beaker,  then  20  c.c. 
hydrochloric  acid  of  sp.  gr.  T12,  and  from  80  to  100  c.c.  of  water  (to 
ensure  uniformity  of  action  the  volume  of  HC1  must  never  be  less 
than  J  or  more  than  J)  ;  the  dichromate  solution  is  then  added  in 
excess,  the  mixture  allowed  to  react  for  a  few  minutes,  and  the 
ferrous  solution  added  until  the  indicator  shows  the  blue  colour. 

*  Poggend.  Annal.  118,  17. 


154  ANTIMONY. 

To  find  the  exact  point  more  closely,  J  or  1  c.c.  dichromate  solution 
may  be  added  and  again  iron,  until  the  precise  ending  is  obtained. 

METHOD  OF  PROCEDURE  :  The  material,  free  from  organic  matter,  organic 
acids,  or  heavy  metals,  is  dissolved  in  the  proper  proportion  of  HC1,  and  titrated 
precisely  as  just  described  for  the  arsenious  solution ;  the  strength  of  the 
dichromate  solution  having  been  found  in  relation  to  As203  the  calculation  as 
respects  Sb2O3  presents  no  difficulty.  Where  direct  titration  is  not  possible  the 
same  course  may  be  adopted  as  with  arsenic  ;  namely,  precipitation  with  H2S  and 
digestion  with  mercuric  chloride. 

In  the  case  of  using  permanganate  it  is  equally  necessary  to  have 
the  same  proportion  of  HG1  present  in  the  mixture,  and  the  standard 
solution  must  be  added  till  the  rose  colour  is  permanent.  The 
permanganate  may  be  safely  used  with  J  the  volume  of  HC1,  with 
the  addition  of  some  magnesic  sulphate,  and  as  the  double  tartrate 
of  antimony  and  potassium  can  readily  be  obtained  pure,  and  the 
organic  acid  exercises  no  disturbing  effect  in  the  titration,  it  is 
a  convenient  material  upon  which  to  standardize  the  solution. 

1  c.c.  of  a  solution  containing  5*27  gm.  of  potassium  per- 
manganate per  litre  =  1%  Sb  in  1  gm.  of  substance. 

3.  Distillation  of  Antimonious  or  Antimonic  Sulphide  with 
Hydrochloric  Acid,  and  Titration  of  the  evolved  Sulphuretted 
Hydrogen  (Schneider). 

When  either  of  the  sulphides  of  antimony  is  heated  with  hydro- 
chloric acid  in  Bunsen's,  Fresenius',  or  Mohr's  distilling 
apparatus  (p.  135),  for  every  eq.  of  antimony  present  as  sulphide, 
3  eq.  of  HaS  are  liberated.  If,  therefore,  the  latter  be  determined, 
the  quantity  of  antimony  is  ascertained. 

METHOD  OF  PROCEDURE  :  The  antimony  to  be  determined  is  brought  into  the 
form  of  ter-  or  penta-sulphide  (if  precipitated  from  a  hydrochloric  acid  solution, 
tartaric  acid  must  be  previously  added  to  prevent  the  precipitate  being  con- 
taminated with  chloride),  which,  together  with  the  filter  containing  it,  is  put  into 
the  distilling  flask  with  a  tolerable  quantity  of  hydrochloric  acid  not  too 
•concentrated.  The  absorption  tube  contains  a  mixture  of  caustic  soda  or  potash 
with  a  definite  quantity  of  N/io  arsenious  acid  solution  in  sufficient  excess  to 
retain  all  the  sulphuretted  hydrogen  evolved.  The  flask  is  then  heated  to 
boiling,  and  the  operation  continued  till  all  evolution  of  sulphuretted  hydrogen 
has  ceased ;  the  mixture  is  then  poured  into  a  beaker,  and  acidified  with 
hydrochloric  acid,  to  precipitate  all  the  arsenious  sulphide.  The  whole  is  then 
diluted  to,  say,  300  c.c.,  and  100  c.c.  taken  with  a  pipette,  neutralized  with 
sodium  carbonate,  some  bicarbonate  added,  and  titrated  for  excess  of  arsenious 
acid  with  N/io  iodine  and  starch.  The  separation  of  antimony  may  generally  be 
ensured  by  precipitation  as  sulphide.  If  arsenic  is  precipitated  at  the  same 
time,  it  may  be  removed  by  treatment  with  ammonium  carbonate. 

4.     Titration  with  Standard  Potassium  Bromate  (Gyory).* 

This  method  was  originally  devised  for  the  valuation  of  Fowler's 
solution  and  of  tartar  emetic,  hence  it  is  applicable  to  both 
arsenic  and  antimony. 

2  KBrO3  +  2  HC1  +  3  Sb203  =  2KCl  +  2  HBr  +  3  Sb2O5. 

*Z.  a.  O.  1893,32,415. 


ANTIMONY.  155 

Reagent  required  : 

Decinormal  potassium  bromate — 2'784  gm.  of  the  pure  crystals  dried  at  110°  C. 
are  dissolved  in  water  and  made  up  to  1  litre. 

1  c.c.  =0-004948  gm.  As2O3  0 • /  /  6"' 

1  c.c.  =0-00721  gm.  Sb20s. 

The  titration  is  performed  in  a  solution  acidified  with  HC1.  For  a  1  %  solution 
of  arsenious  acid  an  equal  volume  of  dilute  HC1  (1:2)  is  used ;  but  in  the  case  of 
antimony  more  acid  must  be  taken  in  order  to  prevent  the  precipitation  of  the 
antimony  with  increasing  dilution  during  titration.  Thus,  for  0'3  gm.  of  tartar 
emetic  25  c.c.  or  more  of  the  dilute  acid  should  be  added.  Methyl  orange  is  added 
to  the  solution  as  indicator.  In  each  case  the  slightest  excess  of  bromate  completely 
decolorizes  the  red  solution  in  consequence  of  the  liberation  of  bromine.  The 
bromate  solution  should  be  added  gradually,  especially  towards  the  end.  The 
presence  of  tin  and  of  considerable  quantities  of  iron  and  copper  interferes  with 
this  method. 

5.    Determination  of  Antimony  Pentoxide  and  its 
Compounds  (W  e  1 1  e  r). 

This  is  the  converse  of  method  1.  The  pentachloride  is  distilled 
with  strong  HC1  and  KI  in  one  of  the  forms  of  distilling  apparatus 
(see  figs.  38  and  39),  and  the  separated  iodine  titrated  with  N/10 
thiosulphate. 

NOTE  ON  THE  FOREGOING  METHODS:  E.  Schmidt*  especially  recommends 
methods  2  and  4  for  technical  purposes. 

Beckettf  finds  that  the  volumetric  determination  of  antimony  with  iodine 
gives  very  concordant  results,  but  these  are  only  correct  when  the  older  atomic 
weight  of  antimony  (Sb=122)  is  used.  When  the  atomic  weight  Sb  =  120'2  is 
taken  the  results  are  about  1  per  cent,  too  low. 


ARSENIC. 

As  =  74-96.     As«03  =  197-92.     AsaO5 = 229'92. 
1.    Oxidation  by  Iodine  (Mohr). 

THE  principle  upon  which  the  determination  of  arsenious  oxide 
by  iodine  is  based  is  explained  on  p.  139. 

Experience  has  shown  that  in  the  determination  of  arsenious 
compounds  by  the  method  there  described  it  is  necessary  to  use 
sodium  bicarbonate  for  rendering  the  solution  alkaline,  as  in  the 
case  of  antimony.  J 

METHOD  OF  PROCEDURE  :  To  a  neutral  aqueous  solution,  add  about  20  c.c.  of 
saturated  solution  of  sodium  bicarbonate  to  every  O'l  gm.  or  so  of  As2O3,  and 
then  titrate  with  N/iO  iodine  and  starch.  When  the  solution  is  acid,  it  may  be 
neutralized  with  sodium  carbonate,  then  the  necessary  quantity  of  bicarbonate 
added,  and  the  titration  completed  as  before. 

PROCEDURE  FOR  ARSENIC  ACID  :  This  is  best  done  by  dissolving  the  acid  in 
water  and  boiling  with  potassium  iodide  hi  the  presence  of  hydrochloric  acid  in 

*  Chem.  Ze&.  1910,  34,  453.  t  C.  N.  1910, 102,  101. 

JWashburn  (J.  A.  C.  S.  1908,  31,  and  Analyst  33,  102)  has  studied  Mohr's 
method.  He  suggests  the  use  of  sodium  phosphate  instead  of  bicarbonate. 


156  ARSENIC. 

large  excess  until  all  iodine  vapours  are  dissipated.  The  H3As04  is  completely 
reduced  to  H3As03.  The  liquid  is  then  cooled,  sodium  carbonate  added  to 
neutrality,  then  some  bicarbonate,  and  the  arsenious  acid  titrated  with  iodine  in 
the  usual  way.  Younger*  has  verified  this  method  and  proved  that  the  reduction 
is  complete  ;  he  also  states  that  when  the  boiled  solution  cools,  the  liberation  of 
a  slight  amount  of  iodine  occurs,  which  may  be  prevented  by  using  a  few  c.c.  of 
glycerine.  Of  course  the  arsenic  acid  must  contain  no  nitric  acid,  nitrates,  or 
similar  interfering  bodies. 

1  c.c.  N/10  iodine =0004948  gm.  As2O3,  or  0'005748  gm.  As205. 

The  Determination  of  Arsenic  in  Alkali  Arsenates. — It  was 
originally  proposed  by  Mohr  to  effect  this  by  passing  sulphur 
dioxide  through  the  solution  so  as  to  reduce  the  arsenic  to  arsenious 
acid,  boiling  off  the  SO2,  and  determining  the  arsenious  acid  by 
iodine  as  just  described.  This  process,  however,  was  not  wddely 
adopted,  as  it  was  found  defective  in  many  instances  for  the  reason 
that  the  mere  passing  of  the  gas  through  the  liquid  did  not  ensure 
complete  reduction  of  the  acid. 

Holthoff  and  McKayJ  have  experimented  largely  on  this 
method  of  determining  arsenic,  and  Holthof  proved  that  various 
forms  of  arsenic,  on  being  converted  into  arsenic  acid,  would  bear 
evaporation  to  dry  ness  with  HC1  without  loss,  and  that  arsenic 
sulphide  could  be  oxidized  by  strong  nitric  acid,  or  with  HC1  and 
KC103,  to  arsenic  acid,  and  reduced  to  the  lower  state  of  oxidation 
by  copious  treatment  with  SO2,  the  method  being  to  add  300 
or  400  c.c.  of  strong  solution  of  SO2,  digest  on  the  water  bath  for 
two  hours,  then  boil  down  to  one-half,  and  when  cool  add  sodium 
bicarbonate,  and  titrate  with  iodine. 

McKay  shortens  the  method  considerably  by  placing  the  mixture 
in  a  well-stoppered  bottle,  tying  down  the  stopper,  and  digesting 
in  boiling  water  for  one  hour.  At  the  end  of  that  time  the  bottle 
is  removed  and  allowed  to  cool  somewhat,  then  emptied  into  a  boiling- 
flask,  diluted  with  about  double  its  volume  of  water,  and  boiled 
down  by  help  of  a  platinum  spiral  to  one-half.  The  liquid  is  cooled, 
diluted,  and  either  the  whole  or  an  aliquot  portion  titrated  with 
iodine  in  the  usual  way. 

For  quantities  of  material  representing  about  O'l  gm.  As,  30  c.c. 
of  saturated  solution  of  SO2  will  suffice,  and  the  reduction  may 
therefore  be  made  in  a  bottle  holding  50  or  60  c.c.  (fig.  40).  The 
results  are  satisfactory.  In  the  case  of  titrating  commercial 
alkali  arsenates,  which  often  contain  small  quantities  of  arsenious 
acid,  this  must  be  determined  first,  and  the  amount  deducted  from 
the  total  obtained  after  reduction. 

A.  Williamson||  has  devised  the  following  ready  method  as 
being  applicable  to  commercial  arsenates,  and  has  made  use  of 
the  reaction  which  takes  place  between  arsenic  and  hydriodic  acids 
in  strong  acid  solution.  In  these  circumstances  arsenic  acid 

*  J.  S.  C.  I.  9,  158.  ^Z.  a.  C.  22,  378.  J  C.  N.  53,  221-243. 

II  Journal  of  the  Society  of  Dyers  and  Colourists,  May,  1896. 


ARSENIC.  157 

is  quantitatively  reduced  with  liberation  of  iodine.     The  reaction 
is 

As2O5 +4HI = As2O3 +2H2O  +2I2. 

It  was  found  that  the  reduction  is  complete  only  in  strongly  acid 
solution,  and  that  if  such  a  solution  be  diluted  the  reverse  reaction 
takes  place  to  a  certain  extent,  a  portion  of  the  arsenious  becoming 
oxidized  to  arsenic  acid.  The  iodine  may,  however,  be  determined 
before  dilution  by  means  of  thiosulphate,  and  in  the  absence  of 
other  bodies  capable  of  liberating  iodine  it  may  be  taken  as  a 
measure  of  the  arsenic  acid.  The  acid  solution  may  then  be 
neutralized,  and  the  arsenite  titrated  with  iodine.  This  serves  as 
a  check  on  the  thiosulphate  titration. 

The  reduction  may  be  effected  either  in  hydrochloric  or  sulphuric 
acid  solution,  but  in  either  case  a  considerable  excess  of  acid  must 
be  present,  otherwise  the  reduction  is  incomplete. 

As  commercial  sodium  arsenate  usually  contains  some  nitrate, 
experiments  were  made  to  ascertain  whether  the  presence  of  this 
salt  interferes  with  the  accuracy  of  the  thiosulphate  titration. 
A  pure  solution  of  arsenate  was  prepared  as  before,  and  1  gm.  of 
sodium  nitrate  added.  25  c.c.  of  this  solution  were  then  treated 
with  potassium  iodide  and  hydrochloric  acid,  and  the  iodine  titrated 
with  thiosulphate  as  before.  The  arsenic  acid  calculated  from  the 
thiosulphate  consumed  was  100'3,  instead  of  100.  It  is  evident 
that  the  presence  of  nitrate  causes  little  or  no  liberation  of  iodine  in 
the  cold,  but  if  the  arsenate  is  digested  with  hydrochloric  acid  and 
potassium  iodide  in  a  closed  bottle  immersed  in  boiling  water  the 
iodine  liberated  is  considerably  in  excess  of  that  corresponding  to 
the  arsenic  acid.  In  this  case,  the  quantity  of  thiosulphate  con- 
sumed is  of  no  value.  The  arsenic  can,  however,  be  accurately 
determined  by  titrating  the  arsenite  after  the  iodine  has  been 
decolorized. 

Instead  of  hydrochloric  acid,  15  c.c.  of  a  mixture  of  sulphuric 
acid  and  water,  in  equal  volumes,  may  be  used.  Since  the  addition 
of  sulphuric  acid  causes  the  solution  to  become  slightly  heated,  it  is 
cooled  before  titrating  the  iodine.  The  results  are  practically  the 
same  as  with  hydrochloric  acid. 

Not  less  than  3  gm.  potassium  iodide  should  be  added,  or  complete 
reduction  is  not  immediately  effected.  The  presence  of  small 
quantities  of  nitrate  does  not  interfere  with  the  accuracy  of  the 
thiosulphate  titration.  Complete  reduction  can  be  brought  about 
with  2  gm.  potassium  iodide  and  10  c.c.  of  sulphuric  acid,  if  the 
solution  is  heated  for  five  minutes  on  the  steam  bath.  A  portion 
of  the  iodine  volatilizes,  but  no  arsenic  is  lost.  The  iodine  is 
exactly  decolorized  with  thiosulphate,  the  solution  neutralized  and 
titrated  with  iodine  in  the  ordinary  manner. 

PROCEDURE  WITH  COMMERCIAL  ARSENATE  OP  SODA  :  10  gm.  are  dissolved 
to  1  litre,  and  the  arsenic  acid  in  25  c.c.  determined  by  one  of  the  methods  given 
above.  The  thiosulphate  titration  records  only  the  arsenic  previously  existing  as 
arsenic  acid.  The  small  proportion  of  As2O3  which  usually  exists  is  ascertained 


\l 


158  ARSENIC. 

by  direct  titration.  When  this  is  calculated  to  arsenic  acid,  and  added  to  that 
found  by  thiosulphate,  the  results  approximate  very  closely  to  those  found  by 
titrating  the  arsenic. 

Determination  of  Arsenic  in  presence  of  Tin. — If  both  these 
elements  are  present  in  the  lower  state  of  oxidation,  the  tin  may  be 
oxidized  with  iodine  in  strong  acid  solution,  the  arsenic  being 
unaffected.  Rochelle  salt  is  then  added,  the  solution  neutralized, 
and  the  arsenite  titrated  with  iodine. 

EXAMPLE  :  25  c.c.  of  N/io  sodium  arsenite  were  mixed  with  25  c.c.  of  hydro- 
chloric acid,  and  3  gm.  stannous  chloride  added.  The  tin  was  then  exactly 
oxidized  with  standard  iodine,  and  the  arsenic  titrated  in  the  alkaline  solution. 
24-9  c.c  of  N/10  iodine  were  required. 

If  they  are  present  in  the  highest  state  of  oxidation,  the  arsenic 
may  be  reduced  by  one  of  the  methods  given  under  the  determination 
of  arsenic  acid.  The  stannic  salt  is  not  affected. 

It  is  thus  possible  to  determine  the  arsenic  in  a  mixture  of  arsenate 
and  stannate  of  soda.  In  presence  of  a  considerable  quantity  of 
tin,  however,  the  complete  reduction  of  the  arsenic  acid  is  not 
effected  quite  as  readily  as  when  tin  is  absent.  The  following 
method  has  given  good  results  : — 

4  or  5  gm.  of  the  mixture  are  dissolved  in  as  small  a  quantity  of  HC1  as 
possible,  an  equal  weight  of  tartaric  acid  is  dissolved  in  the  solution,  which  is 
then  diluted  to  250  c.c.  (Tf  the  tartaric  acid  is  not  added  a  precipitate  forms  on 
dilution  which  contains  both  tin  and  arsenic.)  25  c.c.  of  this  solution  are  then 
mixed  with  3  gm.  potassium  iodide  and  25  c.c.  HC1,  sp.  gr.  1'16,  and  the 
solution  heated  on  the  steam  bath  for  two  or  three  minutes  to  ensure  the 
complete  reduction  of  the  arsenic  acid.  The  liberated  iodine  is  exactly 
decolorized  with  thiosulphate,  and  the  arsenic  determined  by  titration  with 
iodine  in  the  neutralized  solution.  A  mixture  of  arsenate  and  stannate  in  equal 
quantities  and  containing  a  known  percentage  of  arsenic  gave  28 '57  instead  of 
28 '75  per  cent,  of  arsenic  acid. 


2.     Oxidation  by  Potassium  Bichromate  (Kessler). 

This  method  is  exactly  the  same  as  that  fully  described  on  p.  153  for  antimony. 

The  arsenious  compound  is  mixed  with  N/io  dichromate  in  excess  in  presence 
of  hydrochloric  acid  and  water,  in  such  proportion  that  at  least  ^  of  the  total 
volume  consists  of  hydrochloric  acid  (sp.  gr.  1*12). 

The  excess  of  dichromate  is  found  by  a  standard  solution  of  pure  iron,  or  of 
double  iron  salt,  with  potassium  ferricyanide  as  indicator ;  the  quantity  of 
dichromate  reduced  is,  of  course,  the  measure  of  the  quantity  of  arsenious 
converted  into  arsenic  acid. 

1  c.c.  N/10  dichromate =0004948  gm.  As2O3. 

In  cases  where  the  direct  titration  of  the  hydrochloric  acid  solution  cannot 
be  accomplished,  the  arsenious  acid  is  precipitated  with  H2S  (with  arsenates  at 
70°  C.),  the  precipitate  well  washed,  the  filter  and  the  precipitate  placed  in 
a  stoppered  flask,  together  with  a  saturated  solution  of  mercuric  chloride  in 
hydrochloric  acid  of  1-12  sp.  gr.,  and  digested  at  a  gentle  heat  until  the 
precipitate  is  white,  then  water  added  in  such  proportion  that  not  less  than  £  of 
the  volume  of  liquid  consists  of  concentrated  HC1 ;  N/io  dichromate  is  then 
added,  and  the  titration  with  standard  ferrous  solution  completed  as  usual. 


ARSENIC.  159 

3.     Indirect  Determination  by  Distilling  with  Chromic  and 
Hydrochloric  Acids  (Buns en). 

The  principle  of  this  very  exact  method  depends  upon  the  fact 
that  when  potassium  dichromate  is  boiled  with  concentrated 
hydrochloric  acid,  chlorine  is  liberated  in  the  proportion  of  3  eq. 
to  1  eq.  of  chromic  acid. 

If,  however,  arsenious  acid  is  present,  but  not  in  excess,  the 
chlorine  evolved  is  not  in  the  proportion  mentioned  above,  but  so 
much  less  as  is  necessary  to  convert  the  arsenious  into  arsenic  acid. 

As203 + 4C1 + 2H20  =  As2O5 + 4HC1. 

Therefore  every  4  eq.  of  chlorine  short  of  the  quantity  yielded 
when  dichromate  and  hydrochloric  acid  are  distilled  alone  represent 
1  eq.  arsenious  acid.  The  operation  is  conducted  in  some  form 
of  the  apparatus  described  on  p.  135  et  seq, 

4.    By  Precipitation  as  Uranium  Arsenate  (Bodeker). 

METHOD  OF  PROCEDURE  :  The  arsenic  must  exist  in  the  state  of  arsenic 
acid  (As205),  and  the  process  is  in  all  respects  the  same  as  for  the  determination 
of  phosphoric  acid,  devised  by  Neubauer,  Pincus  and  myself  (infra).  The 
strength  of  the  uranium  solution  may  be  ascertained  and  fixed  by  pure  sodium  or 
potassium  arsenate,  or  by  means  of  a  weighed  quantity  of  pure  arsenious  acid 
converted  into  arsenic  acid  by  evaporation  with  strong  nitric  acid,  then  neutralized 
with  alkali,  and  dissolved  in  acetic  acid.  The  method  of  titration  is  precisely 
the  same  as  with  phosphoric  acid  ;  the  solution  of  uranium  should  be  titrated 
upon  a  weighed  amount  of  arsenical  compound,  bearing  in  mind  here,  as  in  the 
case  of  P205,  that  the  titration  must  take  place  under  precisely  similar  conditions 
as  to  quantity  of  liquid,  the  amount  of  sodium  acetate  and  acetic  acid  added, 
and  the  depth  of  colour  obtained  by  contact  of  the  fluid  under  titration  with  the 
ferrocyanide  solution. 

Bo  am*,  who  has  had  large  experience  in  the  examination  of 
arsenical  ores,  recommends  this  method  as  being  rapid  and  accurate, 
and  carries  it  out  as  follows  : — 

METHOD  OF  PROCEDURE  :  1  to  1*5  gm.  of  dried  and  powdered  ore  is  boiled  to 
dryness  with  20-25  c.c.  of  strong  nitric  acid  ;  when  cool  about  30  c.c.  of  30  % 
caustic  soda  solution  is  added  and  boiled  for  a  few  minutes  ;  then  diluted,  filtered 
and  made  up  to  250  c.c.  25  c.c.  of  the  liquid  are  acidified  with  a  solution  containing 
10  per  cent,  of  sodium  acetate  in  50  per  cent,  acetic  acid,  and  heated  nearly 
to  boiling,  then  titrated  with  the  standard  uranium  as  usual.  For  this  latter, 
the  same  authority  recommends  what  he  terms  a  fourth-normal  solution  of 
uranium,  containing  17'1  gm.  uranium  acetate,  and  15  c.c.  glacial  acetic  acid  made 
up  to  2  litres  with  water,  1  c.c.  being  equal  to  1*25  mgm.  As.  But  if  the 
method  has  to  be  considered  accurate,  this  suggestion  can  scarcely  be  adopted, 
since  the  uranium  acetate  of  commerce  is  of  indefinite  hydration ;  and  moreover, 
to  ensure  exactitude,  it  is  necessary  that  the  titration  should  be  carried  out  with 
the  same  proportions  of  saline  matters,  acetic  acid,  etc.  as  existed  when  originally 
standardizing  the  uranium.  I,  therefore,  unhesitatingly  recommend  that  the 
uranium  should  be  standardized  with  a  known  weight  of  pure  arsenic  or  arsenate 
in  the  presence  of  the  same  proportions  of  sodium  hydrate  and  acetate,  acetic 
acid,  etc.  as  will  actually  be  used  in  the  analysis  of  an  ore.  The  method  may 
be  used  for  all  ores  which  can  be  attacked  by  nitric  acid.  It  is  also  available  for 

*C.  N.  61,  219, 


160  ARSENIC. 

iron  pyrites  containing  tolerable  quantities  of  arsenic ;  the  ferric  arsenate  being 
readily  decomposed  by  excess  of  NaHO,  thus  allowing  the  ferric  hydrate  to  be 
filtered  off  free  from  As. 

The  solution  of  arsenic  acid  must,  of  course,  be  free  from  metals 
liable  to  give  a  colour  with  the  indicator  and  from  phosphates. 
Alkalies,  alkaline  earths,  and  zinc  are  of  no  consequence,  but  it  is 
advisable  to  add  nearly  the  required  volume  of  uranium  to  the 
liquid  before  heating.  The  arsenic  acid  must  be  separated  from  all 
bases  which  would  yield  compounds  insoluble  in  weak  acetic 
acid. 

The  AsH3  evolved  from  Marsh's  apparatus  may  be  passed  into 
fuming  HN03,  evaporated  to  dryness,  the  arsenic  acid  dissolved  in 
water  (antimony,  if  present,  is  insoluble),  then  titrated  cautiously 
with  uranium  in  presence  of  free  acetic  acid  and  sodium  acetate  as 
above  described. 


5.     By  Standard  Silver  as  Arsenate. 

This  method  has  been  devised  by  Pierce  of  the  Colorado 
Smelting  Company,  and  is  as  follows  : — 

METHOD  OF  PROCEDURE  :  The  finely-powdered  substance  for  analysis  is 
mixed  in  a  large  porcelain  crucible  with  from  six  to  ten  times  its  weight  of 
a  mixture  of  equal  parts  of  sodium  carbonate  and  potassium  nitrate.  The  mass 
is  then  heated  with  a  gradually  increasing  temperature  to  fusion  for  a  few  minutes, 
allowed  to  cool,  and  the  soluble  portion  extracted  by  warming  with  water  in  the 
crucible,  and  filtering  from  the  insoluble  residue.  The  arsenic  is  in  the  filtrate 
as  alkali  arsenate.  The  solution  is  acidified  with  nitric  acid  and  boiled  to  expel 
C02  and  nitrous  fumes.  It  is  then  cooled  to  the  ordinary  temperature,  and 
almost  exactly  neutralized  as  follows  : — Place  a  small  piece  of  litmus  paper  in  the 
liquid  :  it  should  show  an  acid  reaction.  Now  gradually  add  strong  ammonia  till 
the  litmus  turns  blue,  avoiding  a  great  excess.*  Again  make  slightly  acid  with 
a  drop  or  two  of  strong  nitric  acid  ;  and  then,  by  means  of  very  dilute  ammonia 
and  nitric  acid,  added  drop  by  drop,  bring  the  solution  to  such  a  condition  that 
the  litmus  paper,  after  having  previously  been  reddened,  will,  in  the  course  of 
half  a  minute,  begin  to  show  signs  of  alkalinity.  The  litmus  paper  may  now  be 
removed  and  washed,  and  the  solution,  if  tolerably  clear,  is  ready  for  the  addition 
of  silver  nitrate.  If  the  neutralization  has  caused  much  of  a  precipitate 
(alumina,  etc.),  it  is  best  to  filter  it  off  at  once,  to  render  the  subsequent  filtration 
and  washing  of  the  arsenate  of  silver  easier. 

A  solution  of  silver  nitrate  (neutral)  is  now  added  in  slight  excess ;  and  after 
stirring  a  moment  to  partially  coagulate  the  precipitated  arsenate,  which  is  of 
a  brick-red  colour,  the  liquid  is  filtered,  and  the  precipitate  washed  with  cold 
water.  The  filtrate  is  then  tested  with  silver  and  dilute  ammonia,  to  see  that 
the  precipitation  is  complete. 

The  object  is  now  to  determine  the  amount  of  silver  in  the  precipitate,  and 
from  this  to  calculate  the  arsenic.  The  arsenate  of  silver  is  dissolved  on  the 
filter  with  dilute  nitric  acid  (which  leaves  undissolved  any  chloride  of  silver),  and 
the  filtrate  titrated,  after  the  addition  of  ferric  sulphate,  with  thiocyanate 
(p.  145). 

From  the  formula  3  Ag2O.As«2O5,  647*2  parts  Ag  =  149'92  parts 
As,  or  Ag  :  As  =  l  :  0-2316.  " 

A   modification   of   the   above   method   is   suggested   by  J.    F. 

*  C  a  n  b  y  "neutralizes  with  an  emulsion  of  zinc  oxide. 


ARSENIC.  161 

Bennett*  in  order  to  avoid  some  sources  of  inaccuracy.  He  found 
that  it  was  very  difficult  to  obtain  neutrality  by  either  of  the 
processes  given,  and,  by  avoiding  ammonia,  phenolphthalein  could 
be  used  as  an  indicator. 

METHOD  OF  PROCEDURE  :  0'5  gm.  of  the  finely- powdered  substance  is  fused 
with  3  to  5  gin.  of  sodium  carbonate  and  potassium  nitrate  in  equal  parts,  about 
one-third  being  used  on  the  top.  On  cooling,  the  mass  is  extracted  with  boiling 
\vatcr  and  filtered.  The  nitrate,  which  contains  the  arsenic  as  alkali  arsenate, 
is  strongly  acidified  with  acetic  acid,  then  boiled  to  expel  C02,  and,  after  cooling, 
treated  with  a  few  drops  of  phenolphthalein  and  sufficient  caustic  soda  to  give 
an  alkaline  reaction.  The  purple-red  coloration  produced  by  an  excess  of  alkali 
is  then  discharged  by  acetic  acid.  A  slight  excess  of  neutral  silver  nitrate  is 
then  vigorously  stirred  in,  and  the  whole  left  to  settle,  away  from  direct  sunlight ; 
the  supernatant  liquid  is  poured  off  through  a  filter,  and  the  precipitate  washed 
by  decantation  with  cold  water,  then  thrown  on  the  filter  and  thoroughly  washed. 
The  funnel  is  then  filled  with  water  and  20  c.c.  of  strong  nitric  acid,  this  liquid 
is  run  through  the  filter  into  the  original  beaker,  the  residue  washed  thoroughly 
with  cold  water,  and  the  filtrate  made  up  to  about  100  c.c.,  then  titrated  with 
standard  thiocyanate. 

Owing  to  the  large  amount  of  arsenate  of  silver  formed  from 
a  small  quantity  of  arsenic  (nearly  six  times  by  weight),  it  is  not 
at  all  necessary  or  even  desirable  to  work  with  large  amounts  of 
substance.  0*5  gm.  is  usually  sufficient  for  the  determination  of 
the  smallest  quantity  of  arsenic  ;  and  where  the  percentage  is  high 
as  little  as  O'l  gm.  may  be  taken  with  advantage.  The  method  has 
been  used  with  very  satisfactory  results  on  the  sulphide  of  arsenic 
obtained  in  the  ordinary  course  of  analysis. 

Substances  such  as  molybdic  and  phosphoric  acids,  which  behave 
similarly  to  arssnic  under  this  treatment,  interfere,  of  course,  with 
the  method.  Antimony,  by  forming  sodium  antimoniate,  remains 
practically  insoluble  and  without  effect. 

6.     Determination  of  Arsenious  Sulphide  by  Iodine. 

J.  and  H.  S.  Pattinsonf  have  shown  that  the  separation  of 
arsenic  as  sulphide  from  many  metals,  viz.,  lead,  tin,  cadmium, 
antimony,  and  bismuth,  is  complete  when  made  in  concentrated 
hydrochloric  acid  (sp.  gr.  1'16-1'17).  The  subsequent  determin- 
ation is  carried  out  by  the  authors  as  follows  : — 

METHOD  OF  PROCEDURE  :  The  precipitate  is  collected  on  asbestos  in  a  Gooch 
crucible  and  washed  with  cold  water  until  free  from  hydrochloric  acid.  The 
asbestos  felt  with  the  adhering  precipitate  is  then  placed  in  a  small  beaker ;  the 
crucible  is  wiped  out  with  a  little  clean  ignited  asbestos,  which  is  also  put  into 
the  beaker.  10  or  15  c.c.  of  concentrated  sulphuric  acid  (specific  gravity  about 
1'83),  free  from  arsenic,  are  then  poured  into  the  beaker,  which  is  then  placed 
without  a  cover  on  a  hot  plate  or  on  a  wire  gauze  over  a  small  B  u  n  s  e  n  flame  in 
a  good  draught  closet.  As  soon  as  the  acid  reaches  the  temperature  at  which  it 
begins  to  fume,  the  arsenious  sulphide  becomes  rapidly  decomposed  ;  at  first  both 
sulphuretted  hydrogen  and  sulphurous  acid  are  given  off  (if  a  cover  be  put  on 
the  beaker  the  mutual  decomposition  of  these  two  gases  causes  a  deposition  of 
sulphur  on  the  sides  of  the  beaker).  The  evolution  of  sulphuretted  hydrogen 
is  over  in  a  few  seconds,  but  the  sulphur  dioxide  takes  longer  to  expel, 
*  J.  Am.^C.  S.  21,  431.  t  J.  S.  C.  I.,  1893,' p.  211. 

M 


162  ARSENIC. 

depending  upon  the  quantity  of  arsenious  sulphide  and  of  free  sulphur  that 
may  have  been  mixed  with  the  precipitate.  As  soon  as  the  decomposition  of 
the  arsenious  sulphide  begins,  the  liquid  becomes  darkened  in  colour,  and  the 
heating,  which  may  be  brought  to  and  kept  at  the  verge  of  ebullition  of  the  acid, 
is  continued  until  this  dark  colour  passes  away,  when  it  will  be  found  that  all 
the  sulphurous  acid  has  been  expelled.  It  is  of  the  utmost  importance  that  all 
sulphurous  acid  should  be  eliminated  at  this  stage.  This  takes  about  10  to  20 
minutes  with  precipitates  of  sulphide  weighing  about  0'02  gm.  and  nearly  free 
from  free  sulphur.  Arsenious  acid  remains  in  solution  in  the  sulphuric  acid,  and 
the  amount  is  determined  by  cooling  the  acid,  diluting  with  water,  nearly 
neutralizing  the  acid  with  concentrated  sodium  hydrate  solution,  and  then 
completing  the  neutralization  and  rendering  alkaline  with  an  excess  of  sodium 
bicarbonate,  and  finally  titrating  with  standard  iodine  solution  and  starch.  The 
iodine  solution  must  be  standardized  against  an  approximately  equal  quantity  of 
arsenious  acid,  to  which  the  same  amount  of  sulphuric  acid  has  been  added  as- 
was  used  for  the  decomposition  of  the  arsenious  sulphide  precipitate.  As 
sulphuric  acid  alone  usually  requires  a  few  tenths  of  a  cubic  centimetre  of 
centinormal  iodine  solution  to  be  added  to  it  before  the  blue  colour  of  iodide  of 
starch  forms,  a  blank  experiment  with  the  stock  of  acid  in  use  should  be  made 
once  for  all.  It  was  found  the  best  plan  to  avoid  breaking  up  the  asbestos  felt, 
and  if  possible  to  put  it  in  the  beaker  so  that  the  side  on  which  the  precipitate 
lies  is  on  the  bottom  of  the  beaker.  This  prevents  the  precipitate  from  be- 
coming detached  from  the  felt  and  floating  to  the  top  of  the  acid  or  creeping 
up  the  side  of  the  beaker. 

This  method  was  used  for  six  months  in  the  course  of  daily  work, 
alongside  of  determinations  made  by  weighing  the  sulphide 
precipitate,  or.  after  having  separated  the  arsenic  by  Fischer's- 
distillation  process,  by  titrating  the  distillate  (previously  rendered 
alkaline)  with  iodine  solution,  and  the  results  were  very  concordant. 

Experiments  show  that  there  is  no  loss  of  arsenious  acid  by 
volatilization  when  arsenious  sulphide  is  decomposed  by  heating 
with  strong  sulphuric  acid  in  the  manner  described. 

Determination  of  Arsenic  in  Iron  Ores,  Steel,  and  Pig  Iron 
(J.  E.  Stead).— The  best  method  of  separating  arsenic  from  iron 
solutions  is  undoubtedly  that  of  distilling  with  hydrochloric  acid 
and  ferrous  chloride. 

Stead  found,  after  many  trials  and  experiments,  that  if  the 
distillation  is  conducted  in  a  special  manner  the  whole  of  the 
arsenic  may  be  obtained  in  the  distillate,  unaccompanied  with  any 
traces  of  ferrous  chloride,  and  that  if  the  hydrochloric  acid  is  nearly 
neutralized  with  ammonia,  and  finally  completely  neutralized  with 
sodium  bicarbonate,  the  arsenic  can  be  determined  with  iodine  in 
the  usual  manner. 

The  standard  solutions  required  are  :— 

Arsenious  oxide.  0'66  gm.  ( ==0'5  gm.  elemental  arsenic)  of  pure 
arsenious  oxide  in  fine  powder  is  weighed  and  placed  in  a  flask,  with 
2  gm.  of  sodium  carbonate  and  100  c.c.  of  boiling  distilled  watery 
and  the  liquid  boiled  till  all  the  arsenious  oxide  has  dissolved. 
When  cool,  2  gm.  of  sodium  bicarbonate  are  added  and  diluted  to 
one  litre.  1  c.c.  =0*0005  gm.  As. 

Iodine  solution.  1*7  gm.  of  pure  iodine  is  dissolved  in  2  gm, 
of  potassium  iodide  and  water,  the  solution  diluted  to  one  litre, 


ARSENIC.  163 

then  standardized  by  titrating  2Q  c.c.  of  the  arsenious  solution. 
If  the  iodine  has  been  pure,  20  c.c.  of  the  solution  should  be 
required  just  to  produce  a  permanent  blue  with  starch. 

These  solutions  keep  fairly  well  without  alteration  for  several 
months.  It  is  advisable,  however,  to  ascertain  periodically  the 
value  of  the  iodine  by  titrating  it  with  the  arsenic  solution. 

METHOD  or  PROCEDURE  FOR  STEEL  :  From  1  to  50  gin.  of  the  steel  in 
drillings  are  introduced  into  a  30-ounce  flask,  and  a  sufficient  quantity  of  equal 
parts  of  strong  hydrochloric  acid  and  water  is  added  to  dissolve  it.  The  mouth 
of  the  flask  is  closed  with  a  rubber  stopper  carrying  a  safety  tube  and  a  tube  to 
convey  the  gas  evolved  into  the  Winkler's  spiral  absorption  tubes,  containing 
a  strong  saturated  solution  of  bromine  in  water. 

The  tube  is  filled  to  one-third  of  its  length  with  the  solution,  and  about  £  c.c. 
of  free  bromine  is  run  in  to  replace  the  bromine  which  is  consumed  or  carried 
out  with  the  passing  gas. 

The  contents  of  the  flask  are  now  gently  heated  to  such  a  degree  that  a  steady 
but  not  rapid  current  of  gas  passes  through  the  bromine  solution. 

In  about  one  hour  the  whole  of  the  steel  will  be  dissolved,  and  when  no  more 
evolution  of  hydrogen  can  be  observed,  the  liquid  in  the  flask  is  well  boiled,  s.o 
as  to  completely  drive  all  the  gas  into  and  through  the  bromine  solution. 

The  absorption  tube  is  now  disconnected,  and  the  bromine  solution  containing 
that  part  of  the  arsenic  which  has  passed  off  as  gas  is  rinsed  out  into  a  small 
100  c.c.  beaker,  and  the  excess  of  bromine  is  gently  boiled  off,  and  the  clear 
colourless  solution  is  poured  into  the  flask.  About  0*5  gm.  of  zinc  sulphide  is 
now  dropped  into  the  iron  solution  and  the  contents  are  violently  shaken  for 
about  three  minutes,  by  which  time  the  whole  of  the  arsenic  will  be  in  the 
insoluble  state,  partly  as  sulphide  and  partly  as  a  black  precipitate  of  possibly 
free  arsenic  and  arsenide  of  iron. 

It  has  been  found  that  violent  agitation  for  a  few  minutes  is  quite  as  efficacious 
in  effecting  the  complete  separation  of  arsenic  sulphide  as  the  method  of 
passing  a  current  of  CO2  through  the  solution  to  remove  the  excess  of  hydric 
sulphide,  or  allowing  it  to  stand  ten  or  twenty  hours  to  settle  out. 

The  insoluble  precipitate  is  now  rapidly  filtered  through  a  smooth  filter-paper, 
and  the  flask  is  rinsed  with  cold  distilled  water.  The  precipitate  usually  does 
not  adhere  to  the  filter,  and  in  such  cases  the  paper  is  spread  out  flat  upon 
a  porcelain  slab,  and  the  arsenic  compounds  are  rinsed  off  with  a  fine  jet  of  hot 
water  into  a  small  beaker.  The  precipitate  is  now  dissolved  in  bromine  water, 
and  a  drop  or  two  of  HC1. 

The  bromine  solution  now  containing  all  the  arsenic  is  gently  boiled  to  expel 
the  bromine,  and  it  is  then  poured  into  a  10-ounce  retort  and  is  distilled  with 
ferrous  chloride  and  hydrochloric  acid. 

The  apparatus  used  consists  of  an  ordinary  Liebig's  condenser,  but  the 
retort  has  its  neck  bent  to  an  angle  of  about  150°,  and  this  is  attached  to  the 
condenser,  so  that  any  iron  mechanically  carried  over  may  run  back.  By  this 
device,  the  distillate  will  never  contain  more  than  the  very  slightest  trace  of  iron. 

The  solution  containing  the  arsenic  having  been  run  into  the  retort,  the  beaker 
is  washed  out  and  the  washings  are  also  poured  in.  If  the  solution  is  much 
above  20  c.c.  in  bulk,  it  is  advisable  to  add  a  strong  solution  of  ferrous  chloride 
containing  about  0'5  gm.  of  iron  in  the  ferrous  state,  and  for  this  purpose 
nothing  answers  so  well  as  a  portion  of  the  steel  solution  remaining  after 
separating  the  arsenic,  which  is  first  well  boiled  to  free  it  from  hydric  sulphide, 
and  should  contain  about  10  per  cent,  of  soluble  iron  as  ferrous  chloride.  5  c.c. 
of  this  solution  will  contain  the  necessary  amount  of  iron  to  add  to  the  retort. 
After  adding  the  chloride,  it  is  best  to  boil  down  the  solution  to  about  20  c.c. 
before  adding  any  HC1,  taking  care,  of  course,  to  collect  what  liquid  distils  over. 
When  the  necessary  concentration  has  been  effected,  20  c.c.  of  strong  HC1  are 
run  in,  and  the  distillation  is  continued  till  all  excepting  about  10  c.c.  has  passed 
over.  A  further  quantity  of  20  c.c.  mixed  with  5  c.c.  of  water  is  run  in,  and 

M    2 


164  ARSENIC. 

this  is  all  distilled  over.  At  this  point,  as  a  rule,  all  the  arsenic  will  have  passed 
into  the  distillate,  but  it  is  advisable  to  make  quite  certain,  and  to  add  a  third 
portion  of  acid  and  water,  and  to  distil  it  over.  If  the  distillation  has  not  been 
forced,  the  distillate  will  be  quite  colourless.  The  arsenic  in  the  distillate  will 
exist  as  arsenious  chloride,  accompanied  with  a  large  excess  of  hydrochloric  acid. 
A  drop  of  litmus  is  put  into  this  solution,  and  strong  ammonia  is  run  in  until 
alkaline.  It  is  now  made  slightly  acid  with  a  few  drops  of  HC1,  and  a  slight 
excess  of  solid  bicarbonate  of  soda  is  dropped  in.  The  contents  of  the  flask  are 
now  cooled  by  a  stream  of  water,  and,  after  adding  a  clear  solution  of  starch,  the 
standard  iodine  is  run  in  from  a  burette  till  a  deep  permanent  blue  colouration 
is  produced. 

If  the  steel  or  iron  contains  much  arsenic,  a  smaller  quantity,  say,  1  or  2  gin., 
may  be  dissolved  in  nitric  acid  of  1  '20  specific  gravity  and  the  solution  evaporated 
to  dryness,  the  residue  being  dissolved  in  hydrochloric  acid,  and  the  solution 
transferred  to  the  retort,  and  distilled  directly  with  ferrous  chloride  and  hydro- 
chloric acid,  care  being  taken  that  the  distillation  is  not  forced,  as  as  to  avoid 
any  of  the  iron  solution  passing  over  into  the  distillate. 

METHOD  OF  PROCEDURE  FOR  PIG  IRON  :  In  testing  pig  irons,  they  may  be 
dissolved  in  nitric  acid  and  evaporated  to  dryness,  or  be  treated  in  a  flask  with 
HC1  exactly  in  the  manner  described  above  ;  but  it  is  advisable,  if  the  latter 
method  is  adopted,  after  treating  the  voluminous  mass  of  silica  and  graphite,  etc.. 
with  bromine  and  hydrochloric  acid,  to  filter  off  the  insoluble  matter  and  distil 
the  clear  solution. 

METHOD  OF  PROCEDURE  FOR  IRON  ORES  :  In  testing  ores,  it  is  only  necessary 
to  place  the  powdered  ore  directly  into  the  retort,  and  distil  at  once  with  HC1  and 
ferrous  chloride,  taking  care  to  place  a  few  email  pieces  of  firebrick  also  in  the 
vessel,  to  avoid  bumping. 

If  the  ore  contains  much  manganese,  it  is  advisable  to  dissolve  it  in  a  separate 
vessel  to  liberate  and  expel  the  chlorine,  and  then  to  transfer  it  into  the  retort. 

The  time  taken  to  test  iron  or  steel  need  not  exceed  two  hours,  and  for  iron  or 
other  ores  not  much  more  than  half  an  hour. 

It  is  quite  possible  accurately  to  determine  as  small  a  quantity  as  0'002  per 
cent,  of  arsenic  by  this  method. 

When  dissolving  steels  in  dilute  HC1,  if  there  is  no  rust  on  the  sample  or 
ferric  chloride  present  in  the  acid,  and  the  presence  of  air  is  carefully  avoided, 
as  a  rule  only  about  one-tenth  of  the  total  arsenic  present  passes  off  with  the  gas. 

A  very  simple  and  accurate  method  of  determining  a  small 
amount  of  arsenic  when  it  exists  in  the  form  of  freshly  precipitated 
sulphide  is  suggested  by  F.  Flatten.*  It  consists  in  simply  boiling 
the  sulphide  with  pure  water  for  a  period  of  from  1  to  3  hours,  or 
until  the  liquid  is  quite  colourless,  and  all  the  H2S  dissipated. 
The  arsenic  will  then  exist  wholly  as  As2O3,  and  may  be  titrated 
direct  with  N/10o  iodine,  and  a  slight  amount  of  sodium  bicarbonate 
as  usual. 

Both  this  and  Stead's  method  have  been  proved  to  give  identical 
results,  when  carried  out  by  separate  skilled  operators  on  the  same 
samples  of  material.  | 

7.    Titration  by  Potassium  Bromate  (Gyory). 
See  under  Antimony,  p.  154. 

*  J.  S.  C.  I.  13,  324. 

t  See  also  A.  H.  L  o  w,  "  Determination  of  Antimony  ana  Arsenic  in  Ores,"  J.  A .  C.  S 
1906,  1715  ;  and  Norton  and  Koch,  "Determination  of  Arsenic  and  Antimony  m 
the  presence  of  Organic  Matter,"  J.  A.  C.  S.  1905,  1247. 


ARSENIC.  ]  65 

8.     Titration  by  Potassium  lodate  (Andrews). 

See  p.  134  (Applicable  to  Antimony). 

9.     Determination  of  Arsenic  in  Organic  Compounds. 

Little,  Cahen,  and  Morgan,*  find  the  most  satisfactory 
process  to  be  a  combination  of  Pringsheim'sf  method  of 
oxidation  by  sodium  peroxide  and  Gooch  and  Browning's 
volumetric  method.  J 

METHOD  or  PROCEDURE  : — O2-0-3  gm.  of  the  finely-powdered  substance  is 
mixed  in  a  nickel  crucible  with  10-15  gm.  of  sodium  carbonate  and  sodium 
peroxide  in  equal  proportions,  a  portion  of  these  reagents  being  spread  over  the 
mixture.  After  heating  gently  for  about  15  minutes,  the  fusion  is  completed  by 
heating  to  dull  redness  for  5  minutes.  The  contents  of  the  crucible  are 
extracted  with  water  and  rinsed  into  a  450  c.c.  conical  flask,  treated  with 
25-31  c.c.  of  sulphuric  acid  (1  :  1),  and  the  volume  reduced  to  100  c.c.  by 
boiling.  One  gm.  of  potassium  iodide  is  now  added  and  the  liquid  further 
concentrated  to  40  c.c.  After  removing  the  last  traces  of  iodine  by  means  of 
a  few  drops  of  dilute  sulphurous  acid,  the  green  solution  is  diluted  with  hot  water 
and  saturated  with  hydrogen  sulphide.  The  arsenious  sulphide  is  collected, 
washed,  dissolved  in  20  c.c.  of  N/2  sodium  hydroxide  and  treated  with  30  c.c. 
of  hydrogen  peroxide  (20  vols.),  the  excess  of  the  latter  being  destroyed  by 
heating.  A  few  drops  of  phenolphthalein  are  now  added,  followed  by  11  c.c. 
of  sulphuric  acid  ( 1  :  1)  and  1  gm.  of  potassium  iodide.  The  solution  is  con- 
centrated to  40  c.c.,  decolorised  with  a  few  drops  of  dilute  sulphurous  acid, 
diluted  with  cold  water,  neutralized  with  2N-sodium  hydroxide,  and  slightly 
acidulated  with  sulphuric  acid.  An  1 1  per  cent,  solution  of  sodium  phosphate 
(compare  W^rsh burn,  J.  Amer.  Chern.  Soc.,  1908,  30,  31),  is  then  added,  and 
the  arsenite  solution  titrated  with  iodine  and  starch  in  the  usual  way.  The 
volume  of  sodium  phosphate  solution  added  should  be  about  equal  to  that  of 
N/io  iodine  required  in  the  titration.  Compounds  containing  little  or  no 
oxygen  require  a  proportionately  larger  quantity  of  sodium  peroxide  for 
oxidation.  When  the  arsenic  compound  contains  iodine,  sodium  ioclate  is 
formed  on  oxidation,  and  sufficient  sulphurous  acid  must  be  added  to  the 
acidified  extract  to  reduce  this  salt  to  iodide. 


BARIUM. 

Ba-137'37. 

IN  a  great  number  of  instances  the  determination  of  barium  is 
simply  the  converse  of  the  process  for  sulphuric  acid  (q.  v.),  using 
either  a  standard  solution  of  sulphuric  acid  or  a  neutral  sulphate  in 
a  known  excess,  and  finding  the  amount  by  residual  titration. 

When  barium  can  be  separated  as  carbonate,  the  determination 
is  made  as  on  p.  72. 

Precipitation  as  Barium  Chromate. — A  decinormal  solution  of 
dichromate  for  precipitation  purposes  must  differ  from  that  used 

*  J.  Clicm,  S.  1909,  95,  1177.  t  Jm.  Chcm.  J.  1904,  81,  383. 

J  Am.  J.  Sci.  1890,  (3),  11,  60      Sec  also  p.  226. 


166  BARIUM. 

for  oxidation  purposes.  In  the  present  case  the  solution  is  made 
by  dissolving  7 '37  gm.  of  pure  potassium  dichromate  in  water,  and 
diluting  to  1  litre. 

METHOD  OF  PROCEDURE  :  The  barium  compound,  which  may  contain  alkalies, 
magnesia,  strontia,  and  lime,,  is  dissolved  in  a  good  quantity  of  water,  ammonia 
free  from  carbonate  added,  heated  to  60°  or  70°  C.,  and  the  standard  dichromate 
added  cautiously,  with  shaking,  so  long  as  the  yellow  precipitate  of  barium 
chromate  is  formed,  and  until  the  clear  supernatant  liquid  possesses  a  faint  yellow 
colour.  1  c.c.  N/io  solution  =0*00687  gm.  Ba. 

Titration  of  the  Precipitate  with  Permanganate. — In  this  case  the  precipitate 
of  barium  chromate  is  well  washed,  transferred  to  a  flask,  and  mixed  with  an 
excess  of  ferrous  ammonium  sulphate  ;  the  amount  of  iron  oxidized  by  the  chromic 
acid  is  then  determined  by  titration  with  permanganate  ;  the  quantity  of  iron 
changed  to  the  ferric  state  multiplied  by  the  factor  0'8187  =  Ba. 

Or  the  barium  chromate  may  be  digested  with  HC1  and  KI,  as 
described  on  p.  138.  1  c.c.  N/10  thiosulphate  =  '004579  gm.  Ba.  * 

BISMUTH. 
Bi  =  208. 

THE  determination  of  this  metal  or  its  compounds  volumetrically 
has  occupied  the  attention  of  Pattison  Muir,  to  whom  we  are 
indebted  for  several  methods  of  gaining  this  end.  Two  of  the  best 
are  given  here,  namely,  (1)  precipitation  of  the  metal  as  basic 
oxalate,  and  titration  with  permanganate ;  (2)  precipitation  as 
phosphate  with  excess  of  standard  sodium  phosphate,  and  titration 
of  that  excess  by  standard  uranium  acetate. 

1.     Titration  as  Oxalate. 

Normal  bismuth  oxalate,  produced  by  adding  excess  of  oxalic 
acid  to  a  nitric  acid  solution  of  the  metal,  when  separated  by 
filtration,  and  boiled  with  successive  quantities  of  water  three  or 
four  times,  is  transformed  into  basic  oxalate. 

METHOD  OF  PROCEDURE  :  The  solution  containing  bismuth  must  be  free  from 
hydrochloric  acid,  as  the  basic  oxalate  is  readily  soluble  in  that  acid.  A  largo 
excess  of  nitric  acid  must  also  be  avoided.  Oxalic  acid  must  be  added  in  con- 
siderable excess.  If  the  precipitate  be  thoroughly  shaken  up  with  the  liquid 
and  the  vessel  is  then  set  aside,  the  precipitate  quickly  settles,  and  the  super- 
natant liquid  may  be  poured  off  through  a  filter  in  a  very  short  time.  If  the 
precipitate  be  boiled  for  five  or  ten  minutes  with  successive  quantities  of  about 
50  c.c.  of  water,  it  is  quickly  transformed  into  the  basic  salt.  So  soon  as  the 
supernatant  liquid  ceases  to  show  an  acid  reaction,  the  transformation  is  complete. 
It  is  well  to  employ  a  solution  of  permanganate  so  dilute  that  at  least  50  c.c.  are 
required  for  the  titration  (**7io  strength  suffices).  The  basic  oxalate  may  be 
dissolved  in  dilute  sulphuric  acid  in  place  of  hydrochloric  ;  it  is  more  soluble, 
however,  in  the  latter  acid.  If  the  solution  contains  but  little  hydrochloric  acid, 
there  is  no  danger  of  chlorine  being  evolved  during  the  process  of  titration. 

In  applying  this  process  to  the  determination  of  bismuth  in  a  solution  containing 
other  metals,  it  is  necessary,  if  the  solution  contain  substances  capable  of  acting 
upon,  or  of  being  acted  on  by,  permanganate,  to  separate  the  bismuth  from  the 
other  metals  present.  This  is  easily  done  by  precipitating  in  a  partially 


BISMUTH.  167 

neutralized  solution  with  much  warm  water  and  a  little  ammonium  chloride. 
The  precipitate  must  be  dissolved  in  nitric  acid,  and  the  liquid  boiled  down  once 
or  twice  with  addition  of  the  same  acid  in  order  to  expel  all  hydrochloric  acid, 
before  precipitating  as  oxalate.  The  liquid  should  contain  just  sufficient  nitric 
acid  to  prevent  precipitation  of  the  basic  nitrate  before  oxalic  acid  is  added, 
1  molecule  oxalic  acid  (126-06)  corresponds  to  1  atom  bismuth  (208). 

1  c.c.  N/10  permanganate  =  '01 04  gm.  Bi. 

A  shorter  method,  based  on  the  same  reactions,  has  been  arranged 
by  Muir  and  Robbs.*  In  this  case,  however,  the  double  oxalate 
of  potassium  and  bismuth  is  the  compound  obtained,  the  excess  of 
oxalate  of  potash  being  determined  residually.  Reisf  has  shown 
that  when  normal  potassium  oxalate  is  added  to  a  solution  of 
bismuth  nearly  free  from  mineral  acid,  but  containing  acetic  acid, 
a  double  salt  of  the  formula  Bi2  (C2O4)3,  K2C2O4  is  precipitated. 
In  applying  this  process  for  the  determination  of  bismuth  in  mixtures, 
it  is  necessary  to  separate  the  metal  as  oxychloride,  and  that  it 
should  be  obtained  in  solution  as  nitrate  with  a  small  excess  of 
nitric  acid.  This  is  done  by  evaporating  off  the  greater  part  of  the 
free  acid,  allowing  just  sufficient  to  remain  that  the  bismuth  may 
remain  in  solution  while  hot.  A  large  excess  of  acetic  acid  is  then 
added,  it  is  made  up  to  a  definite  measure,  and  an  aliquot  portion 
taken  for  titration. 

The  solution  of  normal  potassium  oxalate  standardized  by 
permanganate  must  not  be  added  in  great  excess.  It  is  well, 
therefore,  to  deliver  it  into  the  bismuth  liquid  from  a  burette  until 
the  precipitation  is  apparently  complete,  then  add  a  few  extra  c.c., 
and  allow  to  remain  for  some  time  with  shaking.  It  is  then  filtered 
through  a  dry  filter,  a  measured  portion  taken,  and  the  residual 
oxalic  acid  found  by  permanganate. 

For  determining  bismuth  in  ores  the  following  method  has  been 
worked  out  by  Warwick  and  Kyle.J 

One  gm.  of  the  finely  powdered  ore  is  evaporated  to  dryness  with  5  or  10  c.c. 
of  strong  nitric  acid  ;  another  5  c.c.  of  acid  and  25  c.c.  of  water  are  added,  and 
the  whole  is  diluted  to  100  c.c.  Five  gm.  of  ammonium  oxalate  or  oxalic  acid 
are  introduced,  boiled  for  five  mimites,  allowed  to  settle,  and  the  supernatant 
liquid  filtered  off.  The  precipitate  is  boiled  twice  with  50  c.c.  of  water,  and  the 
washings  are  passed  through  the  same  paper.  With  an  ordinary  ten  per  cent. 
ore  this  treatment  should  suffice  to  convert  the  bismuth  oxalate  into  the  basic 
salt ;  but  if  the  filtrate  is  still  acid,  boiling  must  be  repeated  to  neutrality.  The 
precipitate  on  the  paper  is  then  dissolved  in  2  to  5  c.c.  of  1:1  HC1,  receiving 
the  liquid  in  the  beaker  containing  the  bulk  of  the  basic  oxalate ;  this  is  warmed 
till  entirely  dissolved,  and  then  diluted  to  250  c.c.  with  hot  water.  The  solution 
is  neutralized  with  ammonia,  and  the  resulting  precipitate  taken  up  in  1  :  4 
H2S04,  adding  a  few  c.c.  in  excess.  Finally  the. liquid  is  titrated  at  between  70° 
and  100°  C.  with  permanganate.  A  permanganate  solution  in  which  1  c.c.  = 
0-010  gm.  Fo  will  be  equal  to  0-0186  gm.  bismuth  ;  by  diluting  100  c.c.  of  this 
with  86  c.c.  of  water  a  solution  of  permanganate  will  be  obtained,  of  which  1  c.c. 
should  equal  O'OIO  gm.  of  bismuth.  A  permanganate  solution  1  c.c.  =0'010  gm. 
Fe  ;  found  =  0-01868  gm.  Bi.  100  c.c.  permanganate  above +86  c.c.  of  water; 
1  c.c.  found  =0-01017  gm.  Bi.  Lead,  iron,  copper,  zinc,  arsenic,  and  tellurium 

*  J.  C.  S.  41,  1.  f  Xerichte,  14,  1172.  J  C.  N.  75,  3. 


168  BISMUTH. 

do  not  interfere  with  the  process.  Care  must  be  taken  to  avoid  using  too  little- 
or  too  much  nitric  acid.  Hydrochloric  acid  must  not  be  used  to  dissolve  the  ore. 
The  results  are  accurate  enough  for  all  commercial  work,  and  an  analysis  occupies 
little  time.  The  figures  quoted  show  maximum  errors  of  —  0'0032  and"  +0'001  gm. 
in  determining  0'5  gm.  of  bismuth  in  presence  of  lead,  copper,  zinc,  iron,  and 
arsenic. 

2.     Precipitation  as  Phosphate. 

The  necessary  standard  solutions  are — 

(a)  Standard  sodium  phosphate  containing  35*8  gm.  per  litre. 
1  c.c.  =0-0071  gm.  P2O5  or  0*0208  gm.  Bi. 

(b)  Standard  uranium  acetate,  corresponding  volume  for  volume 
with  the  above,  when  titrated  in  the  presence  of  an  approximately 
equal  amount  of  sodium  acetate  and  free  acetic  acid. 

Success  depends  very  much  upon  identity  of  conditions,  as  is- 
explained  under  Phosphates. 

METHOD  OF  PROCEDURE  :  The  bismuth  to  be  determined  must  be  dissolved  in 
nitric  acid  ;  bases  other  than  the  alkalies  and  alkaline  earths  must  be  absent. 
The  absence  of  those  acids  which  interfere  with  the  determination  of  phosphoric 
acid  by  the  uranium  process  (non- volatile,  and  reducing  organic  acidsr 
sulphuretted  hydrogen,  hydriodic  acid,  etc.)  must  be  assured.  As  bismuth  is- 
readily  separated  from  other  metals,  with  the  exception  of  antimony  and  tin,  by 
additions  of  much  warm  water  and  a  little  ammonium  chloride  to  feebly  acid 
solutions,  a  separation  of  the  bismuth  from  those  other  metals  which  are  present 
should  precede  the  process  of  estimation.  If  alkalies  or  alkaline  earths  be  alone 
present,  the  separation  may  be  dispensed  with.  The  precipitated  bismuth  salt  is 
to  be  washed,  dissolved  in  a  little  strong  nitric  acid,  and  the  solution  boiled 
down  twice  with  addition  of  a  little  more  nitric  acid,  in  order  to  remove  the 
whole  of  the  hydrochloric  acid  present. 

Such  a  quantity  of  a  tolerably  concentrated  solution  of  sodium  acetate  is 
added  as  shall  ensure  the  neutralization  of  the  nitric  acid,  and  therefore  the 
presence  in  the  liquid  of  free  acetic  acid.  If  a  precipitate  form,  a  further 
addition  of  acetate  must  be  made.  The  liquid  is  heated  to  boiling  ;  a  measured 
volume  of  the  sodium  phosphate  solution  is  run  in  ;  the  boiling  is  continued  for 
a  few  minutes  •  the  liquid  is  passed  through  a  ribbed  filter,  the  precipitate  being 
washed  repeatedly  with  hot  water ;  and  the  excess  of  phosphoric  acid  is 
determined  in  the  filtrate  by  titration  with  uranium.  If  the  filtered  liquid  be 
received  in  a  measuring  ilask,  which  is  subsequently  filled  to  the  mark  with 
water,  and  if  the  inverted  uranium  method 'be  then  employed,  the  results  are 
exceedingly  accurate.  This  method  is  especially  to  be  recommended  in  the 
determination  of  somewhat  large  quantities  of  bismuth,  since  it  is  possible  that  in 
such  cases  a  large  amount  of  sodium  acetate  will  have  been  used,  which,  as  is- 
well  known,  has  a  considerably  disturbing  effect  on  the  reaction  of  the  indicator. 

If  the  bismuth  solution  contain  a  large  excess  of  nitric  acid,  it  is  better  to- 
neutralize  nearly  Avith  sodium  carbonate  before  adding  sodium  acetate  and 
titrating. 

Fuller  details  of  both  the  above  processes  are  contained  in 
J.  C.  S.  1877  (p.  674)  and  1878  (p.  70). 

BROMINE. 

Br  =  79-92. 

THIS  element,  or  its  unoxidized  compounds,  can  be  determined 
precisely  in  the  same  way  as  chlorine  by  N/10  silver  solution  (p.  141),. 


BROMINE.  169 

or  by  thiocyanate  (p.  145),  but  these  methods  are  seldom  of  any 
avail,  since  the  absence  of  chlorine  or  its  combinations  is  a  necessary 
condition  of  accuracy. 

Bromine  in  aqueous  solution,  or  as  gas,  may  be  determined  by 
absorption  with  solution  of  potassium  iodide,  in  many  cases  by 
mere  digestion,  and  in  other  cases  by  distillation,  in  any  of  the 
forms  of  apparatus  given  on  p.  135  et  seq.,  and  the  operation  is 
carried  out  precisely  as  for  chlorine  (p.  176).  1  eq.  1  =  1  eq.  Br.  or 
I  found  x  0-6297  =  Br. 

A  process  for  the  determination  of  bromine  in  presence  of  chlorine 
is  still  much  wanted  in  the  case  of  examining  kelp  liquors,  etc. 
Heine*  uses  a  colour  method  in  which  the  bromine  is  liberated 
by  free  chlorine,  absorbed  by  ether,  and  the  colour  compared  with 
an  ethereal  solution  of  bromine  of  known  strength.  Fehling 
states  that  with  care  the  process  gives  fairly  accurate  results. 
It  is,  of  course,  necessary  to  have  an  approximate  knowledge  of  the 
amount  of  bromine  present  in  any  given  solution. 

Reimannf  adopts  the  following  method,  which  gives  tolerably 
accurate  results,  but  requires  skill  and  practice. 

The  neutral  bromine  solution  is  placed  in  a  stoppered  vessel,  together  with 
a  globule  of  chloroform  about  the  size  of  a  hazel  nut.  Chlorine  water  of  known 
strength  is  then  added  cautiously  from  a  burette,  protected  from  bright  light, 
in  such  a  way  as  to  ensure  first  the  liberation  of  the  bromine,  which  colours  the 
chloroform  orange  yellow  ;  then  more  chlorine  water  is  added,  until  the  yellowish 
white  colour  of  chloride  of  bromine  appears  (KBr  +C12=KC1  +BrCl). 

The  operation  may  be  assisted  by  making  a  weak  solution  of  potassium  chromate, 
of  the  same  colour  as  a  solution  of  chloride  of  bromine  in  chloroform,  to  serve  as 
a  standard  of  comparison. 

The  strength  of  the  chlorine  water  is  ascertained  by  potassium  iodide  and  N/io 
thiosulphate.  2  eq.  Cl  =  l  eq.  Br. 

In  examining  mother-liquors  containing  organic  matter,  they  must  be  evaporated 
to  dryness  in  presence  of  free  alkali,  ignited,  extracted  with  water  ;  then  neutralized 
with  hydrochloric  acid  before  titrating  as  above. 

CavazziJ  gives  a  method  which  answers  well  for  determining 
bromine  in  small  quantity,  when  mixed  with  large  proportions  of 
alkali  chlorides.  It  is  based  on  the  fact  that,  when  such  a  mixture 
is  heated  to  100°  C.  with  barium  peroxide  and  sulphuric  acid,  the 
whole  of  the  bromine  is  liberated  with  a  mere  trace  of  chlorine  ; 
the  bromine  so  evolved  is  absorbed  in  any  convenient  apparatus, 
such  as  fig.  38. 

The  distillation  is  carried  out  in  a  350  c.c.  flask  with  double-bored  stopper ; 
one  bore  carries  an  open  tube  reaching  to  the  bottom  of  the  flask,  the  other  the 
delivery  tube  which  is  connected  with  the  \J  tubes.  The  first  U  tube  is  empty ; 
the  second  contains  20  c.c.  of  a  standard  solution  of  arsenious  acid  in  hydrochloric 
acid,  containing  0*005  gm.  As203  in  each  c.c.,  and  is  connected  with  an  aspirator 
or  water  pump.  The  apparatus  is  arranged  so  that  the  flask  and  empty  (J  tube 
are  immersed  in  boiling  water,  the  vapours  of  IT202  are  thus  decomposed,  and  the- 
stream  regulated  by  the  aspirator. 

*  Journ-  i.  pracl.  Chem.  38,  184.  t  Annul,  d.  CJicm.  u.  Pliarm.  115,  140. 

J  Gazz.  Chim.  Hal.  13,  171. 


170  BROMINE. 

The  reagents  used  by  the  author  are — 

Barium  peroxide,  containing  63  %  Ba02. 

Dilute  sulphuric  acid  1  :  2. 

Arsenious  acid  dissolved  in  dilute  hydrochloric  acid,  5  gm.  of  pure  As203  per 
litre. 

Standard  permanganate,  3*55  gm.  per  litre. 

It  was  found  that  the  relative  strengths  of  the  arsenic  and  permanganate 
solutions,  when  titrated  together,  diluted,  and  boiling,  were,  18-2  c.c.  of  the  latter 
to  20  c.c.  of  the  former.  Therefore  1  c.c.  of  permanganate  by  calculation  = 
0-00888  gm.  Br. 

The  author  found  that  treating  2  gm.  of  KC1  in  the  apparatus,  without  bromine, 
always  gave  a  faint  trace  of  Cl,  so  that  only  18  c.c.  of  permanganate  were  required 
for  the  20  c.c.  of  arsenic,  instead  of  18-2  c.c.  ;  and  this  he  regards  as  a  constant  for 
that  quantity  of  material.  The  examples  of  analysis  with  from  0'05  to  0'2  gm. 
KBr,  and  all  with  the  correction  of  0'2  c.c.,  are  satisfactory. 

Norman  McCulloch*  has  described  a  method,  devised  by 
himself,  for  the  rapid  and  accurate  determination  of  bromine, 
in  presence  of  iodine  or  chlorine,  in  any  of  the  ordinary  commercial 
forms  or  chemical  combinations  free  from  oxidizing  and  reducing 
agents  and  metals  forming  bromides  insoluble  in  hydrochloric  acid. 
The  author's  explanation  of  the  principles  upon  which  the  method 
is  based  is  complicated  and  voluminous,  and  to  this  the  reader  is 
referred.  I  have  not  been  able  to  verify  the  method,  but  as  the 
author  is  known  to  have  practical  experience,  as  well  as  theoretical 
knowledge,  a  short  summary  is  given  here. 

The  solutions  described  by  the  author  are- 
Standard  permanganate,  3*19  gm.  per  litre; 

Standard  potassium  iodide,  8*278  per  litre. 

The  solutions  should  agree  volume  for  volume,  but  it  is  preferable 
to  verify  them  by  dissolving  2-3  gm.  of  iodine  in  caustic  soda,  in 
a  150  c.c.  stoppered  bottle,  adding  HC1  in  good  excess,  cooling, 
then  adding  the  permanganate  from  a  burette  until  nearly  colour- 
less. A  little  chloroform  as  indicator  is  then  added,  and  the 
permanganate  cautiously  run  in,  with  shaking,  until  the  violet 
colour  of  the  iodine  is  discharged,  owing  to  production  of  IC1,  due 
to  the  reaction  of  Cl  liberated  by  the  permanganate  from  HC1. 
The  iodine  equivalent  of  the  permanganate  is  calculated  to  bromine 
by  the  coefficient  x  0*6713  and  each  c.c.  permanganate  should 
represent  about  0*004  gm.  of  Br. 

The  other  reagents  are : — Chloroform,  purified  by  adding  some 
permanganate,  then  HC1  till  the  colour  is  discharged,  then  a  little  KI 
and  the  I  so  liberated  again  discharged  with  permanganate,  the 
chloroform  being  finally  washed  free  from  all  acid; 

A  three  per  cent,  solution  of  hydrocyanic  acid,  made  by  decom- 
posing a  solution  of  pure  potassium  cyanide  with  excess  of  HC1  and 
adding  permanganate  till  a  faint  pink  colour  remains.  40  gm.  of 
KCN  in  400  c.c.  of  water  with  70  c.c.  of  HC1  will  give  such  a  solution. 
Owing  to  its  poisonous  nature  great  caution  must  be  used  in  'making 
this  solution,  and  to  avoid  as  much  as  possible  the  evolution  of 
prussic  acid  the  temperature  must  be  kept  down  by  ice  or  a  freezing 

*  C.  N.  60,  259. 


BROMINE.  171 

mixture  of  nitre  and  sal  ammoniac.  If  the  cyanide  contains,  as  is 
often  the  case,  some  alkali  carbonate,  this  should  be  removed 
previously  by  BaCl2,  as  otherwise  CO2  will  be  liberated  and  a  loss 
•of  HCN  occur.  Finally  the  cool  solution  is  rendered  faintly  pink 
with  some  permanganate ; 

Solution  of  manganous  chloride,  made  by  dissolving  500  gm.  of 
MnCl2  +  4H2O  in  250  c.c.  of  warm  water.  This  solution  is  used  to 
prevent  the  liberation  of  free  chlorine  from  the  HC1  in  the  analysis. 

METHOD  OF  PROCEDURE  :  The  weighed  bromide,  containing  from  O'Oo  to 
•O'lo  grn.  of  Br,  is  dissolved  in  15  c.c.  of  water  in  a  150  c.c.  stoppered  bottle,  and 
-about  30  c.c.  of  the  manganese  solution  added;  permanganate  is  then  run  in 
in  excess  of  the  required  quantity,  and  the  bottle  cooled  rapidly  to  10°  C.  by  ice 
or  a  freezing  mixture.  When  cooled,  the  bottle  is  shaken  by  a  rotary  motion, 
and  about  15  c.c.  of  moderately  strong  HC1  slowly  added,  with  motion  of  the 
bottle  to  dissolve  the  manganic  hydroxide,  2-4  c.c.  of  hydrocyanic  solution  are 
then  delivered  in,  the  bottle  closed  and  returned  to  the  cooling  mixture  for  about 
half  an  hour.  The  liquid  is  then  titrated  with  the  standard  potassium  iodide, 
until  nearly  decolorized  from  the  decomposition  of  the  manganic  chloride,  and 
then  slightly  coloured  from  liberation  of  free  I.  Lastly,  the  slight  excess  of 
iodide  is  determined  by  adding  a  little  chloroform,  and  the  titration  finished  with 
permanganate.  The  bromine  is  calculated  by  taking  the  difference  between  the 
amounts  of  bromine,  represented  by  total  permanganate  and  iodide  used.  If 
iodine  is  present  it  is,  of  course,  recorded  as  bromine,  and  its  amount,  if  required, 
must  be  ascertained  by  some  other  method  capable  of  its  determination  in  the 
presence  of  bromine. 

The  author  gives  several  very  good  results  obtained  with  pure 
sodium  bromide. 


CADMIUM. 

Cd  =  112-4. 

THIS  metal  may  be  determined,  as  may  many  others,  by 
precipitating  as  sulphide  and  decomposing  the  sulphide  with 
a  ferric  salt,  the  iron  being  reduced  to  the  ferrous  state  in  proportion 
to  the  amount  of  sulphide  present. 

Follenius  has  found  that  when  cadmium  is  precipitated  as 
sulphide  in  acid  liquids  the  precipitate  is  apt  to  be  contaminated 
to  a  small  extent  with  salts  other  than  sulphide.  The  separation 
as  sulphide  is  best  made  by  passing  H2S  into  the  hot  liquid  which 
•contains  the  cadmium,  and  which  should  be  acidified  with  10  per 
cent,  of  concentrated  sulphuric  acid  by  volume.  From  hydro- 
chloric acid  solutions  the  metal  is  only  completely  separated  by 
H2S  when  the  hot  solution  contains  not  more  than  5  per  cent,  of 
acid  of  sp.  gr.  I'll,  or  14  per  cent,  if  the  liquid  is  cold. 

Ferric  chloride  is  to  be  preferred  for  the  decomposition  of  the 
cadmium  sulphide,  and  the  titration  is  carried  out  precisely  as  in 
the  case  of  Zinc  (q.  v.). 

P.  von  Berg*  gives  a  good  technical  process  for  the  determination 
of  either  cadmium  or  zinc  as  sulphide,  by  means  of  iodine,  as 
follows  : — 

*  Z.  a.  C.  28,  23. 


172  CADMIUM. 

METHOD  OF  PROCEDURE  :  The  washed  sulphide  of  zinc  or  cadmium  is 
allowed  to  drain  upon  the  filter,  and  then  transferred,  together  with  the  filter,  to 
a  stoppered  flask  containing  800  c.c.  of  water  deprived  of  air  by  boiling  and  the 
passage  of  carbonic  acid  gas.  The  whole  is  well  shaken  in  order  to  break  up  the 
precipitate  and  bring  it  into  the  most  finely  divided  condition  possible,  so  that  the 
sulphide  may  not  be  protected  from  the  action  of  the  iodine  by  separated  sulphur. 
A  moderate  quantity  of  hydrochloric  acid  is  added,  there  being  no  necessity  to 
entirely  dissolve  the  sulphide,  and  then  an  excess  of  iodine  solution  of  known 
strength.  The  residual  free  iodine  is  then  titrated  with  thiosulphate  without 
loss  of  time.  The  whole  operation,  from  the  transference  of  the  sulphide  to  the 
flask  to  the  final  titration,  occupies  about  five  minutes,  and  gives  results  varying 
between  98-8  and  100'2  per  cent.  The  reaction  proceeds  according  to  the 
equation,  ZnS  +2HC1  +1,  --ZnCU  +  2HI  +S. 


CALCIUM. 

Ca  =  40-09. 

1  c.c.  N/,0  permanganate  =  0'002S05  gm.  CaO 

-0-005005  gm.  CaC03 
=0-00861  gm.  CaS04H-2H20 
.,  normal  oxalic  acid =0*02805  gm.  CaO 

Cry st.  oxalic  acid  x  0*444  =CaO 

Double  iron  salt  x 0*07143          =CaO 

THE  determination  of  calcium  alkalimetrically  has  already  been 
described  (p.  72),  but  that  method  is  of  limited  application,  unless 
calcium  oxalate,  in  which  form  Ca  is  generally  separated  from  other 
bases,  be  converted  into  carbonate  or  oxide  by  ignition,  and  thus 
determined  with  normal  nitric  acid  and  alkali.  This"and  the  follow- 
ing method  by  Hemp  el  are  as  exact  in  their  results'as  the  determi- 
nation by  weight ;  and  where  a  series  of  determinations  have  to 
be  made,  the  method  is  very  convenient. 

Titration  with  Permanganate. — The  readiness  with  which  calcium 
can  be  separated  as  oxalate  facilitates  the  use  of  this  method,  so 
that  it  can  be  applied  successfully  in  a  great  variety  of  instances. 
It  is  not  necessary  here  to  enter  into  detail  as  to  the  method  of 
precipitation  ;  except  to  say  that  it  may  take  place  in  either 
ammoniacal  or  weak  acetic  acid  solution,  and  that  it  is  absolutely 
necessary  to  remove  all  excess  of  ammonium  oxalate  from^the 
precipitate  by  washing  with  warm  water  previous  to  titration. 

METHOD  OF  PROCEDURE  :  When  the  clean  precipitate  is  obtained,  a  hole  is 
made  in  the  filter,  and  the  bulk  of  the  precipitate  is  washed  through  the  funnel 
into  a  flask ;  the  filter  is  then  treated  with  small  quantities  of  hot  dilute 
sulphuric  acid,  and  again  washed  into  the  flask.  Hydrochloric  acid  in  moderate 
quantity  may  safely  be  used  for  the  solution  of  the  oxalate,  since  there  is  not  the 
danger  of  liberating  free  chlorine  which  exists  in  the  case  of  iron  (Fleischer, 
Titrirmdhodc,  p.  70),  but  sulphuric  is  better. 

When  the  precipitate  is  completely  dissolved,  the  solution  is  freely  diluted 
with  water,  and  further  acidified  with  sulphuric  acid,  warmed  to  00°  or  70°,  and 
the  standard  permanganate  cautiously  delivered  into  the  liquid  with  constant 


CALCIUM.  1 73 

agitation  until  a  faint  permanent  pink  tinge  occurs,,  precisely  as  in  the  case  of 
standardizing  permanganate  with  oxalic  acid  (p.  123). 

PROCEDURE  FOR  LIME  IN  BLAST  FURNACE  SLAGS  :  Place  about  1  gm.  of  the 
very  finely-ground  slag  into  a  beaker,  cover  with  water,  and  boil  gently,  then  add 
gradually  strong  HC1  until  the  whole  is  dissolved,  including  SiO2.  Dilute  the 
liquid,  nearly  neutralize  with  ammonia,  and  add  a  solution  of  ammonium  acetate. 
The  silica  and  alumina  form  a  flocculent  precipitate  which  is  easily  washed  on 
a  filter.  The  filtrate  and  washings  are  concentrated  somewhat,  and  the  CaO 
precipitated  with  ammonium  oxalate  and  free  ammonia ;  the  precipitate  is 
dissolved  as  before  described  in  hot  dilute  sulphuric  acid,  and  titrated  with 
permanganate.  If  much  manganese  is  present,  the  calcium  oxalate  must  be 
re-dissolved  and  re-precipitated  before  the  titration  is  made. 

In  all  cases  where  a  clean  oxalate  precipitate  can  be  obtained, 
such  as  mineral  waters,  manures,  etc.,  very  exact  results  are 
obtainable  ;  in  fact,  quite  as  accurate  as  by  the  gravimetric  method. 
Ample  testimony  on  this  point  is  given  by  Fresenius,  Mohr, 
Hempel,  and  others.  When  much  iron,  alumina,  magnesia,  etc., 
is  present,  it  will  be  preferable  to  re-precipitate  the  oxalate,  so  as 
to  free  it  from  adhering  contaminations  previous  to  titration. 

Indirect  Titration. — In  the  case  of  calcium  salts  soluble  in  water 
and  of  tolerably  pure  nature,  the  determination  by  permanganate 
can  be  made  by  adding  to  the  solution  a  measured  excess  of 
normal  oxalic  acid,  then  ammonia  in  slight  excess,  and  heating  to 
boiling,  so  as  rapidly  to  separate  the  precipitate.  The  mixture 
is  then  cooled,  diluted  to  a  measured  volume,  filtered  through  a  dry 
filter,  and  an  aliquot  portion  titrated  with  permanganate,  after 
acidifying  with  sulphuric  acid  as  usual.  A  great  variety  of  calcium 
salts  may  be  converted  into  oxalates  by  a  short  or  long  treatment 
with  oxalic  acid  or  ammonium  oxalate,  including  calcium  sulphate, 
phosphate,  tartrate,  citrate,  etc. 


CERIUM. 

Ce  =  140-25. 

STOLE  A*  states  that  the  moist  cerium  oxalate  may  be  titrated 
precisely  as  in  the  case  of  calcium  oxalate  with  permanganate, 
and  with  accurate  results.  No  examples  or  details,  however,  are 
given.  It  is  probable  that  it  is  only  correct  in  the  case  of  the  pure 
substance. 

This  method  has,  however,  been  examined  by  P.  E.  Browning 
and  A.  Lynch,|  who  prepared  the  oxalate  from  pure  cerium 
chloride  by  ammonium  oxalate.  Definite  volumes  of  the  cerium 
solution,  the  exact  strength  of  which  was  known  (from  O'l  to  0'2 
gm.  of  CeCl2),  were  used  for  precipitation  at  a  moderate  temperature, 
some  trials  being  made  on  neutral  portions  and  some  on  portions 

*  Z.  a.  C.  19,  194.  t  Amer.  Journ.  Science,  8,  No.  48. 


17i  CERIUM. 

slightly  acid  with  HC1  (in  which  case  about  1  gm.  of  manganous. 
sulphate  was  used).  The  precipitate,  after  being  carefully  washed, 
was  dissolved  in  about  10  c.c.  of  hot  dilute  H2SO4,  and  then 
made  up  to  about  500  c.c.  with  hot  water  at  about  80°  C.  when 
the  titration  with  permanganate  was  immediately  made.  The 
results  obtained  were,  both  in  the  neutral  and  acid  solutions,  very 
near  the  amounts  of  cerium  taken. 

Bun  sen's  method,  originated  many  years  ago,  showed  that  the 
oxide  of  cerium  obtained  by  ignition  of  the  oxalate  might  be 
determined  volumetrically  by  dissolving  it  in  strong  HC1  with  a  few 
crystals  of  KI  in  a  small  sealed  flask,  which  was  heated  on  a  water- 
bath  till  the  oxide  was  dissolved  and  the  free  iodine  liberated. 
The  iodine  was  then  titrated  with  thiosulphate  in  the  usual  way 
and  the  amount  of  cerium  calculated  therefrom. 

1  c.c.  N/10  thiosulphate  =0-017225  gm.  CeO2. 

A  modification  of  this  method  was  adopted  with  satisfactory 
results  by  Browning,  Hanford,  and  Hall,  as  follows  : — 

METHOD  OF  PROCEDURE  :  Weighed  portions  of  the  pure  cerium  dioxide, 
about  0-1  to  0-15  gm.,  were  placed  in  small  glass-stoppered  bottles  of  about 
100  c.c.  capacity,  together  with  1  gm.  of  potassium  iodide  free  from  iodate  and 
a  few  drops  of  water  to  dissolve  the  iodide.  A  current  of  C02  was  passed  into- 
the  bottle  for  about  five  minutes  to  expel  the  air,  10  c.c.  of  pure  strong  HC1 
were  added,  the  stopper  inserted,  and  the  bottle  heated  gently  upon  a  steam 
radiator  for  about  one  hour,  until  the  dioxide  dissolved  completely  and  the  iodine- 
was  set  free.  After  cooling  the  bottle,  to  prevent  loss  of  iodine  upon  removing 
the  stopper,  the  contents  were  carefully  washed  into  about  400  c.c.  of  water,  and 
titrated  with  N/io  sodium  thiosulphate  to  determine  the  amount  of  iodine- 
liberated  according  to  the  reaction — 

2Ce02  +  8HC1  +  2KI  =2CeCl3  +  2KC1  +  4H20  + 12. 

A  few  blank  determinations  were  carried  through  in  the  bottles  without  the 

.  cerium  dioxide  to  determine  the  amount  of  iodine  set  free  under  these  conditions. 

The  amount  obtained  was  uniformly  equal  to  0'04  c.c.  of  the  N/io<  thiosulphate 

solution,  which  was  taken  as  the  correction  and  applied  to  all  the  determinations. 

Determination  in  the  Presence  of  other  Rare  Earths.  G.  vonKnorre.*  This 
is  based  on  the  fact  that  the  yellow  eerie  salts  are  reduced  by  hydrogen  peroxide 
in  the  presence  of  free  acid  to  colourless  cerous  salts,  as  in  the  equation  : 

2Ce(S04)2  +H202  -Ce2(S04)3  +H2S04  +02. 

The  cold  solution  of  the  eerie  salt  is  mixed  with  an  excess  of  a  dilute  solution  of 
hydrogen  peroxide,  of  which  the  strength  is  known,  and  when  all  colour  has 
disappeared  the  excess  of  peroxide  is  titrated  back  with  permanganate. 

If  the  permanganate  be  standardized  on  iron,  the  amount  of  cerium  present 
may  be  expressed  in  terms  of  iron,  55*85  parts  of  the  latter  being  equivalent  to 
140'25  parts  of  cerium.  It  is  advisable  to  use  a  dilute  solution  of  permanganate 
(not  more  than  2  gm.  of  KMn04  per  litre). 

Notwithstanding  Rose's  statement  that  permanganate  is  slowly  decolorized 
by  a  solution  of  cerous  sulphate,  the  author  finds  that  the  end-reaction  can  be 
readily  recognised.  With  a  freshly  prepared  acidified  solution  of  a  eerie  salt  the 
reduction  takes  place  instantaneously,  but  if  the  solution  has  been  exposed  to 
the  air  for  some  time,  as  long  as  fifteen  minutes  may  be  necessary  for  complete 
decolorization.  The  results  obtained,  however,  in  both  cases  are  identical.  By 
boiling  an  old  solution  after  the  addition  of  sulphuric  acid,  and  cooling  before 

*  Z.  a.  C.,  1897,  085-688. 


CERIUM.  175 

adding  the  hydrogen  peroxide,   the  rate  of  reduction  is  accelerated,   and  the 
reaction  takes  place  almost  as  rapidly  as  in  a  freshly  prepared  solution. 

Either  sulphuric  or  nitric  acid  may  be  used,  but  it  is  essential  that  the 
acidification  shall  take  place  before  the  addition  of  the  hydrogen  peroxide,  since 
otherwise  by-reactions  occur  and  the  results  are  too  high. 

A  method  of  Gibbs*  is  modified  by  Job|  as  follows  :— 

A  known  volume  of  the  cerium  solution  is  treated  in  the  cold  with  peroxide 
of  lead,  and  a  large  excess  of  concentrated  nitric  acid,  in  order  to  oxidize  any 
cerous  salts  present,  then  the  mixture  is  agitated,  filtered,  and  the  filtrate  titrated 
with  dilute  hydrogen  peroxide.  The  determination  of  cerium  by  this  method 
may  be  carried  out  equally  well  in  presence  of  thorium,  lanthanum,  and 
didymium,  and  should  thus  be  of  great  use  in  directly  determining  cerium  in 
the  crude  oxalates  from  monazite  sand. 


CHLORINE. 

01  =  35-46. 

1  c.c.  N/10  silver  solution =0'003546  gm.  Cl. 

=0-005846  gm.  NaCl. 

THE  powerful  affinity  existing  between  chlorine  and  silver  in 
solution,  and  the  ready  precipitation  of  the  resulting  chloride , 
seem  to  have  led  to  the  earliest  important  volumetric  process  in 
existence,  viz.,  the  assay  of  silver  by  the  wet  method  of  Gay 
Lussac.  The  details  of  the  process  are  more  particularly  described 
under  the  article  relating  to  the  assay  of  Silver  q.v. ;  the 
determination  of  chlorine  is  just  the  converse  of  the  process  there 
described,  and  the  same  precautions,  and  to  a  certain  extent  the 
same  apparatus,  are  required. 

The  solutions  required,  however,  are  systematic,  and  for  exactness 
and  convenient  dilution  are  of  decinormal  strength  as  described 
on  p.  141.  In  many  cases  it  is  advisable  to  possess  also  centinormal 
solutions,  made  by  diluting  100  c.c.  of  N/10  solution  to  1  litre. 

1.     Direct  Precipitation  with  N/10  Silver  solution. 

Very  weak  solutions  of  chlorides,  such  as  good  drinking  waters,  are  not  easily 
examined  for  chlorine  by  direct  precipitation,  unless  they  are  considerably 
concentrated  by  evaporation  previous  to  treatment,  owing  to  the  fact  that,  unless- 
a  tolerable  quantity  of  chloride  can  be  formed,  it  will  not  collect  together  and 
separate  so  as  to  leave  the  liquid  clear  enough  to  tell  whether  on  the  addition 
of  fresh  silver  a  distinct  formation  of  chloride  occurs.  The  best  effects  are 
produced  when  the  mixture  contains  chlorine  equal  to  from  1|  to  2  gm.  of  salt 
per  100  c.c.  Should  the  proportion  be  much  less  than  this,"  the  difficulty  of 
precipitation  may  be  overcome  by  adding  a  quantity  of  freshly  precipitated 
chloride,  made  by  mixing  equal  volumes  of  N/io  salt  and  silver  solutions,  shaking 
vigorously,  pouring  off  the  clear  liquid,  and  adding  the  chloride  to  the  mixture 
under  titration.  The  best  vessel  to  use  for  the  trial  is  a  well  stoppered  round 
white  bottle,  holding  from  100  to  150  c.c.,  and  fitting  into  a  paper  case,  so  as  to 
prevent  access  of  strong  light  during  the  titration.  Supposing,  for  instance, 
a  neutral  solution  of  potassium  chloride  requires  titration,  20  or  30  c.c.  are  measured 
into  the  shaking  bottle,  a  few  drops  of  strong  nitric  acid  added  (free  acid  must 

*  Z.  a.  C.,  1864,  p.  SC5.  1  Compf.  Rend.,  1899,  p.  101. 


176  CHLORINE. 

always  be  present  in  direct  precipitation),  and  a  round  number  of  c.c.  of  silver 
solution  added  from  the  burette.  The  bottle  is  placed  in  its  case  (or  may  be 
enveloped  in.  a  dark  cloth)  and  vigorously  shaken  for  half  a  minute,  then  un- 
covered, and  gently  tapped  upon  a  table  or  book,  so  as  to  start  the  chloride 
downward  from  the  surface  of  the  liquid  where  it  often  swims.  A  quick 
clarification  indicates  excess  of  silver.  The  nearer  the  point  of  exact  counter- 
balance the  more  difficult  to  obtain  a  clear  solution  by  shaking,  but  a  little  practice 
soon  accustoms  the  eye  to  distinguish  the  faintest  precipitate. 

In  case  of  overstepping  the  balance  in  any  trial,  it  is  only  necessary 
to  add  to  the  liquid  under  titration  a  definite  volume  of  N/10  salt 
solution,  and  finish  the  titration  in  the  same  liquid,  deducting,  of 
course,  the  same  number  of  c.c.  of  silver  as  has  been  added  of 
salt  solution. 

Fuller  details  and  precautions  are  given  under  Silver. 

2.     Precipitation  by  N/10  Silver  in  Neutral  Solution  with 
Chromate  Indicator  (see  p.  142.) 

3.    Titration  with  N/10  Silver  and  Thiocyanate  (see  p.  145). 

This  method  gives  very  accurate  results  if,  after  the  chlorine  is 
precipitated  with  excess  of  N/10  silver,  the  silver  chloride  is  filtered 
off,  washed  well,  and  the  filtrate  and  washings  titrated  with  N/10 
thiocyanate  for  the  excess  of  silver. 

METHOD  OF  PROCEDURE  :  The  material  to  be  titrated,  such  as  water  residues, 
beer  ash,  or  other  substances  in  which  the  chlorine  is  to  be  determined,  being 
brought  into  clear  solution,  a  known  volume  of  N/io  silver  in  excess  is  added, 
the  mixture  having  been  previously  acidified  with  nitric  acid  ;  the  mixture  is  well 
stirred,  and  the  supernatant  liquid  filtered  off  through  a  small  filter,  the 
chloride  well  washed,  and  to  the  filtrate  and  washings  5  c.c.  of  ferric  indicator 
(p.  146)  and  the  same  volume  of  nitric  acid  (p.  146)  are  added.  The  flask  is 
then  brought  under  the  thiocyanate  burette,  and  the  solution  delivered  in  with 
a  constant  gentle  movement  of  the  liquid  until  a  permanent  light-brown  colour 
appears.  If  the  silver  chloride  is  not  removed  from  the  liquid  previous  to 
titration  a  serious  error  may  occur,  owing  to  the  ready  solubility  of  the  chloride 
in  the  thiocyanate  solution. 

4.     By  Distillation  and  Titration  with  Thiosulphate 
or  Arsenite. 

In  cases  where  chlorine  is  evolved  directly  in  the  gaseous  form 
or  as  the  representative  of  some  other  body  (see  p.  135),  a  very  useful 
absorption  apparatus  is  shown  in  fig.  38.  The  little  flask  a  is  used 
as  a  distilling  vessel,  connected  with  the  bulb  tubes  by  an  india- 
rubber  joint  ;*  the  stoppers  for  the  tubes  are  also  of  the  same 
material,  the  whole  of  which  should  be  cleansed  from  sulphur  by 
boiling  in  weak  alkali.  A  fragment  of  solid  magnesite  may  with 
advantage  be  added  to  the  acid  liquid  in  the  distilling  flask  ;  in  all 
other  respects  the  process  is  conducted  exactly  as  is  described  . 
on  p.  135  et  seq. 

*  India-rubber  and  especially  vulcanized  rubber  is  open  to  some  objection  in  these 
analyses,  and  apparatus  is  now  readily  to  be  had  with  glass  connections. 


CHLORIMETBY.  177 

This  apparatus  is  equally  well  adapted  to  the  absorption  of 
-ammonia  or  other  gases,  and  possesses  the  great  recommendation 
,that  there  is  scarcely  a  possibility  of  regurgitation. 

Mohr's  apparatus  (fig.  39),  is  also  serviceable  for  this  method. 


CHLORINE  GAS  AND  BLEACHING  COMPOUNDS. 

1  c.c.  N/10  sodium  arsenite  or  thiosulphate  solution  =  0'003546 
:gm.  Cl. 

1  litre  of  chlorine  at  0°  C.,  and  760  mm.,  weighs  3-219  gm. 

CHLORINE  water  may  be  titrated  with  thiosulphate  by  adding 
~a  measured  quantity  of  it  to  a  solution  of  potassium  iodide,  then 
^delivering  the  thiosulphate  from  a  burette  till  the  colour  of  the 

free  iodine  has  disappeared  ;  or  by  using  an  excess  of  the  reducing 

agent,    then    starch,    and   titrating    residually   with   N/10    iodine. 

When  arsenious  solution  is  used  for  titration,  the  chlorine  water  is 
-delivered  into  a  solution  of  sodium  carbonate,  excess  of  arsenious 

solution  added,  then  starch  and  N/10  iodine  till  the  colour  appears, 

or  iodized  starch-paper  may  be  used. 

Bleaching  Powder. — This  important  substance,  which  is  also 
called  chloride  of  lime,  is  made  by  the  action  of  chlorine  gas  on 
slaked  lime.  Its  composition  appears  to  be  best  represented  by 
-the  formula  Ca  (OC1)  Cl,*  which  is  due  to  Odling.  When  treated 
with  water,  it  is  resolved  into  calcium  chloride  and  hypochlorite, 
-thus  :— 

2Ca  (OC1)  Cl  =  CaCl2+Ca  (OC1)2. 

'The  calcium  hypochlorite  constitutes  the  bleaching  agent. 

The  technical  analysis  is  confined  to  the  determination  of  the 
"*'  available "  or  "  bleaching "  chlorine,  which  in  England  and 
America  is  always  expressed  as  percentage  by  weight  on  the 
bleaching  po\vder.  In  France,  however,  its  strength  is  given  in 
<*ay-Lussac  degrees,  which  indicate  the  number  of  litres  of 
'chlorine  gas,  at  0°  C.  and  760  mm.,  capable  of  being  evolved  from 
one  kilogram  of  bleaching  powder. 

100  French  Degrees  =  31*78  per  cent,  chlorine. 

1.     Titration  by  Arsenious  Solution  (Penot). 

The  first  thing  to  be  done  in  determining  the  value  of  a  sample 
vof  bleaching  powder  is  to  bring  it  into  solution,  which  is  best 
managed  as  follows  : — 

'The  sample  is  well  and  quickly  mixed,  and  7 '09  gm.  weighed,  put  into 
;a  mortar,  a  little  water  added,  and  the  mixture  rubbed  to  a  smooth  cream ;  more 
water  is  then  stirred  in  with  the  pestle,  allowed  to  settle  a  little  while,  then 
poured  off  into  a  litre  flask  ;  the  sediment  again  rubbed  with  water,  poured  off, 
.and  so  on  repeatedly,  until  the  whole  of  the  chloride  has  been  conveyed  into  the 

*Calcium  chloro-hypochlorite. 


178  CHLORTMETRY. 

flask  without  loss,  and  the  mortar  washed  quite  clean.  The  flask  is  then  filled 
to  the  mark  with  water,  well  shaken,  and  50  c.e.  of  the  milky  liquid  (  =0'3546  gm. 
bleaching  powder)  taken  out  with  a  pipette,  emptied  into  a  beaker,  and  the  N/io 
arsenious  solution  delivered  in  from  a  burette  until  a  drop  of  the  mixture  taken  out 
with  a  glass  rod  and  brought  in  contact  with  iodized  starch-paper  (p.  140)  gives 
no  blue  stain. 

The  starch-paper  may  be  dispensed  with  by  adding  arsenious  solution  in  excess, 
then  starch,  and  titrating  residually  with  N/io  iodine  till  the  blue  colour  appears. 
The  number  of  c.c.  of  arsenite  used  gives  percentage  of  available  chlorine  (35  % 
available  chlorine  is  a  common  guarantee). 


2.     Buns  en's  Method. 

10  or  20  c.c.  of  the  chloride  of  lime  solution,  prepared  as  above,  are  measured 
into  a  beaker,  and  an  excess  of  solution  of  potassium  iodide  added ;  the  mixture 
is  then  diluted  somewhat,  acidified  with  acetic  acid,  and  the  liberated  iodine 
titrated  with  N/io  thiosulphate  and  starch;  1  eq.  iodine  so  found  represents 
1  eq.  chlorine. 

The  presence  of  chlorate  does  not  affect  the  result  Avhen  acetic 
acid  is  used.  If  it  be  desired  to  determine  the  amount  of  chlorate 
in  bleach,  the  following  method  has  been  devised  by  R.  Fresenius. 
It  depends  on  the  fact  that  hypochlorites  are  decomposed  by  lead 
acetate  with  formation  of  lead  peroxide,  whilst  the  chlorate  which 
may  be  present  is  unaffected. 

METHOD  OF  PROCEDURE  :  20  gm.  of  bleaching  powder  are  ground  up  with 
water  in  repeated  quantities  and  made  up  to  a  litre  ;  after  settling,  50  c.c. 
(  =1  gm.  of  bleach)  are  filtered  off  through  a  dry  filter,  put  into  a  flask,  and  mixed 
with  a  solution  of  lead  acetate  in  some  excess.  There  is  formed  at  first  a  white 
precipitate  of  lead  chloride  and  lead  hydroxide  ;  these  being  acted  on  by  the 
hypochlorite  become  first  yellow,  then  brown,  with  liberation  of  chlorine  and 
passing  into  lead  peroxide.  After  the  precipitate  has  settled,  more  lead  solution 
is  added,  to  make  sure  that  the  conversion  is  complete.  The  mixture  is  allowed 
to  stand  in  the  open  flask,  with  frequent  shaking,  till  all  smell  of  chlorine  has 
disappeared,  which  occurs  in  from  eight  to  ten  hours.  The  precipitate  is  then 
filtered  off  and  washed  till  the  wash-water  is  free  from  acid.  The  washings  are 
evaporated  somewhat,  added  to  the  filtrate,  and  the  whole  mixed  with  sodium 
carbonate  in  slight  excess,  to  precipitate  the  lead  and  lime  as  carbonates — these 
are  well  washed,  the  filtrate  and  washings,  which  contain  the  chlorate  as  the 
sodium  salt,  evapoiated  nearly  to  dryness,  then  transferred  to  either  a  Fresenius 
or  Mohr  apparatus  (fig.  38  or  39)  and  distilled  with  HC1  as  directed  on  p.  135 
et  seq.  1  eq.  :  I  =CI2O5. 

Mixtures  of  Chlorides,  Hypochlorites,  and  Chlorates. — It  is  known 
that  chlorine  acting  upon  alkali  and  alkaline- earthy  hydrates 
gives  rise  to  chlorides,  and  at  the  same  time  to  chlorates,  or  to 
hypochlorites,  according  as  the  temperature  and  the  concentration 
are  higher  or  lower.  Under  average  conditions  the  three  kinds  of 
salts  are  formed  simultaneously. 

A  mixture  of  the  same  salts  is  produced  if  solutions  of  sodium 
chloride  are  submitted  to  electrolysis,  according  to  the  processes 
recently  used  for  the  manufacture  of  free  chlorine  and  of  caustic 
soda,  or  of  chlorates,  or  hypochlorites. 

In  these  various  cases  it  is  of  great  industrial  importance  to 
determine  easily  the  proportion  of  each  of  the  salts  present. 


CHLORINE.  179 

For  the  analysis  of  such  a  mixture  of  salts,  the  subjoined  method 
is  recommended  as  at  once  expeditious  and  accurate.  All  the 
determinations  are  performed  successively  upon  the  same  solution.* 

METHOD  OF  PROCEDURE  :  1.  The  mixture  of  Hypochlorite,  chlorate,  and 
chloride  is  poured  into  a  beaker.  There  is  then  run  into  it  from  a  burette  a  standard 
solution  of  alkali  arsenite  until  the  hypochlorite  is  completely  reduced.  To 
find  the  exact  moment  when  the  reduction  is  completed,  a  drop  of  the  liquid 
is  placed  upon  a  porcelain  plate  in  contact  with  a  drop  of  solution  of  potassium 
iodide  and  starch. 

On  the  mixture  of  the  two  drops  there  appears  a  blue  colour  as  long  as  there 
remains  any  hypochlorite  not  reduced.  As  soon  as  the  mixture  ceases  to  become 
coloured,  the  volume  of  the  arsenite  liquid  is  noted,  and  the  proportion  of 
hypochlorite  or  hypochlorous  acid  which  has  transformed  it  into  arsenic  acid 
is  obtained  ;  or  consequently,  that  of  the  corresponding  chlorine. 

As20,  +CaCl202  =As205  +  CaCl2. 
or 

As203  +2NaC10  =As205  +2NaCl. 

2.  The  liquid  (which  now  contains  merely  chlorate  and  chloride  )  is  slightly 
acidified   with   sulphuric  acid,  and   a   quantity   of  ammonium-ferrous   sulphate 
added,  at  least  twenty  times  as  much  as  the  chlorate  supposed  to  be  present. 
Heat  to  about  100°,  adding  in  small  successive  quantities  5  c.c.  of  sulphuric  acid 
diluted  with  15  c.c.  of  water.     It  is  best  to  use  a  tap-funnel,  letting  the  acid  fall 
in  drop  by  drop.     After  having  stoppered  the  vessel,  to  avoid  contact  of  air,  it 
is  allowed  to  cool  for  a  short  time,  and  the  excess  of  ferrous  salt  is  then  titrated 
with  permanganate.     As  the  quantity  of  ferrous  salt  which  was  introduced  is 
known,  by  difference  the  quantity  which  has  been  peroxidized  at  the  expense  of 
the  chlorate  reduced  to  the  state  of  chloride  is  found 

NaC103+6FeO=NaCl+Fe203.     . 

It  is  thus  easy  to  calculate  the  proportion  of  chlorate  or  of  chloric  acid,  or  the- 
corresponding  quantity  of  chlorine. 

3.  The  total  chlorine,  which  is  now  present  entirely,  in  the  state  of  chloride, 
is   determined   as   follows  : — The  rose   tint   produced   by  the   permanganate'  is 
removed  by  adding  a  trace  of  ferrous  sulphate.     Then  add  a  measured  •  volume  of 
standard  silver  nitrate,  more  than  enough  to  precipitate  all  the  chlorine,  and 
determine  the  excess  of  the  silver  salt  by  means  of  standard  thiocyanate  (p.  145). 
The  ferric  salt  previously  formed  by  the  peroxidation  of  the  ferrous  salt  serves 
as  an  indicator,  by  producing  a  permanent  red  colouration  as  soon  as  there  is 
no  more  silver  salt  to  precipitate.     The  arsenic  acid  produced  in  the  first  operation 
does  not  interfere  in  the  least. 

In  order  to  avoid  the  use  of  too  large  a  quantity  of  silver  nitrate,  which 
would  be  necessary  on  account  of  the  large  proportion  of  chlorine  to  be 
precipitated,  an  aliquot  part  of  the  solution  may  be  taken. 

The  chlorine  found  in  the  state  of  a  chloride  in  the  original  liquid  is  easily 
calculated  by  deducting  from  the  total  chlorine  just  determined  the  two 
quantities  already  found  in  the  state  of  hypochlorite  and  of  chlorate. 

The  three  operations  succeed  each  other  without  interruption,  and  without 
separate  preparation,  and  are  completed  in  a  short  time. 

In  a  number  of  experiments  with  mixtures,  the  discrepancies  found  between 
the  experimental  results  and  the  calculated  numbers  rarely  reached  1  mgm.  when 
operating  upon  from  250  to  500  mgm. 

Mixtures  of  Chlorides,  Chlorates,  and  Perchlorates. — A.  C  a  r  n  o  1. 1  Perchlorates 
are  found  with  chlorides  and  chlorates  in  the  products  of  the  calcination  of  chlorates. 
Hypochlorites  are  only  produced  in  the  cold  or  by  wet  methods  ;  but  in  such  cases 
no  perchlorates  are  formed,  nor  can  the  latter  be  reduced  by  the  usual  reagents  in 
solution,  dry  heat  being  necessary  to  accomplish  this  result. 

*  A.  Cam  ot,  Compt.  Rend.  122,  449.  t  Compt.  Rend.  122,  452. 

N    2 


180  CHLORINE. 

In  analysing  such  mixtures,  the  chlorides  and  chlorates  are  determined  first  by 
titrating  one  portion  of  the  solution  for  the  chlorides  by  Vol hard's  method, 
and  the  other  part  for  the  total  chlorine  after  reduction  of  the  chlorates  by  the 
aid  of  ferrous  sulphate ;  or,  as  an  alternative  method,  both  titrations  can  be 
performed  on  the  same  liquid,  the  chlorides  first — with  sodium  arsenate  as 
indicator  in  preference  to  potassium  chromate,  which  would  interfere  with 
the  subsequent  reaction — and  then  the  total  chlorine  after  reduction  of  the 
chlorates. 

The  perchlorates  are  determined  by  heating  the  powdered  substance,  mixed 
with  four  or  five  times  its  weight  of  purified  quartz-sand,  in  a  platinum  crucible, 
the  mixture  being  covered  by  a  layer  of  the  same  sand  1  or  2  cm.  deep.  The 
bottom  of  the  crucible  is  kept  at  a  red  heat  for  about  twenty  to  thirty  minutes, 
and  this  is  sufficient  completely  to  reduce  the  chlorates  and  perchlorates, 
volatilization  of  the  chloride  being  prevented  by  the  condensing  effect  of  the 
upper  layer  of  sand.  An  aqueous  solution  is  then  made,  the  total  chlorides 
titrated  as  before,  and  the  perchlorate  found  by  difference. 

Determination  of  Perchlorate  in  Chili  Saltpetre. — A  h  r  e  n  s  and  H  e  1 1*.  20  gm.  of 
the  powdered  sample  are  introduced  into  a  flat  200  c.c.  platinum  dish,  moistened  with 
2-3  c.c.  of  cold  saturated  caustic  soda,  1  gm.  of  pure  manganese  dioxide  added, 
And  the  whole  evaporated  to  dryness  ;  the  dish  is  then  covered  and  heated  to 
redness.  When  cold,  the  fused  mass  is  treated  with  100  c.c.  of  hot  water,  allowed 
to  cool,  and  then  made  up  to  250  c.c.  ;  50  c.c  of  the  filtrate  are  acidified  with  10-15 
c.c.  of  nitric  acid  of  sp.  gr.  1'20,  and  a  solution  of  permanganate  is  added  drop  by 
•drop  until  the  colour  is  permanent  for  a  minute,  showing  that  all  the  nitrous  acid 
lias  been  oxidized.  The  chlorine  is  then  determined  by  Volhard  's  process,  and 
the  difference  between  the  amounts  of  chlorine  found  before  and  after  fusion  is 
•calculated  into  perchlorate.  Iodides  present  in  the  sample  do  not  interfere,  as 
they  are  oxidized  to  iodates  by  the  permanganate. 

The  lodimetric  Determination  of  Chloric  and  Nitric  Acids.— 
The  following  methods  by  McGowanf  depend  on  the  principle 
that  when  a  fairly  concentrated  solution  of  a  nitrate  or  chlorate 
is  warmed  with  an  excess  of  pure,  strong  hydrochloric  acid,  a  nitrate 
is  completely  decomposed,  and  the  production  of  nitrosyl  chloride 
and  chlorine  is  quantitative,  the  reaction  being 

HN03+3HC1=NOC1+C12+2H20. 

If  the  operation  is  conducted  in  an  atmosphere  of  carbonic  acid, 
and  the  escaping  gases  are  passed  through  a  solution  of  potassium 
iodide,  an  amount  of  iodine  is  liberated  exactly  equivalent  to  the 
whole  of  the  chlorine  present  (free  and  combined),  nitric  oxide 
escaping.  1  mol.  of  nitric  acid  thus  yields  3  atoms  of  chlorine  or 
iodine.  The  iodine  can  then  be  titrated  in  the  usual  manner  with 
thiosulphate.  With  chlorates  only  chlorine  is  evolved.  D  e 
Koninck  and  NihoulJ  give  details  of  a  process  depending  upon 
the  same  principle. 

METHOD  OF  PROCEDURE  FOR  NITRATES  :  It  is,  of  course,  absolutely  essential 
that  air  should  be  completely  excluded  from  the  apparatus  as,  if  any  were 
present,  the  escaping  nitric  oxide  would  be  re-oxidized  to  nitrogen  trioxide  or 
tetroxide,  and  this  would  in  its  turn  liberate  a  further  quantity  of  iodine  from 
the  iodine  solution. 

*  Chcm.  Centr.  1898,  2,  ,558.  t  J-  C.  S.  69,  530  and  .7.  C.  A?.  61,  87. 

J  Zcit.  fvr  angcw.  Chem.  August  15th,  1890. 


CHLORINE. 


181 


The  apparatus  required  is  very  simple,  and  can  readily  be  made  by  any  ono 
moderately  expert  at  glass-blowing.  The  main  point  to  be  attended  to  is  to 
have  no  corks  or  rubber  stoppers,  etc.,  for  the  escaping  chlorine  to  act  upon. 
Fig.  41  is  a  sketch  of  the  apparatus.  The  condensing  arrangement  for  the  chlorine 
does  its  work  perfectly,  and  may  therefore  be  used  with  advantage,  not  only  for 
this,  but  also  for  other  similar  methods  in  which  iodine  is  set  free.  The- 
measurements  given  are  those  of  the  apparatus  as  used  by  the  author. 

A  is  a  small,  round-bottomed  flask,  into  the  neck  of  which  a  glass  stopper,  a-,, 
is  accurately  ground  (with  fine  emery  and  oil).  The  capacity  of  the  bulb  is- 
about  46  c.c.,  and  the  length  of  the  neck,  from  x  to  y,  90  mm.  The  first  condenser 
is  a  simple  tube,  slightly  enlarged  at  the  foot  into  two  small  bulbs.  The  length 
from  a  to  b  is  300  mm.,  from  b  to  e  180  mm.,  and  from  e  to  /  30  mm.  The 
capacity  of  the  bulb  B  is  25  c.c.,  and  the  total  capacity  of  the  two  bulbs  and  tube, 
up  to  the  top  of  C,  41  c.c.  This  condenser  is  immersed,  up  to  the  level  of  e  in 
a  beaker  of  water.  DisaGeissler  bulb  apparatus,  and  E  a  chloride  of  calcium 
tube,  filled  with  broken  glass,  which  acts  as  a  tower,  g  is  a  small  funnel,  attached 
by  rubber  and  lip  to  the  branch  tube  h.  Between  the  tube  i  and  the  wash-bottle 
for  the  carbonic  acid  is  placed  a  short  piece  of  glass  tubing,  s,  containing  a  strip 
of  filter-paper,  slightly  moistened  with  iodide  of  starch  solution.  This  tube  s  is 
really  hardly  necessary,  as  no  chlorine  escapes  backwards  if  a  moderate  current 
of  carbonic  acid  is  kept  passing,  but  it  serves  as  a  check.  The  joints  p  and  q 
are  of  narrow  rubber  tubing.  The  joint  o  is  made  by  grinding  one  tube  into  the 
other,  k  is  the  outlet  tube. 


Fig.  41. 

The  operation  is  performed  in  the  following  manner  : — The  evolution  flask  is- 
washed  and  thoroughly  dried,  and  the  nitrate  (say  about  0'25  gm.  of  potassium, 
nitrate)  is  dropped  into  it  from  the  weighing  tube.  1  to  2  c.c.  of  water  are  now 
added,  and  the  bulb  is  gently  warmed,  so  as  to  bring  the  nitrate  into  solution,, 
after  which  the  stopper  of  the  flask  is  firmly  inserted  into  it.  About  15  c.c. 
of  a  solution  of  potassium  iodide  (1  in  4)  are  run  into  the  first  condensing  tube 
any  iodide  adhering  to  the  upper  portion  of  the  tube  being  washed  down 
with  a  little  water,  and  5  c.c.  of  the  same  solution,  mixed  with  8  to  10  c.c.  of 
'water,  are  sucked  into  the  Geissler  bulbs,  whilst  the  glass  in  tower  E  is  abo 


182  CHLORINE. 

thoroughly  moistened  with  the  iodide.  The  Geissler  bulbs  should  be  so 
arranged  that  gas  only  bubbles  through  the  last  of  them,  the  liquid  in  the  others 
remaining  quiescent. 

All  the  joints  having  been  made  tight,  the  C0.2  is  turned  011  briskly,  and 
passed  through  the  apparatus  until  a  small  tubeful  collected  at  /,  over  caustic 
potash  solution,  shows  that  no  appreciable  amount  of  air  is  left  in  it.  The  small 
outlet  tube  I  is  now  replaced  by  a  chloride  of  calcium  tube,  fdled  with  broken 
glass  which  has  been  moistened  with  the  above  iodide  solution,  and  closed  by 
a  cork  through  which  an  outlet  tube  passes,  the  object  of  this  "  trap  "  tube  being 
•to  prevent  any  air  getting  back  into  the  apparatus  ;  and  the  brisk  current  of 
C02  is  continued  for  a  minute  or  two  longer,  so  as  practically  to  expel  all  the  air 
from  this  last  tube.  The  stream  of  gas  is  now  stopped  for  an  instant,  and  about 
15  c.c.  of  pure  concentrated  hydrochloric  acid,  free  from  chlorine,  run  into  A 
through  the  funnel  g  (into  the  tube  of  which  it  is  well  to  have  run  a  few  drops 
of  water  before  beginning  to  expel  the  air  from  the  apparatus),  and  A  is  shaken 
so  as  to  mix  its  contents  thoroughly.  A  slow  current  of  CO2  is  now  turned  ort 
again  (1  to  2  bubbles  through  the  wash-bottle  per  second),  and  A  is  gently 
warmed  over  a  burner.  It  is  a  distinct  advantage  that  the  reaction  does  not 
begin  until  the  mixed  solutions  are  warmed,  when  the  liquid  becomes  orange- 
coloured,  the  colour  again  disappearing  after  the  nitrosyl  chloride  and  chlorine 
have  been  expelled.  The  warming  should  be  very  gentle  at  first,  in  order  to 
make  sure  of  the  conversion  of  all  the  nitric  acid,  and  also  because  the  first 
escaping  vapours  are  relatively  very  rich  in  chlorine  ;  after  which  the  liquid  in 
A  is  briskly  boiled.  A  very  little  practice  enables  the  operator  to  judge  as  to  the 
proper  rate  of  warming.  When  the  volume  of  liquid  in  A  has  been  reduced  to 
about  7  c.c.  (by  which  time  it  is  again  colourless),  the  stream  of  C0£  is 
slightly  quickened,  and  the  apparatus  allowed  to  cool  down  a  little.  The  burner 
is  now  set  aside  for  a  few  minutes,  and  2  c.c.,  or  so,  more  of  hydrochloric  acid 
previously  warmed  in  a  test-tube,  run  in  gently  through  g  ;  there  is  no  fear 
either  of  the  iodide  solution  running  back,  or  of  any  bubbles  of  air  escaping 
through  y,  if  this  is  done  carefully.  This  is  a  precautionary  measure,  in  case 
a  trace  of  the  liberated  chlorine  might  have  lodged  in  the  comparatively  cool 
liquid  in  tube  li.  The  C02  is  once  more  turned  on  slowly,  and  the  liquid  in  A  is 
boiled  again  until  it  is  reduced  to  about  5  c.c.  It  is  now  only  necessary  to  allow 
the  apparatus  to  cool  down,  passing  C02  all  the  time,  after  which  the  contents 
of  the  condensers  are  transferred  to  a  flask  and  titrated  with  thiosulphate.  At 
the  end  of  a  properly  conducted  experiment,  the  glass  in  the  upper  part  of  tower 
E  should  be  quite  colourless,  and  there  should  only  be  a  mere  trace  of  iodine 
showing  in  the  lower  part  of  the  tower,  while  the  liquid  in  the  last  bulb  of  the 
Geissler  apparatus  ought  to  be  only  pale  yellow.  During  the  operation  the 
stopper  of  A  and  the  various  joints  can  be  tested  for  tightness  from  time  to  time 
by  means  of  a  piece  of  iodide  of  starch  paper,  and,  before  disjointing,  it  is  well  to 
test  the  escaping  gas  (say,  at  m)  in  the  same  way,  to  make  sure  that  all  nitric 
oxide  has  been  thoroughly  expelled. 

EXAMPLE  :  0*2627  gin.  of  pure  KN03  was  taken.  The  liberated  iodine  required 
38-55  c.c.  of  thiosulphate  (of  which  1  c.c.  =0-006805  gm.  KN03)  for  conversion. 
This  gave  0-2623  gm.  nitrate  found,  or  99*86  per  cent. 

METHOD  OF  PROCEDURE  FOR  CHLORATES  :  The  apparatus  employed  is  the  same 
as  for  nitrates,  but  since  it  is  unnecessary  in  this  determination  previously  to 
expel  the  air  present  by  a  current  of  CO2,  those  tubes  which  come  after  the 
tower  E  are  dispensed  with.  The  details  of  the  operation  are  also  practically 
the  same  as  in  the  case  of  a  nitrate,  only  simpler.  Comparatively  dilute  hydro- 
chloric acid  may  be  employed,  and  the  C02  is  required  merely  to  ensure  a  regular 
passage  of  the  vapours  through  the  iodine  solution,  and  to  prevent  any  chlorine 
escaping  backwards.  This  is  tested,  as  before,  by  the  small  piece  of  iodide  of 
starch  paper  in  tube  s,  which  should  be  so  placed  as  never  to  get  warm. 

The  chlorate  is  weighed  out  into  the  dry  evolution  flask  A,  then  dissolved  in 
8  to  10  c.c.  of  water,  and,  after  all  the  necessary  connections  have  been  made, 
8  to  10  c.c.  of  pure  concentrated  hydrochloric  acid  are  run  in  through  the  funnel  g. 
Since  the  reaction  begins  in  the  cold,  the  CO2  must  be  turned  on  immediately, 


CHLOJRINE.  1 83 

and  kept  passing  at  the  rate  of  about  four  bubbles  per  second.  Care  should  be 
taken  to  heat  very  gently  at  first,  until  the  bulk  of  the  chlorine  has  coine  over, 
after  which  the  lamp  flame  may  be  gradually  turned  up  and  the  liquid  boiled, 
exactly  as  in  the  case  of  the  nitrate  ;  this  ensures  that  no  chlorine  escapes  back- 
wards. And,  as  before,  after  all  the  chlorine  has  been  apparently  driven  out,  and 
the  solution  has  become  colourless,  a  second  quantity  of  warm  hydrochloric  acid 
(1  in  2)  is  run  in,  and  the  boiling  repeated  for  a  few  minutes. 

Chlorates,  lodates,  and  Bromates. 

C12O5  =  150-92.  I2O5  =  333-84.  Br2O5  =  239'84.  The  compounds 
of  chloric,  iodic,  and  bromic  anhydrides  may  all  be  determined  by 
distillation  or  digestion  with  excess  of  hydrochloric  acid  ;  with 
chlorates  the  quantity  of  acid  must  be  considerably  in  excess. 

In  each  case  1  eq.  of  the  respective  anhydrides  taken  as  monobasic, 
or  their  compounds,  liberates  6  eq.  of  chlorine,  and  consequently 
6  eq.  of  iodine  when  decomposed  in  the  digestion  flask.  In  the 
case  of  distillation,  however,  iodic  and  bromic  acids  only  set  free 
4  eq.  iodine,  while  iodous  and  bromous  chlorides  remain  in  the  retort. 
In  both  these  cases  digestion  is  preferable  to  distillation. 

EXAMPLE  :  0*2043  gm.  pure  potassium  chlorate,  equal  to  the  sixth  part  of 
Tote  eq.,  was  decomposed  by  digestion  with  potassium  iodide  and  strong  hydro- 
chloric acid  in  the  bottle  shown  in  fig.  40.  After  the  reaction  was  complete  and 
the  bottle  cold,  the  stopper  was  removed  and  the  contents  washed  out  into 
a  beaker,  starch  added,  and  103  c.c.  N/1O  thiosulphate  delivered  in  from  the 
burette  ;  then  again  23'2  c.c.  of  N/ioo  iodine  solution,  to  reproduce  the  blue 
colour  ;  this  latter  was  therefore  equal  to  2'32  c.c.  N/io  iodine,  which  deducted 
from  the  103  c.c.  thiosulphate  gave  100-68  c.c.,  which  multiplied  by  the  factor 
0-002043,  gave  0-2056  gm.,  instead  of  0-2043  gm. 


CHROMIUM. 


1.     Reduction  by  Iron. 

THE  determination  of  chromates  is  very  simply  and  successfully 
performed  by  the  aid  of  ferrous  sulphate,  being  the  converse  of  the 
process  devised  by  Penny  for  the  determination  of  iron  (seep.  126). 

METHOD  or  PROCEDURE  :  A  very  small  beaker  or  other  convenient  vessel  is 
partly  or  wholly  filled,  as  may  be  requisite,  with  perfectly  dry  and  granular  double 
sulphate  of  iron  and  ammonia  ;  the  exact  weight  then  taken  and  noted.  The 
chromium  compound  is  brought  into  solution,  not  too  dilute,  acidified  with 
sulphuric  acid,  and  small  quantities  of  the  iron  salt  added  from  time  to  time  with 
a  dry  spoon,  taking  care  that  none  is  spilled,  and  stirring  with  a  glass  rod,  until 
the  mixture  becomes  green  and  the  iron  is  in  excess,  best  shown  by  a  small  drop 
being  brought  in  contact  with  a  drop  of  potassium  ferricyanide,  when,  if  a  blue 
colour  appears  at  the  point  of  contact,  the  iron  is  in  excess.  It  is  necessary  to 
determine  this  excess,  which  is  most  conveniently  done  by  N/iO  dichromate  being 
added  until  the  blue  colour  produced  by  contact  with  the  indicator  disappears. 
The  vessel  containing  the  iron  salt  is  again  weighed,  the  loss  noted  ;  the  quantity 
of  the  salt  represented  by  the  N/iO  dichromate  deducted  from  it,  and  the  remainder 
multiplied  by  the  factor  required  by  the  substance  sought.  A  freshly  made 
standard  solution  of  iron  salt,  well  acidified  with  sulphuric  acid,  may  be  used  in 
place  of  the  dry  salt. 


184  CHROMIUM. 

EXAMPLE  :  0*5  gm.  pure  potassium  dichromate  was  taken  for  analysis,  and  to- 
its  acid  solution  4*15  gin.  double  iron  salt  added.  3*3  c.c.  of  N/io  dichromato 
were  required  to  oxidize  the  excess  of  iron  salt ;  it  was  found  that  0'7  gm.  of  th<^ 
salt  =17*85  c.c.- dichromate.  consequently  3'3  c.c.  of  the  latter  were  equal  to 
0*1299  gm.  iron  salt ;  this  deducted  from  the  quantity  originally  used  left  4'0201  gin. ,. 
which  multiplied  by  0*1255  gave  0*504  gm.  instead  of  0*5  gm. 

In  the  case  of  lead  chromate  being  determined  in  this  way,  it  is 
best  to  mix  both  the  chromate  and  the  iron  salt  together  in  a  mortar-, 
rubbing  them  to  powder,  adding  hydrochloric  acid,  stirring  well 
together,  then  diluting  with  water  and  titrating  as  before.  Where 
pure  double  iron  salt  is  not  at  hand,  a  solution  of  iron  wire  in 
sulphuric  acid,  freshly  made,  and  of  ascertained  strength,  may  be 
used. 


2.     Determination  of  Chromates  by  Distillation 
with  Hydrochloric  Acid. 

When  chromates  are  boiled  with  an  excess  of  strong  hydrochloric- 
acid  in  one  of  the  apparatus  (fig.  38  or  39),  every  1  eq.  of  chromic- 
acid  liberates  3  eq.  chlorine.  For  instance,  with  potassium 
dichromate  the  reaction  may  be  expressed  as  follows— 

K2Cr207  +  14HC1  -  2KC1 +Cr2Cl6  +  7H2O  +  3C12. 

If  the  liberated  chlorine  is  conducted  into  a  solution  of  potassium 
iodide,  3  eq.  of  iodine  are  set  free,  and  can  be  determined  by  N/1(> 
arsenite  or  thiosulphate.  3  eq.  of  iodine  so  obtained  (  =  380*76}. 
represent  1  eq.  chromic  acid  (  =  100).  The  same  decomposition 
takes  place  by  mere  digestion,  as  described  on  page  138. 

3.     Chrome  Iron  Ore,  Steel,  etc. 

The  ore  varies  in  quality,  some  samples  being  very  rich  in 
chromium,  while  others  are  very  poor.  In  all  cases  the  sample  is 
to  be  first  of  all  brought  into  extremely  fine  powder.  About  a  gram 
is  rubbed  tolerably  fine  in  a  steel  mortar,  then  finished  fractionally 
in  an  agate  mortar. 

Chris tomanos  recommends  that  the  coarse  powder  should  be 
ignited  for  a  short  time  on  platinum  previous  to  powdering  with  the 
agate  mortar  ;  after  that  it  should  be  sifted  through  the  finest 
material  that  can  be  used,  and  the  coarser  particles  returned  to  the 
mortar  for  regrinding. 

Previous  to  analysis  it  should  be  again  ignited,  and  the  analysis 
made  on  the  dry  sample. 

O'Neill's  Process. — The  very  finely  powdered  ore  is  fused  with  ten  times  ilw- 
weight  of  potassium  bisulphatc  for  twenty  minutes,  taking  care  that  it  does  not 
rise  over  the  edge  of  the  platinum  crucible  ;  when  the  fusion  is  complete,  the  molten 
mass'  is  caused  to  flow  over  the  sides  of  the  crucible,  so  as  to  prevent  the  formation? 
of  a  sol'.d  lump,  and  the  .crucible  set  aside  to  cool.  The  mass  is  transferred  to 
a  porcelain  dish,  and  lixiviated  with  warm  water  until  entirely  dissolved  (there  must 
be  no  black  residue,  otherwise  the  ore  is  not  completely  decomposed)  ;  sodium 


CHROMIUM.  185- 

carbonate  is  then  added  to  the  liquid  until  it  is  strongly  alkaline  ;  it  is  then  brought 
on  a  filter,  washed  slightly,  and  the  filter  dried.  When  perfectly  dry,  the  precipitate- 
is  detached  from  the  filter  as  much  as  possible  ;  the  filter  burned  separately  ; 
the  ashes  and  precipitate  mixed  with  about  twelve  times  the  weight  of  the  original 
ore  of  a  mixture  of  two  parts  potassium  chlorate  and  three  parts  sodium  carbonate, 
and  fused  in  a  platinum  crucible  for  twenty  minutes  or  so  ;  the  resulting  mass 
is  then  treated  with  boiling  water,  filtered,  and  the  filtrate  titrated  for  chromic- 
acid  as  in  (1) 

The  ferric  oxide  remaining  on  the  filter  is  titrated,  if  required,  by 
any  of  the  methods  described  under  Iron. 

Britton's  Process. — Reduce  the  mineral  to  the  finest  possible  state  of 
division  in  an  agate  mortar.  Weigh  off  0'5  gm.,  and  add  to  it  4  gm.  of  flux, 
previously  prepared,  composed  of  one  part  potassium  chlorate  and  three  parts 
soda-lime;  thoroughly  mix  the  mass  by  triturating  in  a  porcelain  mortar,  and 
then  ignite  in  a  covered  platinum  crucible  at  a  bright-red  heat  for  an  hour  and 
a  half  or  more.  20  minutes  is  sufficient  with  the  gas  blowpipe.  The  mass  wilt 
not  fuse  but  when  cold  can  be  turned  out  of  the  crucible  by  a  few  gentle  taps, 
leaving  the  interior  of  the  vessel  clean  and  bright.  Triturate  in  the  mortar 
again  and  transfer  the  powder  to  a  tall  beaker,  add  about  20  c.c.  of  hot  water, 
and  boil  for  two  or  three  minutes  ;  when  cold  add  15  c.c.  of  HC1,  and  stir  with 
a  glass  rod,  till  the  solid  matter,  with  the  exception  probably  of  a  little  silica 
in  flakes,  becomes  dissolved.  Both  the  iron  and  chromium  will  then  be  in  the 
highest  state  of  oxidation — Fe2O3  and  Cr2^3-  Pour  the  fluid  into  a  white  porcelain 
dish,  and  dilute  with  washings  of  the  beaker  to  about  100  c.c.  Immediately 
after,  add  cautiously  1  gm.  of  metallic  iron  of  known  purity,  or  an  equivalent 
quantity  of  double  iron  salt  previously  dissolved  in  dilute  sulphuric  acid,  and 
further  dilute  with  cold  water  to  about  150  c.c.  Titrate  with  N/io  permanganate- 
the  amount  of  ferrous  oxide  remaining.  The  difference  between  the  amount  of 
iron  found  and  of  the  iron  weighed  will  be  the  amount  oxidized  to  sesquioxide  by 
the  chromic  acid.  Every  one  part  so  oxidized  will  represent  0'31035  of  Cr  or 
0'45359  of  Cr20;!>  m  which  last  condition  the  substance  usually  exists  in  the  ore. 

If  the  amount  of  iron  only  in  the  ore  is  to  be  determined,  the  process  is  stilt 
shorter.  After  the  fluxed  mineral  has  been  ignited  and  reduced  to  powder  as- 
already  directed,  dissolve  it  by  adding  first,  10  c.c.  of  hot  water  and  applying, 
a  gentle  heat,  and  then  15  c.c.  of  HC1,  continuing  the  heat  to  incipient  boiling 
till  complete  decomposition  has  been  effected  ;  cool  by  immersing  the  tube  in 
a  bath  of  cold  water,  add  pieces  of  pure  metallic  zinc  sufficient  to  bring  the  iron 
to  the  condition  of  protoxide  and  the  chromium  to  sesquioxide,  and  apply  heat 
till  small  bubbles  of  hydrogen  cease  and  the  zinc  has  become  quite  dissolved  ; 
then  nearly  fill  the  tube  with  cold  water,  acidulated  with  one-tenth  of  sulphuric- 
acid,  and  pour  the  contents  into  the  porcelain  dish,  add  cold  water  to  make  up  the- 
volume  to  about  250  c.c.,  and  titrate  with  standard  permanganate  or  dichromate.. 

Sell's  Process. — This  method*  is  carried  out  by  first  fusing  the- 
finely  ground  ore  with  a  mixture  of  sodium  bisulphate  and  fluoride 
in  the  proportion  of  1  mol.  bisulphate  and  2  mol.  fluoride,  and 
subsequent  titration  of  the  chromic  acid  by  standard  thiosulphate 
and  iodine. 

From  O'l  to  0'5  gm.  of  the  ore  is  placed  on  the  top  of  ten  times  its  weight  of" 
the  above-mentioned  mixture  in  a  large  platinum  crucible,  and  ignited  for  fifteen 
minutes ;  an  equal  weight  of  sodium  bisulphate  is  then  added,  and  well  in- 
corporated by  fusion  and  stirring  with  a  platinum  wire;  then  a  further  like 
quantity  of  bisulphate  added  in  the  same  way.  When' complete  decomposition 
has  occurred,  the  mass  is  boiled  with  water  acidulated  with  sulphuric  acid,  and 
the  solution  diluted  to  a  definite  volume  according  to  the  quantity  of  ore  originally 
taken. 

*  J.  C.  S.  1879,  p.  292. 


1 86  CHROMIUM. 

To  ensure  the  oxidation  of  all  the  chromium  and  iron  previous  to  titration, 
a  portion,  or  the  whole,  of  the  solution  is  heated  to  boiling,  and  permanganate 
added  until  a  permanent  red  colour  is  produced.  Sodium  carbonate  is  then 
added  in  slight  excess,  and  sufficient  alcohol  to  destroy  the  excess  of  permanganate  ; 
the  manganese  precipitate  is  then  filtered  off,  and  the  clear  solution  titrated  with 
N/io  thiosulphate  and  iodine. 

The  author  states  that  the  analysis  of  an  ore  by  this  method  may 
be  accomplished  in  one  hour  and  a  half. 

For  the  oxidation  of  salts  of  chromium,  the  same  authority 
recommends  boiling  with  potash  or  sodium  carbonate  (to  which 
a  small  quantity  of  hydrogen  peroxide  is  added)  for  15  minutes. 

For  the  preliminary  fusion  and  oxidation  of  chrome  iron  ore, 
Dittmar  recommends  a  mixture  of  two  parts  borax  glass  and 
one  and  a  half  part  each  of  sodium  and  potassium  carbonates.  These 
are  fused  together  in  a  platinum  crucible  until  all  effervescence 
ceases,  then  poured  out  into  a  large  platinum  basin  or  upon  a  clean 
iron  plate  to  cool,  broken  up,  and  preserved  for  use. 

Ten  parts  of  this  mixture  are  used  for  one  part  of  chrome  ore, 
and  the  fusion  made  in  a  platinum  crucible,  closed  for  the  first  five 
minutes,  then  open  for  about  forty  minutes,  frequently  stirring 
with  a  platinum  wire,  and  using  a  powerful  Buns  en  flame.  The 
gas  blowpipe  hastens  this  method  considerably. 

The  above  described  methods  of  treating  the  ores  of  chromium  so 
as  to  obtain  complete  decomposition  are  apparently  now  superseded 
to  a  great  extent  by  the  use  of  sodium  peroxide,  but  the  action  of  this 
reagent  upon  platinum,  gold,  silver,  nickel,  or  porcelain  is  so  energetic 
that  its  use  requires  great  care.  Many  well  known  authorities  on 
the  analysis  of  chrome  ores  use  a  basic  mixture  such  as  was  first 
suggested  by  Clark,  but  modified  by  Stead,  i.e.,  magnesia  or 
lime  four  parts,  potassium  and  sodium  carbonates  of  each  one  part. 
Clark's  original  mixture  of  magnesia  and  caustic  soda  acts  on 
platinum,  but  Stead's  mixture  does  not. 

The  fusion  is  made  by  mixing  the  very  finely  ground  sample  with  ten  times 
its  weight  of  the  basic  mixture  in  a  platinum  crucible,  and  heating  to  bright 
redness  at  the  back  of  a  gas  muffle  for  about  an  hour.  When  the  crucible  is 
removed  and  cool  the  mass  is  found  sintered  together.  It  is  removed  to  a  beaker, 
and  the  crucible  washed  out  with  water  and  dilute  sulphuric  acid.  The 
decomposition  is  generally  complete,  but  if  any  black  specks  are  found  they 
must  be  separated  by  filtration,  dried,  and  again  fused  with  some  of  the  basic 
mixture ;  finally  the  whole  is  mixed  with  excess  of  ferrous  salt,  and  the 
imoxidized  iron  titrated  with  dichromate  as  before  described 

Rideal  and  Rosenblum*  give  a  series  of  experiments  on  the 
determination  of  chromium  in  ores,  steels,  etc.,  and  on  the  use  of 
sodium  peroxide,  which  latter  they  find  has  a  most  destructive 
effect  on  all  kinds  of  vessels  in  which  the  decomposition  is  made. 
Nickel  seems  the  best  material  if  not  exposed  to  too  high  a  tempera- 
ture, but  they  found  also  that  a  good  deal  of  nickel  was  dissolved 
from  the  crucibles  by  the  sulphuric  acid  used  to  dissolve  the  melt, 
and  they  therefore  attach  great  importance  to  the  filtration  of  the 

*  j.  s.  c.  i.  14. 1017. 


CHROMIUM.  187 

aqueous  solution  of  the  melt,  so  as  to  remove  nickel  and  iron  oxides, 
which  otherwise  interfere  with  the  titration  by  masking  the  colour 
of  the  indicator. 

Ferrochrome,  Chromium  Steel,  etc. — Sp tiller  and  Brenner* 
describe  an  improved  method  which  gives  better  results  than  the 
previous  method  advocated  by  Sp  tiller  and  K  aim  an. 

METHOD  OF  PROCEDURE  FOR  FERROCHROME  :  0*35  gm.  of  the  finely  powdered 
sample,  mixed  in  a  silver  dish  with  2  gm.  of  dry  powdered  sodium  hydroxide 
and  covered  with  4  gm.  of  sodium  peroxide,  is  heated  until  the  mixture  begins  to 
melt,  when,  as  a  consequence  of  the  strong  chemical  action,  the  whole  mass  soon 
becomes  liquefied.  The  dish  is  then  again  heated  for  ten  minutes  over  a  powerful 
burner,  and  5  gm.  of  sodium  peroxide  is  cautiously  added,  stirring  all  the  while 
with  a  silver  spatula.  After  heating  for  thirty  minutes  more,  another  5  gm.  of 
sodium  peroxide  is  added  and  the  heating  continued  for  twenty  minutes,  when 
a  final  5  gm.  of  the  peroxide  is  added. 

When  cold,  the  silver  basin  is  placed  in  a  deep  porcelain  dish  and  filled  with 
water  ;  when  the  lixiviation  is  completed,  which  takes  a  few  minutes  only,  the 
silver  dish  is  lifted  out  and  well  rinsed  with  hot  water.  A  brisk  current  of  C02 
is  then  passed  through  the  liquid  for  half  an  hour,  the  whole  allowed  to  cool, 
introduced  into  a  litre  measure,  and  made  up  to  the  mark  with  water.  After 
shaking  and  filtering,  250  c.c.  are  taken  and  the  chromic  acid  titrated  by 
a  permanganate  solution  of  which  1  c.c.  equals  about  0'005  gm.  of  iron,  and 
-a  solution  of  ferrous  ammonium  sulphate  containing  7  gm.  of  the  salt  in  500  c.c. 
The  chromium  solution  is  diluted  with  1  litre  of  cold  water  which  has  been 
previously  boiled  and  acidified  with  20  c.c.  of  sulphuric  acid  (1  :  5  by  volume) ; 
100  c.c.  of  ferrous  ammonium  sulphate  are  added,  and  the  mixture  titrated  back 
with  permanganate.  The  strength  of  the  ferrous  solution  is  determined  by 
a  blank  experiment  under  similar  conditions.  If  the  solution  of  the  melt  appears 
green,  it  is  advisable  to  add  first  a  few  c.c.  of  permanganate  and  then  some  more 
sodium  peroxide,  when  a  pure  yellow  liquid  will  be  obtained. 

METHOD  OF  PROCEDURE  FOR  CHROME  STEEL  :  2  gm.  of  the  sample  is  dissolved 
in  20  c.c.  of  warm  hydrochloric  acid  contained  in  a  porcelain  dish,  10  c.c.  of 
dilute  sulphuric  acid  (1  :  1)  are  added,  and  the  whole  evaporated  to  dryness  ;  the 
residue  is  then  transferred  to  a  silver  dish  and  heated  with  2  gm.  of  sodium 
hydroxide  and  5  gm.  of  sodium  peroxide,  until  the  sulphates  are  decomposed  and 
the  mass  begins  to  cake.  A  strong  heat  is  now  applied  and  another  5  gm.  of  the 
peroxide  is  added.  When  the  mass  begins  to  fuse,  it  is  well  stirred  with  a  silver 
spatula,  and  after  20  minutes  another  5  gm.  of  peroxide  is  added  ;  after  another 
20  minutes,  when  the  oxidation  is  complete,  a  further  addition  of  5  gm.  of  the 
soda  is  made  and  the  mass  is  allowed  to  cool.  The  melt  is  then  extracted  as  in 
the  former  case,  but  the  liquid  is  made  up  to  500  c.c.  only,  and  250  c.c.  of  the 
filtre  (  =  1  gm.  of  sample)  are  taken  for  the  titration  of  the  chromium.  In  this 
case,  the  authors  prefer  titrating  according  to  Zulkow sky's  method,  i.e.,  the 
liquid  is  put  into  a  tall,  narrow  beaker,  mixed  with  10  c.c.  of  a  10  per  cent, 
solution  of  potassium  iodide,  and  acidified  with  pure  hydrochloric  acid.  To 
another  beaker  containing  20  c.c.  of  a  solution  of  potassium  dichromate 
(0-9833  gm.  per  litre),  250  c.c.  of  water  are  added,  then  10  c.c.  of  a  10  per  cent, 
solution  of  potassium  iodide  and  a  little  hydrochloric  acid.  After  being  left  for 
15  minutes  in  a  dark  place,  both  liquids  are  titrated  with  solution  of  sodium 
thiosulphate  containing  4-96  gm.  of  the  salt  per  litre.  The  amount  of  chromium 
being  known  in  the  one  solution,  the  quantity  contained  in  the  other  is  readily 
calculated. 

Rideal  and  Rosenblum  have  obtained  excellent  results  with 
ferrochrome  by  fusion  with  sodium  peroxide  alone.  The  manner 
of  procedure  was  as  follows  : — 

*  CJicm.  Zeit.,  1897.  2,  p.  3,  4. 


1 88  CHROMIUM. 

About  0-5  gm.  of  a  very  finely  powdered  ferrochrome  was  mixed  with  3  gin.  of 
sodium  peroxide  and  heated  very  gently  in  a  nickel  crucible,  until  the  mass 
began  to  melt,  and  then  to  glow  by  itself.  The  heating  was  then  continued  for 
ten  minutes,  and  after  the  mass  was  partially  cooled  1  gm.  of  sodium  peroxide 
was  added  and  the  heating  continued  for  another  five  minutes. 

The  crucible,  when  still  moderately  warm,  was  placed  in  a  suitable  porcelain 
basin,  which  was  then  half  filled  with  hot  water  and  covered  with  a  clock  glass. 
The  melt  easily  dissolved  in  the  hot  water,  the  solution  obtained  being  of  a  deep 
purple  colour,  due  to  sodium  ferrate,  which  is  abundantly  formed  during  the 
fusion.  The  solution  also  contained  sodium  manganate,  resulting  from  the 
oxidation  of  the  manganese  which  is  present  in  ferrochromc. 

To  decompose  both  these  salts  a  small  quantity  of  sodium  peroxide  was  added, 
on  which  the  solution  immediately  lost  its  purple  colour.  The  solution  was  then 
boiled  for  ten  minutes  to  decompose  the  excess  of  sodium  peroxide,  and  the 
insoluble  residue  of  iron,  nickel,  and  manganese  oxide  was  filtered  off.  An  excess 
of  sulphuric  acid  was  then  added  to  the  solution,  and  after  cooling  it  was  titrated 
in  the  usual  manner  with  permanganate. 

Galbraith's  method,  modified  somewhat  by  Stead*,  is  con- 
sidered the  most  rapid  method  for  the  determination  of  chromium 
in  irons  and  steels. 

The  sample  (2  gm.)  is  dissolved  in  30  c.c.  dilute  sulphuric  acid  (1  to  3),  filtered, 
the  solution  diluted  to  about  300  c.c.  and  heated  to  boiling.  Strong  solution  of 
potassium  permanganate  is  now  added  until  the  red  colour  is  permanent  for  ten 
minutes,  then  80  c.c.  of  10  per  cent,  hydrochloric  acid,  and  the  liquid  heated  until 
decolorized  ;  150  c.c.  of  water  are  added,  about  100  c.c.  boiled  off  to  expel  the 
chlorine,  and  the  chromium  is  then  titrated  with  ferrous  sulphate  and  dichromate 
as  on  p.  126.  The  residue  insoluble  in  dilute  sulphuric  acid  is  mixed  with  0.5  gm. 
of  the  basic  mixture  previously  mentioned,  and  heated  to  intense  redness  for  half 
an  hour ;  the  chromium  is  afterwards  titrated  in  hydrochloric  acid  solution  with 
ferrous  sulphate  and  dichromate. 

Another  process  consists  in  dissolving  2  gm.  of  the  sample  in  hydrochloric 
acid ;  without  filtering,  the  liquid  is  nearly  neutralized  with  a  2  per  cent, 
solution  of  caustic  soda,  and,  after  diluting  to  300  c.c.,  10  c.c.  of  a  5  per  cent, 
solution  of  sodium  phosphate  and  30  gm.  of  sodium  thiosulphate  are  added. 
After  boiling  to  expel  the  S02,  20  c.c.  of  a  saturated  solution  of  sodium  acetate 
are  added,  and  the  boiling  continued  for  five  minutes  ;  the  precipitated  chromium 
phosphate  is  then  washed  with  a  2  per  cent,  solution  of  ammonium  nitrate,  dried, 
calcined,  and  fused  with  the  basic  mixture.  The  melt,  dissolved  in  30  c.c.  of 
hydrochloric  acid  and  150  c.c.  of  water,  is  boiled  for  ten  minutes  and  titrated. 
The  process  may  be  used  in  presence  of  vanadium.  In  this  case,  the  chromium 
must  be  titrated  by  means  of  ferrous  sulphate  and  permanganate  in  presence  of 
sulphuric  acid. 

Rideal  and  Rosenblum's  experiments  appear  to  show  that 
sodium  peroxide,  if  certain  conditions  be  observed  in  its  use,  is  a 
very  valuable  reagent  for  the  analysis  of  chrome  ore,  ferrochrome, 
and  chrome  steel,  as  it  removes  the  two  main  defects  of  former 
methods,  viz.,  the  necessity  of  repeated  fusion  to  effect  complete 
decomposition  and  the  inconvenient  slowness  of  these  processes. 
The  conditions  which  should  be  observed  are  summarized  by 
them  as  follows  :— 

(1)  Great  care  should  be  taken  to  reduce  the  chrome  ore  or  the  ferrochrome 
to  an  almost  impalpable  powder.  This  can  be  done  without  much  difficulty  if 
the  ore  or  the  alloy  be  crushed  in  a  steel  mortar  until  a  powder  is  obtained 
which  will  pass  through  a  linen  bag.  This  powder  is  then  ground  in  an  agate 

*Jour.  Iron  and  Steel  Institute,  1893,  153. 


CHHOMIUM.  1M> 

mortar  to  the  required  degree  of  fineness,  a  little  water  being  added  to  facilitate 
the  grinding. 

(2)  The  water  solution  of  the  melt,  before  acidulation,  must  be  freed  fronran 
excess  of  sodium  peroxide.     Whenever  sodium  ferrate  or  sodium  manganate  is 
formed  during  the  fusion  it  must  be  decomposed  in  the  water  solution  of^the 
melt. 

(3)  As  the  result  of  the  analysis  depends  to  a  large  extent  upon  the  titration, 
and  especially  upon  a  clear  perception  of  its  final  point,  it  is  important  that  the 
solution  in  which  the  chrome  is  to  be  determined  should  be  as  free  as  possible 
from  other  metallic  salts,   as  for  instance,   iron,   manganese,   and  nickel  salts. 
We  have  also  observed  that  the  ferricyanide  solution  which  is  used  as  an  indicator 
is  most  satisfactory  when  it  contains  no  more  than  1  per  cent,  of  ferricyanide.    3 

lodimetric  Determination  of  Chromic  Acid. — H.  P.  Seubert 
and  Henke*  have  devised  a  method  which  depends  upon  the 
reaction  : 

K2Cr2O7  +6KI +  7H2S04 =4K2SO4  +Cr2(SO4)8  +  7H2O  +  3I2 

Under  ordinary  circumstances  the  action  takes  considerable  time. 
The  authors  have  made  an  exhaustive  investigation  of  its  rate  of 
progress  when  the  different  bodies  are  present  in  different  quantities. 
Increasing  the  proportion  of  acid  accelerates  the  reaction  more  than 
increasing  the  proportion  of  potassium  iodide  does  ;  dilution  greatly 
retards   it.      The   following   are   convenient   proportions   to   use  : 
Dichromate,  0'05  gm.  ;  potassium  iodide,  0'5  gm.  ;  sulphuric  acid, 
T8  gm.  ;  total  volume,  100  c.c.     If  less  than  0*05  gm.  of  dichromate 
be  present,  the  other  quantities  should  still  be  kept  the  same.     If 
there  be  more  dichromate,  the  iodide  and  acid  should  be  proportion- 
ately increased  without  adding  more  water,  unless  there  be  more  than 
0'25  gm.     The  reaction  is  complete  in  about  six  minutes.     The     j 
liquid  should  then  be  diluted  and  titrated  with  thiosulphate  solution,     y 
using  starch  as  indicator. 


COBALT. 

Co  =  58-97. 

Determination  by  Permanganate  and  Mercuric  Oxide 
(C.  Winkler). 

THE  volumetric  determination  of  cobalt,  especially  in  the  presence 
of  other  metals,  is  not  yet  very  satisfactory.  The  method  here 
mentioned  is  worthy  of  notice,  and  with  an  alteration  suggested  by 
H.  B.  Harrisf  is  capable  of  giving  fair  technical  results.  This 
alteration  consists  in  carrying  out  the  titration  in  a  hot  solution 
instead  of  a  cold  one,  as  appears  to  have  been  done  by  Winkler. 
If  an  aqueous  solution  of  cobaltous  chloride  or  sulphate  be  treated 
with  an  emulsion  of  precipitated  mercuric  oxide,  no  decomposition 
ensues  ;  but  on  the  addition  of  permanganate  to  the  mixture, 
hydra  ted  cobaltic  and  manganic  oxides  are  precipitated,  and  the 

*  Zcit.  f.  any.  C.,  1930,  1147.         t  ./.  Am.  C.  S.,  1898,  173. 


190  COBALT. 

mercuric  oxide  is  simply  used  to  mechanically  separate  the  resulting 
oxides..  It  is  probable  that  no  definite  equation  can  be  given  for 
the  reaction,  and  therefore  practically  the  working  effect  of  the 
permanganate  is  best  determined  by  a  standard  solution  of  cobalt 
of  known  strength,  say  metallic  cobalt  dissolved  as  chloride,  or 
neutral  cobaltous  sulphate. 

METHOD  OF  PROCEDURE  :  The  solution  of  about  O'l  to  0'2  gm.  of  the  metal, 
free  from  any  great  excess  of  acid  is  placed  in  a  flask,  diluted  to  about  200  c.c., 
and  a  tolerable  quantity  of  mercuric  emulsion  (precipitated  from  the  nitrate  or 
perchlorate  by  alkali  and  washed)  added.  Permanganate  from  a  burette  is  then 
slowly  added  to  the  hot  solution  with  constant  shaking  until  the  rose  colour 
appears  in  the  clear  liquid  above  the  bulky  brownish  precipitate. 

The  appearance  of  the  mixture  is  somewhat  puzzling  at  the 
beginning,  but  as  more  permanganate  is  added  the  precipitate  settles 
more  freely,  and  the  end  as  it  approaches  is  very  easily  distinguished. 
The  process  is  complete  when  the  rose  colour  is  persistent  for  a 
minute  or  two  ;  subsequent  bleaching  must  not  be  regarded. 

The  actual  decomposition  as  between  cobaltous  chloride  and  per- 
manganate may  be  formulated  thus — 

6CoCl2  +  5HgO  +K2Mn2O8  +H2O  =  3Co2(OH)6  +  5HgCl2 
+  2KCl+2MnO2H2O 

but  as  this  exact  decomposition  cannot  be  depended  upon  to  take 
place  in  all  mixtures,  it  is  not  possible  to  accept  systematic  numbers 
calculated  from  normal  solutions. 

Solutions  containing  manganese,  phosphorus,  arsenic,  active 
chlorine,  oxygen  compounds,  or  organic  matter,  cannot  be  used  in 
this  method  of  determination  ;  moderate  quantities  of  copper  or  lead 
are  of  no  consequence.  Nor  is  antimony  when  its  quantity  is  double 
or  more  than  the  cobalt,  but  if  less  the  results  are  too  high. 

A  further  modification  of  this  method  was  advocated  by  von 
Reis  and  Wiggert,  and  possesses  the  advantage  of  being  easy 
and  simple  in  execution.  Very  fair  results  were  obtained  by  H.  B. 
Harris  on  trial  at  the  same  time  as  the  examination  of  Winkle  r's 
method. 

METHOD  OF  PROCEDURE  :  The  solution  of  cobalt  is  mixed  witli  an  emulsion 
of  zinc  oxide  and  heated  to  boiling.  A  standard  solution  of  permanganate 
is  then  added  in  known  quantity,  but  more  than  enough  to  precipitate  the 
oxidized  cobalt.  The  latter  precipitate  settles  to  the  bottom,  the  excess  of 
permanganate  is  then  found  by  titration  with  a  standard  solution  of  ferrous 
ammonium  sulphate. 

R.  L.  Taylor*  has  experimented  on  Rose's  method  of  separating  cobalt 
from  nickel,  and  has  improved  it  by  using  a  perfectly  neutral  solution  instead 
of  a  strongly  acid  one  as  used  by  Rose  ;  the  latter  neutralized  the  solution  by 
calcium  or  barium  carbonate,  but  the  C02  so  produced  retarded  or  altogether 
stopped  the  precipitation  of  the  cobalt  oxide.  By  the  new  process  the  result  is 
that  a  dilute  neutral  solution  of  cobalt  may  be  quantitatively  precipitated  by 
barium  or  calcium  carbonate  in  presence  of  bromine  water.  If  the  liquid  from 
which  the  cobalt  is  to  be  precipitated  is  acid,  the  acid  must  be  neutralized  by 
an  excess  of  carbonate  and  well  boiled  to  expel  all  the  C02,  and  then  cooled 

»C.  N.  88,  184. 


COBALT.  191 

before  the  bromine  water  is  added.  Not  only  C02  but  zinc  also  stops  the 
precipitation. 

The  composition  of  the  black  oxide  is  not  quite  clear,  but  it  is  fairly  constant 
in  composition,  and  if  dissolved  with  HC1  and  KI  the  amount  is  ascertained  by 
titrating  the  liberated  iodine.  The  method  has  been  tested  by  J-.  H.  Davidson 
in  the  assay  of  cobalt  ores,  and  he  finds  it  much  more  rapid  than  the  usual  methods 
and  sufficiently  accurate  for  assay  purposes. 

For  other  methods  for  the  determination  of  Cobalt,  •  see  under 
Nickel. 

COPPER. 

Cu  =  63-57. 

1  c.c.  N/10  solution =0-006357  gm.  Cu. 
Iron  x  1-138  =Cu. 

Double  Iron  Salt      xO'1622  =  Cu. 

1.     Reduction  by  Grape  Sugar  and  subsequent  titration  with 
Ferric  Chloride  and  Permanganate  (S  c  h  w  a  r  z). 

THIS  process  is  based  upon  the  fact  that  grape  sugar  precipitates 
cuprous  oxide  from  an  alkaline  solution  of  the  metal  containing 
tartaric  acid  ;  the  oxide  so  obtained  is  collected  and  mixed  with 
ferric  chloride  and  hydrochloric  acid.  The  result  is  the  following 
decomposition  :— 

Cu20+Fe2Cl6+2HCl-2CuCl2+2FeCl2+H20. 

Each  equivalent  of  copper  reduces  one  equivalent  of  ferric  to  ferrous 
chloride,  which  is  determined  by  permanganate  with  due  precaution. 
The  iron  so  obtained  is  calculated  into  copper  by  the  requisite  factor. 

METHOD  OF  PROCEDURE  :  The  weighed  substance  is  brought  into  solution  by 
nitric  or  sulphuric  acid  or  water,  in  a  porcelain  dish  or  glass  flask,  and  most  of 
the  acid  in  excess  neutralized  with  sodium  carbonate  ;  neutral  potassium  tartrate 
is  then  added  in  not  too  large  quantity,  and  the  precipitate  so  produced  dissolved 
to  a  clear  blue  liquid  by  adding  caustic  potash  or  soda  in  excess  ;  the  vessel  is 
next  heated  cautiously  to  about  50°  C.  in  the  water  bath,  and  sufficient  grape 
sugar  added  to  precipitate  the  copper  present ;  the  heating  is  continued  until  the 
precipitate  is  of  a  bright  red  colour,  and  the  upper  liquid  is  brownish  at  the  edges 
from  the  action  of  the  alkali  on  the  sugar ;  the  tempsraturc  must  never  exceed 
00°  C.  When  the  mixture  has  somewhat  cleared,  the  upper  fluid  is  poured  through 
a  moistened  filter,  and  afterwards  the  precipitate  brought  on  the  same,  and  washed 
with  hot  water  till  thoroughly  clean  ;  the  precipitate  which  may  adhere  to  the 
dish  or  flask  is  well  wrashed,  and  the  filter  containing  the  bulk  of  the 
protoxide  put  with  it,  and  an  excess  of  solution  of  ferric  chloride  (free  from  nitric 
acid  or  free  chlorine)  added,  together  with  a  little  sulphuric  acid  ;  the  whole  is 
then  warmed  and  stirred  until  the  cuprous  chloride  is  all  dissolved.  It  is  then 
filtered  into  a  good-sized  flask,  the  old  and  new  filters  being  well  washed  with  hot 
water,  to  which  at  first  a  little  free  sulphuric  acid  should  be  added,  in  order  to 
be  certain  of  dissolving  all  the  oxide  in  the  folds  of  the  paper.  The  entire  solution 
is  then  titrated  with  permanganate  in  the  usual  way.  Bichromate  may  also  be 
used,  but  the  end  of  the  reaction  is  not  so  distinct  as  usual,  from  the  turbidity 
produced  by  the  presence  of  copper. 

A  modification  of  this  permanganate  method,  which  gives  very 


192  COPPER. 

•good  technical  results,  has  been  devised  by  R.  K.  Meade,*  in 
which  the  copper  is  precipitated  as  thiocyanate.  The  author 
•considers  it  superior  in  accuracy  to  the  iodide  method  ;  but  with 
this  I  cannot  agree,  except  in  certain  cases. 

METHOD  OF  PROCEDURE  :  The  copper  is  brought  into  solution  as  a  sulphate, 
cither  by  dissolving  it  in  sulphuric  acid  or  by  evaporation  of  its  solution  with 
-sulphuric  acid.  The  greater  part  of  the  free  acid  is  neutralized  by  ammonia,  the 
.solution  warmed,  sulphurous  acid  added  until  the  solution  smells  strongly  of  the 
reagent,  and  then  a  slight  excess  of  ammonium  or  potassium  thiocyanate.  The 
copper  is  immediately  precipitated  as  cuprous  thiocyanate.  Stirring  and  warming 
renders  the  precipitate  heavy  and  easily  handled.  The  solution  is  filtered  through 
asbestos,  using  the  pump,  and  well  washed.  The  precipitate  and  filter  arc  thrown 
into  the  beaker  in  which  the  precipitation  was  made  and  heated  with  a  solution 
of  caustic  soda  or  caustic  potash.  Double  decomposition  takes  place.  Hydrated 
•cuprous  oxide  and  potassium  or  sodium  thiocyanate  result — 

2CuSCN  +2KOH  =Cu2(OH)2  +2KSCN. 

The  oxide  is  filtered  on  asbestos  and  washed  well  with  hot  water.  The 
precipitate  and  filter  are  again  placed  in  the  same  beaker  and  an  excess  of  ferric 
chloride  or  ferric  sulphate  (free  from  nitric  acid,  free  chlorine,  or  ferrous  salts), 
together  with  a  little  dilute  sulphuric  acid,  added.  The  copper  oxide  reduces 
•a  corresponding  amount  of  iron  from  the  ferric  to  the  ferrous  condition — 

Cu20  +Fe2Cl6  +2HC1  =2CuCl2  +  2FeCL  +  H2O. 

The  beaker  is  warmed  and  stirred  until  all  the  copper  oxide  is  dissolved.  The 
•solution  is  then  poured  through  a  perforated  platinum  disc,  and  the  asbestos 
which  remains  behind  upon  it  washed  with  water,  to  which  has  been  added  a  little 
sulphuric  acid  and  a  little  ferric  chloride  or  sulphate.  The  solution  is  then 
titrated  with  permanganate.  The  iron  equivalent  to  the  permanganate  used 
multiplied  by  1-138  gives  the  weight  of  copper  in  the  sample. 

Instead  of  sulphurous  acid,  ammonium  or  sodium  bisulphite  may  be  used  to 
reduce  the  copper.  A  solution  of  equal  weights  of  sodium  bisulphite  and 
potassium  thiocyanate  answers  well  as  a  reagent  for  the  precipitation  of  the 
metal.  Since  copper  is  the  only  metal  precipitated  by  an  alkali  thiocyanate 
from  an  acid  solution,  the  presence  of  arsenic,  antimony,  bismuth,  zinc,  and  other 
elements  which  render  the  electrolytic,  the  cyanide,  and  the  iodine  methods 
inaccurate  will  not  affect  the  results. 

The  caustic  alkali  solution,  used  to  convert  the  cuprous  thiocyanate  into 
cuprous  hydroxide,  must  not  be  too  strong,  or  some  of  the  metal  will  go  into 
solution,  colouring  the  liquid  blue.  About  a  half  normal  solution  of  caustic 
potash,  made  by  dissolving  28  gm.  of  the  salt  in  a  litre  of  water,  is  a  convenient 
strength.  Either  ferric  sulphate  or  ferric  chloride  may  be  used  to  dissolve  the 
cuprous  oxide.  The  former  is  probably  the  safer,  but  the  latter  appears  to  dissolve 
the  precipitate  the  more  readily  of  the  two. 

2.     Reduction  by   Zinc  and  subsequent  titration  with  Ferric 
Chloride  and  Permanganate  (F  1  e  i  t  m  a  n  n). 

The  metallic  solution,  free  from  nitric  acid, -bismuth,  and  lead, 
is  precipitated  with  clean  sticks  of  pure  zinc  ;  the  copper  collected, 
washed,  and  dissolved  in  a  mixture  of  ferric  chloride  and  hydro- 
chloric acid  ;  a  little  sodium  carbonate  may  be  added  to  expel  the 
atmospheric  air.  The  reaction  is — 

Cu  +Fe2Cl6 =CuCl2  +  2FeCl2. 

*  J.  Am.  C.  S.  20,  610. 


COPPER.  193 

When  the  copper  is  all  dissolved,  the  solution  is  diluted  and 
titrated  with  permanganate  ;  55'85  Fe  =  31*79  Cu,  or  1  Fe  =  '5692  Cu. 

If  the  original  solution  contains  nitric  acid,  bismuth,  or  lead,  the 
decomposition  by  zinc  must  take  place  in  an  ammoniacal  solution, 
from  which  the  precipitates  of  either  of  the  above  metals  have  been 
removed  by  filtration  ;  the  zinc  must  in  this  case  be  finely  divided 
and  the  mixture  warmed.  The  copper  is  all  precipitated  when  the 
colour  of  the  solution  has  disappeared.  It  is  washed  first  with  hot 
water,  then  with  weak  HC1  and  water  to  remove  the  zinc,  again 
with  water,  and  then  dissolved  in  the  acid  and  ferric  chloride  as 
before. 


3.    Determination  as  Cuprous  Iodide. 

This  excellent  method  is  based  on  the  fact  that  when  potassium 
iodide  is  mixed  with  a  salt  of  copper  in  acid  solution  cuprous  iodide 
is  precipitated  as  a  dirty  white  powder  and  iodine  set  free.  If  the 
latter  is  then  immediately  titrated  with  thiosulphate  and  starch,  the 
corresponding  quantity  of  copper  is  found. 

2CuS04  +  4KI = Cu2I2 + 2K2S04  + 12. 

The  solution  of  the  metal,  if  it  contain  nitric  acid,  is  evaporated 
with  sulphuric  acid  till  the  former  is  expelled,  or  the  nitric  acid 
is  neutralized  with  sodium  carbonate,  and  acetic  acid  added  ;  the 
sulphate  solution  must  be  neutral,  or  only  faintly  acid  ;  excess  of 
acetic  acid  is  of  no  consequence,  and  therefore  it  is  always  necessary 
to  get  rid  of  all  free  mineral  acids  and  work  only  with  free  acetic 
acid. 

J.  W.  Westmoreland,*  who  has  had  very  large  experience  in 
examining  a  variety  of  copper  products,  and  has  worked  the  process 
in  my  own  laboratory,  strongly  recommends  it  for  the  determination 
of  copper  in  its  various  ores,  etc.  The  metal  may  very  conveniently 
be  separated  from  a  hot  sulphuric  acid  solution  by  sodium  thio- 
sulphate :  this  gives  a  flocculent  precipitate  of  subsulphide  mixed 
with  sulphur,  which  filters  readily,  and  can  be  washed  with  hot 
water.  Arsenic  and  antimony,  if  present,  are  also  precipitated ;  tin, 
zinc,  iron,  nickel,  cobalt,  and  manganese  are  not  precipitated.  On 
igniting  the  precipitate  most  of  the  arsenic  and  the  excess  of  sulphur 
are  expelled,  an  impure  subsulphide  of  copper  being  left.  Sulphur- 
etted hydrogen  may  of  course  be  used  instead  of  the  thiosulphate, 
but  its  use  is  objectionable  to  many  operators ;  moreover,  in  some 
circumstances,  a  small  amount  of  copper  remains  in  the  solution, 
and  iron  in  small  quantity  is  also  precipitated  with  the  copper 
and  cannot  be  entirely  removed  by  washing.  If  H2S  is  used  it 
should  be  passed  for  some  time,  and  the  precipitate  allowed  to 
stand  a  few  hours  to  settle ;  after  filtration  and  washing  the  CuS 
should  be  redissolved  in  HNO3  and  reprecipitated  with  the  gas, 
it  is  then  quite  free  from  iron. 

*  j.  s.  c.  /.,  5,  51. 


194  COPPER. 

Standardizing  the  Thiosulphate  Solution. — This  may  be  done  on 
pure  electrotype  copper,  but  this  is  not  always  to  be  had  pure,  and 
the  safest  standard  is  high  conductivity  wire,  first  dissolving  in 
nitric  acid,  boiling  to  expel  nitrous  fumes,  diluting,  neutralizing 
with  sodium  carbonate  till  a  precipitate  appears,  then  adding  acetic 
acid  till  clear.  The  liquid  is  then  made  up  to  a  definite  volume, 
and  a  quantity  equal  to  about  O5  gm.  Cu  taken  in  a  flask  or  beaker, 
about  ten  times  the  copper  weight  of  potassium  iodide  added,  and 
when  dissolved  the  thiosulphate  is  run  in  from  a  burette  until  the 
free  iodine  is  nearly  removed,  some  starch  then  added,  and  the 
titration  finished  in  the  usual  way.  The  thiosulphate  will  of  course 
need  to  be  checked  occasionally. 

If  strictly  N/10  thiosulphate  is  used,  each  c.c.  =0*006357  gm.  Cu. 

METHOD  OF  PROCEDURE  :  For  determining  the  copper  in  iron  pyrites  or  burnt 
ore  5  gm.  of  the  substance  should  be  taken,  2  gm.  for  30-40  %  mattes  or  1  gm. 
for  60  %  mattes,  and  with  precipitates  it  is  best  to  dissolve  say  5  gm.  and  dilute 
to  a  definite  volume,  and  take  as  much  as  would  represent  from  0*5  to  0-7  gm.  of 
Cu  for  titration.  The  solution  is  made  with  nitric  acid,  to  which  hydrochloric  is 
also  added  later  on,  and  then  evaporated  to  dryness  with  excess  of  sulphuric  acid 
to  convert  the  bases  into  sulphates ;  the  residue  is  treated  with  warm  water  and 
any  insoluble  PbS04,  etc.,  filtered  off.  The  filtrate  is  heated  to  boiling  and 
precipitated  with  thiosulphate,  this  precipitate  is  filtered  off,  washed  with  hot 
water,  dried,  and  roasted ;  the  resulting  copper  oxide  is  then  dissolved  in  nitric 
acid,  and  after  the  excess  of  acid  has  been  mainly  removed  by  evaporation,  sodium 
carbonate  is  added  so  as  to  precipitate  part  of  the  copper  and  ensure  freedom 
from  mineral  acid,  acetic  acid  is  added  till  a  clear  solution  is  obtained  ;  about 
ten  parts  of  potassium  iodide  to  one  of  copper  supposed  to  be  present  are  then 
added,  and  the  titration  carried  out  in  the  usual  way. 

An  excellent  modification  of  this  method,  much  used  in  America, 
is  described  by  A.  H.  Low.*  A  solution  of  thiosulphate  is  used 
containing  about  19  gm.  per  litre,  which  is  standardized  upon 
about  0-2  gm.  of  pure  copper  foil  dissolved  in  5  or  6  c.c.  of  nitric 
acid  sp.  gr.  1-2  in  a  250  c.c.  flask  in  the  following  manner  : — 

Heat  the  nitric  acid  solution  to  boiling,  add  5  c.c.  strong  bromine  water,  and 
again  boil  till  all  nitrous  fumes  and  bromine  are  expelled.  As  soon  as  the  incrusted 
matter  has  dissolved,  add  a  slight  excess  of  ammonia,  boil  off  the  excess,  then  add 
3  to  4  c.c.  of  acetic  acid,  which  dissolves  any  precipitated  copper  hydroxide 
(boiling  again  may  be  necessary),  after  cooling  and  diluting  to  about  50  c.c.  add 
3  gm.  of  KI  and  titrate  with  thiosulphate  as  usual. 

METHOD  OF  PROCEDURE  FOR  ORES  :  Treat  0-25-0-5  gm.  of  finely  ground  ore 
with  5  or  6  c.c.  of  .nitric  acid  1*42  sp.  gr.  and  evaporate  nearly  to  dryness.  Dissolve 
all  incrusted  matter  by  heating  with  5  c.c.  strong  hydrochloric  acid,  add  7  c.c. 
sulphuric  acid  and  heat,  to  expel  volatile  acids,  till  the  sulphuric  acid  fumes 
freely.  After  cooling  and  diluting  with  25  c.c.  of  water,  heat  to  dissolve  any 
anhydrous  ferric  sulphate,  and  filter.  The  filtrate  and  washings,  which  should 
not  much  exceed  75  c.c.,  are  received  in  a  small  beaker.  The  copper  is  now 
precipitated  by  means  of  aluminium  as  follows : — Place  in  the  beaker  two  pieces 
of  stout  sheet  aluminium,  about  one-sixteenth  of  an  inch  in  thickness,  which, 
for  the  sake""of  convenience  in  subsequent  washing,  should  be  1£  inch  square 
with'the  four  corners  bent,  for  about  a  quarter  of  an  inch,  alternatively  up  and 
down  at  right  angles.  -  This  prevents  the  pieces  from  lying  flat  against  each 
other  or  upon  the  bottom  of  the  beaker.  The  same  pieces  may  be  used  repeatedly, 

*  J.  Am.  C.  S.  18,  458,  and  24,  1082. 


COPPER.  195 

as  they  are  but  little  attacked  each  time.  Cover  the  beaker  and  boil  gently  for 
7-10  minutes.  Unless  the  bulk  of  the  solution  is  excessive,  this  will  be  quite 
sufficient  for  all  percentages  of  copper.  Ordinarily  the  aluminium  will  be  found 
to  be  clean,  and  nearly  or  quite  free  from,  precipitated  copper.  If,  by  chance, 
the  copper  adheres  to  any  considerable  extent,  it  will  usually  become  loosened 
by  a  little  additional  boiling,  or  it  may  be  removed  by  the  aid  of  a  glass  rod. 
Rinse  the  cover  and  sides  of  the  beaker  with  cold  water.  To  prevent  oxidation 
of  finely-divided  copper  during  subsequent  washing  and  at  the  same  time  to 
remove  any  traces  of  copper  still  in  solution,  add  about  15  c.c.  strong  H2S  water. 
Transfer  the  solution  back  to  the  original  flask,  and  by  means  of  a  jet  of  H2S 
water  from  a  wash-bottle  rinse  in  also  as  much  of  the  copper  as  possible,  leaving 
the  aluminium  behind.  Drain  the  beaker  as  completely  as  possible,  and 
temporarily  set  aside  with  the  aluminium,  which  may  still  retain  a  little  copper. 
Allow  the  copper  in  the  flask  to  settle,  and  then  decant  the  liquid  through  a  filter. 
Again  wash  the  copper  similarly  two  or  three  times  with  about  20  c.c.  H2S  water 
each  time,  retaining  it  as  completely  as  possible  in  the  flask.  Finally,  wash  the 
filter  once  or  twice  and  endeavour  to  rinse  all  metallic  particles  down  into  the 
point.  Now  pour  upon  the  aluminium  in  the  beaker  5  c.c.  of  nitric  acid*  1*2 
sp.  gr.,  and  warm  the  beaker  gently,  but  do  not  heat  to  boiling,  as  the  aluminium 
would  be  thereby  unnecessarily  attacked.  See  that  any  copper  present  is  dis- 
solved, and  pour  the  hot  solution  very  slowly  through  the  filter,  thus  dissolving 
any  contained  particles  of  copper,  and  receive  the  filtrate  in  the  flask  containing 
the  main  portion  of  the  copper.  Before  washing  the  filter  pour  upon  it  5  c.c.  of 
bromine  water,  and  wash  the  filter  and  beaker  with  hot  water.  The  bromine 
must  be  in  sufficient  excess  to  give  a  slight  tinge  to  the  filtrate.  Boil  the  filtrate 
to  remove  bromine,  add  excess  of  ammonia,  and  proceed  as  described  above  for 
copper  foil  in  standardizing.  If  the  percentage  of  Cu  in  the  ore  does  not  exceed 
20  per  cent.,  the  precipitated  metal  may  be  washed  with  H2S  water  upon  the 
filter,  instead  of  by  decantation,  care  being  taken  that  the  filter  is  kept  filjed  till 
the  washing  is  complete,  to  avoid  oxidation. 

A.  M.  Fair  lief  uses  ammonium  thiocyanate,  in  preference  to 
sodium  thiosulphate  or  aluminium,  to  separate  the  copper. 

The  cuprous  thiocyanate  is  dissolved  in  strong  nitric  acid,  the  solution  boiled 
till  red  fumes  are  no  longer  evolved,  then  neutralized  with  ammonia,  acidified 
with  acetic  acid,  KI  added  and  the  titration  carried  out  in  the  usual  way.  The 
presence  of  much  ammonium  acetate  must  be  avoided. 


4.    Determination  by  Potassium  Cyanide  (P  a  r  k  e  s    and 

C.  Mohr). 

(A  solution  containing  40  grams  potassium  cyanide  per  litre  is  used) 
1  c.c.  =  (about)  '01  gm.  copper. 

This  well-known  and  much-used  process  for  determining  copper 
depends  upon  the  decoloration  of  an  ammoniacal  solution  of  copper 
by  potassium  cyanide.  The  reaction  (which  is  not  absolutely 
uniform  with  variable  quantities  of  ammonia)  is  such  that  a  double 
cyanide  of  copper  and  ammonia  is  formed ;  cyanogen  is  also  liberated, 
which  reacts  on  the  free  ammonia,  producing  urea,  oxalate  of  urea, 
ammonic  cyanide  and  formate  (Lie big).  Owing  to  the  influence 

*  Vide^ren  (Z.  a.  C.,  1909,  539)  prefers  to  dissolve  in  a  mixture  of  potassium 
chlorate  and  hydrochloric  and  sulphuric  acids.  If  arsenic  or  antimony  are  present, 
sodium  acetate  is  added  before  titrating.  The  end-points  obtained  are  said  to  Lbe 
mucb  sharper  than  with  L  o  w '  s  method  of  solution. 

.  and  Mining  J.,  1904,  787. 

o  2 


196  COPPER. 

exercised  by  variable  quantities  of  ammonia,  or  its  neutral  salts, 
upon  the  decoloration  of  a  copper  solution  by  the  cyanide,  it  has 
been  suggested  by  Beringer  to  substitute  some  other  alkali  for 
ammonia  in  neutralizing  the  free  acid  in  the  copper  solution.  The 
suggestion  has  been  adopted  by  Davies*  and  by  Fessendenf 
who  both  recommend  sodium  carbonate.  My  own  experiments 
completely  confirm  their  statement  that  none  of  the  irregularities 
observed  with  variable  quantities  of  ammonia  or  its  salts  occur  with 
s.oda  or  potash.  Suppose,  for  example,  that  copper  has  been  separated 
as  sulphide,  and  brought  into  solution  by  nitric  acid,  the  free  acid  is 
neutralized  with  Na2CO3,  and  an  excess  of  it  added  to  redissolve 
the  precipitate.  The  cyanide  solution  is  then  cautiously  run  into 
the  light  blue  solution  until  the  colour  is  just  discharged.  My  own 
experience  is  that  it  is  impossible  to  redissolve  the  whole  of  the  pre- 
cipitate without  using  a  very  large  excess  of  soda  ;  but  there  is  no 
need  to  add  such  an  excess,  as  the  precipitate  easily  dissolves  when 
the  cyanide  is  added.  I  have  used  a  modification  of  this  method 
which  gives  excellent  results,  viz.,  to  neutralize  the  acid  copper 
solution  either  with  Na2CO3  or  NaHO,  add  a  trifling  excess,  and 
then  1  c.c.  of  ammonia  0'960  sp.  gr.  ;  a  deep  blue  clear  solution  is  at 
once  given,  which  permits  of  very  sharp  end-reaction  with  the 
cyanide. 

J.  J.  and  C.  BeringerJ  have  already  adopted  the  method  of 
neutralizing  the  acid  copper  solution  with  soda,  then  adding 
ammonia,  but  the  proportion  they  recommend  is  larger  than 
necessary. 

In  standardizing  the  cyanide,  it  is  advisable  so  to  arrange  matters 
that  copper  is  precipitated  with  soda  exactly  as  in  the  titration  of 
a  copper  ore  ;  that  is  to  say,  free  nitric  or  nitro-sulphuric  acid  should 
be  added,  then  neutralized  with  slight  excess  of  soda,  cleared  with 
1  c.c.  of  ammonia,  then  titrated  with  cyanide.  Large  quantities 
of  nitrate  or  sulphate  of  soda  or  potash,  however,  make  very  little 
difference  in  the  quantity  of  cyanide  used. 

It  has  generally  been  thought  that  where  copper  and  iron  occur  together,  it  is 
necessary  to  separate  the  latter  before  using  the  cyanide.  F.  Field,]]  however, 
has  stated  that  this  is  not  necessary ;  and  I  can  fully  endorse  his  statement  that 
the  presence  of  the  suspended  ferric  oxide  is  no  hindrance  to  the  determination  of 
the  copper ;  in  fact,  it  is  rather  an  advantage,  as  it  acts  as  an  indicator  to  the 
end  of  the  process. 

While  the  copper  is  in  excess,  the  oxide  possesses  a  purplish -brown  colour,  but 
as  this  excess  lessens,  the  colour  becomes  gradually  lighter,  until  it  is  orange 
brown.  If  it  be  now  allowed  to  settle,  which  it  does  very  rapidly,  the  clear  liquid 
above  will  be  found  nearly  colourless.  A  little  practice  is  of  course  necessary  to 
enable  the  operator  to  hit  the  exact  point. 

It  is  impossible  to  separate  the  ferric  oxide  by  filtration  without 
leaving  some  copper  in  it,  and  no  amount  of  washing  will  remove 
it.  For  example,  10  c.c.  of  a  copper  solution  with  10  c.c.  of  ferric 

*  C.  N.  58,  131.    t  C.  N.  61,  131.    See  also  Fernekes  and  Koch,  J.  A.  C.  8.,  27, 1224. 
J  C.  AT.  49,  3.  ||  C.  N.  1,  25. 


COPPER.  197 

solution  were  directly  titrated  with  cyanide  after  treatment 
with  NaHO  in  slight  excess  and  1  c.c.  of  ammonia.  The  cyanide 
required  was  12  c.c.  Another  10  c.c.  of  the  same  copper  and  iron 
solutions  were  then  precipitated  with  soda  and  ammonia  in  same 
proportions.  This  gave  a  complete  solution  of  the  copper  with  the 
ferric  oxide  suspended  in  it.  The  solution  was  filtered  and  the 
ferric  oxide  well  washed  with  hot  water,  then  the  filtrate  cooled  and 
titrated  with  cyanide,  9\5  c.c.  only  being  required.  On  treating 
the  ferric  oxide  on  the  filter  with  nitric  acid,  neutralizing  with  NaHO 
and  NH3  in  proper  proportions  exactly,  2'5  c.c.  of  cyanide  were 
required,  showing  that  the  ferric  oxide  had  retained  20  per  cent, 
of  the  copper. 

I  strongly  recommend  that  operators  who  have  to  deal  with 
copper  determination  in  samples  containing  much  iron  should 
practise  the  use  of  the  cyanide  method  in  the  presence  of  the  iron, 
and  accustom  their  eyes  to  the  exact  colour  which  the  ferric  oxide 
takes  when  the  titration  is  finished,  always,  however,  with  this 
proviso,  that  the  cyanide  solution  is  standardized  upon  a  known 
weight  of  copper  in  the  presence  of  a  moderate  amount  of  iron. 

The  solution  of  potassium  cyanide  should  be  titrated  afresh  at 
intervals  of  a  few  days.  Further  details  of  this  process  are  given 
on  p.  201  (8). 

Dulin*  advocates  the  cyanide  process  for  copper  ores  as  follows  : 

METHOD  OF  PROCEDURE  :  The  ore  is  treated  in  the  way  described  on  p.  194  to 
obtain  a  solution  of  the  copper  practically  free  from  silver  and  lead.  The  copper 
is  then  precipitated  upon  aluminium  foil  as  there  mentioned.  Should  cadmium 
be  present  it  is  also  precipitated  to  some  extent,  but  only  after  the  copper  is  thrown 
down.  If  care  be  taken  to  stop  the  boiling  immediately  after  the  copper  is 
precipitated,  which  a  practised  eye  will  readily  detect,  the  amount  of  cadmium 
precipitated  is  so  small  as  to  cause  no  sensible  error.  The  liquid  being  decanted 
from  the  copper  and  foil,  the  latter  are  washed  well  with  hot  water,  taking  care  to 
lose  no  metal ;  when  quite  clean,  dilute  nitric  acid  is  added  and  boiled  till  the 
copper  is  dissolved,  the  liquid  then  neutralized  with  excess  of  ammonia,  and 
titrated  with  cyanide  in  the  usual  way. 

5.    Determination  as  Sulphide  (P  e  1  o  u  z  e). 
It  is  first  necessary  to  have  a  solution  of  pure  copper  of  known 
strength,  which  is  best  made  by  dissolving  39'286  gm.   of  pure 
re-crystallized  cupric  sulphate  in  1  litre  of  water ;    each   c.c.  will 
contain  O'Ol  gm.  Cu. 

Precipitation  in  Alkaline  Solution. — This  process  is  based  on  the 
fact  that  if  an  ammoniacal  solution  of  copper  is  heated  to  from 
40°  to  80°  C.,  and  a  solution  of  sodium  sulphide  added,  the  whole 
of  the  copper  is  precipitated  as  oxysulphide,  leaving  the  liquid 
colourless.  The  loss  of  colour  indicates,  therefore,  the  end  of  the 
process,  and  this  is  its  weak  point.  Special  practice,  however,  will 
enable  the  operator  to  hit  the  exact  point  closely. 

Casamajorf   uses   instead   of   ammonia   the   alkaline   tartrate 

*  Jour.  Amer.  Chem.  Soc.  17,  346.  t  C.  N.  45.  167. 


198  COPPER. 

solution  of  Fehling,  adding  a  slight  excess  so  as  to  make  a  clear 
blue  solution.  The  addition  of  the  sulphide  gives  an  intense  black 
brown  precipitate,  the  liquid  being  stirred  vigorously  till  clear.  The 
copper  sulphide  agglomerates  into  curds,  and  the  reagent  is  added 
until  no  further  action  occurs  with  a  drop  of  the  sodium  sulphide. 
This  modification  can  also  be  used  for  lead.  PbS04  is  easily  soluble 
in  the  tartrate  solution,  and  can  be  determined  by  the  sodium 
sulphide  in  the  same  way  as  copper. 

The  colour  of  the  solution  is  not  regarded,  but  the  clotty  pre- 
cipitate of  sulphide,  which  is  easily  made  to  agglomerate  by 
vigorous  stirring.  Very  good  results  may  be  gained  by  this 
modification. 

Copper  can  also  be  first  separated  by  glucose,  or  as  thiocyanate 
(Rivot),  then  dissolved  in  HNO3,  and  treated  with  the  tartrate. 

Precipitation  in  Acid  Solution. — The  copper  solution  is  placed  in 
a  stoppered  flask  (400  or  500  c.c.),  freely  acidified  with  hydrochloric 
acid,  then  diluted  with  about  200  c.c.  of  hot  water. 

The  alkali  sulphide  is  then  delivered  in  from  a  burette,  the 
stopper  replaced,  and  the  mixture  well  shaken  ;  the  precipitate  of 
copper  sulphide  settles  readily,  leaving  the  supernatant  liquid  clear  ; 
fresh  sulphide  solution  is  then  added  at  intervals  until  no  more 
precipitate  is  produced.  The  calculation  is  the  same  as  in  the 
case  of  alkaline  precipitation,  but  the  copper  is  precipitated  as 
sulphide  instead  of  oxysulphide. 

6.     Determination  by  Stannous  Chloride  (Weil).* 

This  process  is  based  on  the  fact  that  a  solution  of  a  cupric  salt  in 
large  excess  of  hydrochloric  acid  at  a  boiling  heat  shows,  even  when 
the  smallest  trace  is  present,  a  greenish-yellow  colour.  If  to  such 
a  solution  stannous  chloride  is  added  in  minute  excess,  a  colourless 
cuprous  chloride  is  produced,  and  the  loss  of  colour  indicates  the  end 
of  the  process. 

2CuCl2  +SnCl2 =Cu2Cl2  -f  SnCl4. 

The  change  is  easily  distinguishable  by  the  eye,  but  should  any 
doubt  exist  as  to  whether  stannous  chloride  is  in  excess,  a  small 
portion  of  the  solution  may  be  tested  with  mercuric  chloride.  Any 
precipitate  of  calomel  indicates  the  presence  of  stannous  chloride. 

The  tin  solution  is  prepared  as  described  on  p.  128. 

A  standard  copper  solution  is  made  by  dissolving  pure  copper 
sulphate  in  distilled  water,  in  the  proportion  of  39 "286  gm.  per 
litre  =  10  gm.  of  Cu. 

Method  of  Procedure  for  Copper  alone.— 10  c.c.  of  the  copper  solution  (  =0-1  gm. 
of  Cu)  are  put  into  a  white-glass  flask,  25  c.c.  of  pure  strong  hydrochloric  acid 
added,  placed  on  a  sand-bath  and  brought  to  boiling  heat ;  the  tin  solution  is 
then  quickly  delivered  in  from  a  burette  until  the  colour  is  nearly  destroyed, 
finally  a  drop  at  a  time  till  the  liquid  is  as  colourless  as  distilled  water.  No 
oxidation  will  take  place  during  the  boiling,  owing  to  the  flask  being  filled  with 
acid  vapours. 

*  Z.  Anal.  Chem.  9,  297. 


COPPER.  199 

A  sample  of  copper  ore  is  prepared  in  the  usual  way  by  treatment  with  nitric 
acid,  which  is  afterwards  removed  by  evaporating  with  sulphuric  acid.  Silica, 
lead,  tin,  silver,  or  arsenic,  are  of  no  consequence,  as  when  the  solution  is  diluted 
with  water  to  a  definite  volume,  the  precipitates  of  these  substances  settle  to  the 
bottom  of  the  measuring  flask,  and  the  clear  liquid  may  be  taken  out  for  titration. 
In  case  antimonic  acid  is  present  it  will  be  reduced  with  the  copper,  but  on 
exposing  the  liquid  for  a  night  in  an  open  basin,  the  copper  will  be  completely 
re-oxidized  but  not  the  antimony ;  a  second  titration  will  then  show  the  amount 
of  copper. 

Method  of  Procedure  for  Ores  containing  Copper  and  Iron. — In  the  case  of  copper 
ores  where  iron  is  also  present,  the  quantity  of  tin  solution  required  will  of  course 
represent  both  the  iron  and  the  copper.  In^this  case  a  second  titration  of  the 
original  solution  is  made  with  zinc  and  permanganate,  and  the  quantity  so  found 
is  deducted  from  the  total  quantity ;  the  amount  of  tin  solution  corresponding  to 
copper  is  thus  found. 

EXAMPLE  :  A  solution  was  prepared  from  10  gm.  of  ore  and  diluted  to  250  c.c.  ; 
10  c.c.  required  26*75  c.c.  of  tin  solution  whose  strength  was  16-2  c.c^for  O'l  gm. 
of  Cu. 

10  c.c.  of  ore  solution  were  diluted,  zinc  and  platinum  added,  warmed  till 
reduction  was  complete,  and  the  solution  titrated  with  permanganate,  of  which 
the  quantity  used  =0-0809  gm.  of  Fe. 

The  relative  strength  of  the  tin  solution  to  iron  is  found  thus : — 

63-57  :  0-1      =  55-85  :  x 

x     =  -0879 

i.e.,  0-1  gram  Cu      =  -0879  gram  Fe. 

=  16-2  c.c.  SnCl2. 
Again, 

•0879  :   -0809      =  16-2  :  y 

y     =  14-9 

i.e.,  0-0809  Fe  found  above      =  14-9  c.c.  SnCl2 

Hence,  iron  +  copper      =  26*75  c.c.  SnCl2 

Iron      =  14-9 


Copper  11-85 

Finally, 

16-2  :  11-85      =     0-1:2 

z     =     0-07315. 
That  is,  10  c.c.  of  ore  solution  contain  0*07315  gram  Cu,  or  250  c.c.  (  =10  grams 

100 
of  ore)  contain  -07315  x — =1-829  gm. 

The  percentage  of  copper  is,  therefore,  18-29. 

A  gravimetric  determination  as  a  control  gave  18-34  per  cent.  Cu. 

Fe  determined  volumetrically  gave  20-25  %,  gravimetrically  20-10  %. 

The  method  is  specially  adapted  for  the  technical    analysis    of 
fahl-ores. 


7.    Precipitation  as  Cuprous  Thiocyanate,  Volhard's  method. 

The  necessary  standard  solutions  are  described  on  p.  145.  Each 
c.c.  of  N/10  thiocyanate  represents  0:006357  gm.  Cu. 

2CuSO4  +  2KSCN + S02 + 2H20  =  2CuSCN + 2H2SO4 + K2SO4. 

METHOD  OF  PROCEDURE  :  The  copper  in  sulphuric  or  nitric  acid  solution  is 
evaporated  to  remove  excess  of  acid,  or  if  the  acid  is  small  in  quantity  neutralized 
with  sodium  carbonate,  washed  into  a  500  c.c.  flask,  and  enough  aqueous  solution 


200  COPPEB. 

of  S02  added  to  dissolve  the  traces  of  basic  carbonate  and  leave  a  distinct  smell 
of  S02.  Heat  to  boiling,  and  run  in  the  thiocyanate  from  a  burette  until  the 
addition  produces  no  change  of  colour,  add  3  or  4  c.c.  more,  and  note  the  entire 
quantity ^used,  allow  to  cool,  fill  to  mark,*  and  shake! well.  100  c.c.  are  then 
filtered  through  a  dry  filter,  10  c.c.  of  ferric  indicator  with  some  nitric  acid  added, 
then  titrated  with  N/1O  silver  nitrate  till  colourless  :  then  again  thiocyanate  till 
the  reddish  colour  appears.  The  volume  of  silver  solution,  less  the  final  correction 
with  thiocyanate,  deducted  from  the  original  thiocyanate,  will  give  the  volume 
of  the  latter  required  to  precipitate  the  copper. 

The  process  is  not  accurate  in  presence  of  Fe,  Ag,  Hg,  01,  I,  Br, 
or  As205. 

Several  modern  processes  for  the  volumetric  determination  of 
copper,  chiefly  due  to  American  chemists,  have  been  based  upon 
the  separation  of  the  metal  as  cuprDus  thiocyanate.  One  (Fair  lie's) 
has  already  been  mentioned  as  a  modification  of  the  iodide  method. 
In  another  (Meade's,  see  p.  192),  the  iron  equivalent  of  the 
separated  copper  is  titrated  with  permanganate.  In  all  cases 
the  precipitation  as  thiocyanate  is  effected  as  already  described. 

GAERIGTJE'S  ACIDIMETRIC  METHOD.*  The  washed  thiocyanate  precipitate, 
with  the  filter,  is  decomposed  by  boiling  with  caustic  alkali  (see  M'eade'  s  method, 
p.  192),  excess  of  normal  soda  being  used,  and  the  excess,  after  filtering  off  the 
cuprous  hydroxide,  titrated  by  normal  acid,  with  methyl  orange  as  indicator. 
The  method  is  said  to  be  especially  useful  in  alloy  assay. 

PARR'S  PERMANGANATE  METHOD.!  The  precipitated  cuprous  thiocyanate 
is  decomposed  by  caustic  alkali,  the  copper  oxidized  in  alkaline  solution  by 
permanganate  without  decomposition  of  the  alkali  thiocyanate,  and  the  thiocyanic 
acid  titrated  in  acid  solution  with  permanganate,  the  whole  being  effected  in  one 
operation  as  follows : — The  washed  precipitate  of  cuprous  thiocyanate  and  the 
asbestos  pulp  or  filter  paper  are  treated  with  10  c.c.  of  a  10  %  solution  of  caustic 
potash  and  10  c.c.  of  ammonia  (sp.  gr.  0'96),  and  then  immediately  titrated  with 
permanganate  until,  upon  warming  to  about  50°  C.,  the  green  colour  of  the  supernatant 
liquid  remains.  About  one-third  or  one-fourth  of  the  quantity  of  permanganate 
necessary  for  this  is  then  run  in,  the  mixture  allowed  to  stand  for  5  minutes,  then 
acidified  with  25  c.c.  sulphuric  acid  (1  :  1  or  2)  and  titrated  to  a  pink  colouration 
with  permanganate.  10  atoms  of  cuprous  Cu  are  oxidized  by  7  molecules  of 
permanganate.  The  copper  value  of  the  latter  is  found  by  multiplying  the  iron 
factor  by  0'1602. 

IODATE  METHOD.  J  The  application  of  the  potassium  iodate  method  of  Andrews 
(p.  132)  has  been  found  to  afford  a  simple  and  accurate  process  for  the  titration 
of  cuprous  thiocyanate.  The  washed  precipitate,  together  with  the  asbestos  or 
filter-paper  on  which  it  was  filtered,  is  placed  in  a  bottle,  5  c.c.  of  chloroform, 
20  c.c.  of  water,  and  30  c.c.  of  hydrochloric  acid  are  added,  and  a  standard  solution 
of  potassium  iodate,  containing  11 '784  grams  of  the  salt  per  litre,  is  run  in  with 
constant  shaking,  until  the  colour  first  formed  in  the  chloroform  disappears. 
The  reaction  proceeds  according  to  the  equation  : 

4CuSCN  +  7KI03  +  14HC1  =4CuS04  +  7KC1  +  7IC1  +4HCN  +5H20. 

One  c.c.  of  the  standard  solution  corresponds  to  0'0020  gram  of  copper. 
The  chloroform  may  be  used  a  second  time. 

To  apply  the  method  to  ores,  the  sample  is  dissolved  in  nitric  acid  or  aqua 
regia,  and  boiled  down  with  sulphuric  acid  until  fumes  of  the  latter  are  given  off. 
The  residue  is  diluted  and  filtered  after  the  addition  of  1  or  2  drops  of  hydrochloric 

»  J.  A.  C.  S.,  19,  940. 

t  J.  A.  C.  S.  22,  685  and  24,  580.  See  also  H  awley,  Eng.  andMining  J.  1908,  1155, 
and  J.  S.  C.  /.,  1909, 163. 

t  Jamieson,  Levy,  and  Wells,  J.  A.  C.  S.,  1908,  760. 


COPPER. 


201 


acid  if  silver  is  present.  The  small  quantities  of  lead  and  antimony  left  in 
solution  do  not  interfere  with  the  subsequent  precipitation  of  the  copper  by 
means  of  sulphur  dioxide  and  ammonium  thiocyanate.  A  complete  determination 
can  be  made  in  one  hour. 

Potassium  iodate  can  be  obtained  pure,  and  its  solution  is  very  stable. 


8.    Technical  Examination  of  Copper  Ores  (Steinbeck's 

Process). 

In  1867  the  Directors  of  the  Mansfield  Copper  Mines  offered 
a  premium  for  the  best  method  of  examining  these  ores,  the  chief 
conditions  being  tolerable  accuracy,  simplicity  of  working,  and  the 
possibility  of  one  operator  making  at  least  eighteen  assays  in  the  day. 


Fig.  42. 

The  fortunate  competitor~was^Dr.  Steinb'eck,  whose  process 
completely  satisfied  the  requirements.  The  whole  report  is  con- 
tained in  Z.  a.  C.  viii.  1,  and  is  also  translated  in  G.  N.  xix.  181. 
The  following  is  a  condensed  account  of  the  process,  the  final  titration 
of  the  copper  being  accomplished  by  potassium  cyanide  as  on  p.  195. 


202  COPPER. 

A  very  convenient  arrangement  for  filling  the  burette  with  standard 
solution  where  a  series  of  analyses  has  to  be  made,  and  the  burette 
continually  emptied,  is  shown  in  fig.  42  ;  it  may  be  refilled  by  simply 
blowing  upon  the  surface  of  the  liquid. 

(a)  The  extraction  of  the  Copper  from  the  Ore.— 5  gm.  of  pulverized  ore  are  put 
into  a  flask  with  from  40  to  50  c.c.  of  hydrochloric  acid  (specific  gravity  1*16), 
whereby  all  carbonates  are  converted  into  chlorides,  while  carbonic  acid  is  expelled. 

After  a  while  there  is  added  to  the  fluid  in  the  flask  6  c.c.  of  a  special  nitric  acid, 
prepared  by  mixing  equal  bulks  of  water  and  pure  nitric  acid  of  1-42  sp.  gr.  As 
regards  certain  ores,  however,  specially  met  with  in  the  district  of  Mansfield, 
some,  having  a  very  high  percentage  of  sulphur  and  bitumen,  have  to  be  roasted 
previous  to  being  subjected  to  this  process  ;  and  others,  again,  require  only  1  c.c. 
of  nitric  acid  instead  of  6.  The  flask  containing  the  assay  is  digested  on  a  sand-bath 
for  half  an  hour,  and  the  contents  boiled  for  about  fifteen  minutes ;  after  which 
the  whole  of  the  copper  occurring  in  the  ore,  and  all  other  metals,  are  in  solution 
as  chlorides.  The  blackish  residue,  consisting  of  sand  and  schist,  has  been  proved 
by  numerous  experiments  to  be  either  entirely  free  from  copper,  or  to  contain  at 
the  most  only  0*01  to  0'03  per  cent. 

(6)  Separation  of  the  Copper. — The  solution  of  metallic  and  alkaline  earthy 
chlorides,  and  some  free  HC1,  obtained  as  just  described,  is  separated  by  filtration 
from  the  insoluble  residue,  and  the  fluid  run  into  a  covered  beaker  of  about  400  c.c. 
capacity.  In  this  beaker  a  rod  of  metallic  zinc,  weighing  about  50  gm.,  has 
been  previously  placed,  fastened  to  a  piece  of  stout  platinum  foil.  The  zinc  to 
be  used  for  this  purpose  should  be  as  free  as  possible  from  lead,  and  at  any 
rate  should  not  contain  more  than  from  O'l  to  0-3  per  cent,  of  the  latter  metal. 
The  precipitation  of  the  copper  in  the  metallic  state  sets  in  during  the 
filtration  of  the  warm  and  concentrated  fluid,  and  is,  owing  especially  also  to  the 
entire  absence  of  nitric  acid,  completely  finished  in  from  half  to  three-quarters 
of  an  hour  after  the  beginning  of  the  filtration.  If  the  fluid  be  tested  with  SH2 
no  trace  of  copper  can  or  should  be  detected ;  the  spongy  metal  partly  covers 
the  platinum  foil,  partly  floats  about  in  the  liquid,  and  in  case  either  the  ore 
itself  or  the  zinc  applied  in  the  experiment  contained  le.ad,  small  quantities  of 
that  metal  will  accompany  the  precipitated  copper.  After  the  excess  of  zinc  (for 
an  excess  must  always  be  employed)  has  been  removed,  the  metal  is  repeatedly 
and  carefully  washed  by  decantation  with  fresh  water,  and  care  taken  to  collect 
together  every  particle  of  the  spongy  mass. 

(c)  Determination  of  the  precipitated  Copper. — To  the  spongy  metallic  mass 
in  the  beaker,  wherein  the  platinum  foil  is  left,  since  some  of  the  metal  adheres 
to  it,  8  c.c.  of  the  special  nitric  acid  are  added,  and  the  copper  dissolved  by  the  aid 
of  moderate  heat  in  the  form  of  cupric  nitrate,  which,  in  the  event  of  any  small 
quantity  of  lead  being  present,  will  of  course  be  contaminated  with  lead. 

When  copper  ores  are  dealt  with  containing  above  6  per  cent,  of  copper,  which 
may  be  approximately  determined  from  the  bulk  of  the  spongy  mass  of  precipitated 
metal,  16  c.c.  of  nitric  acid,  instead  of  8,  are  applied  for  dissolving  the  metal. 
The  solution  thus  obtained  is  left  to  cool,  and  next  mixed,  immediately  before 
titration  with  cyanide,  with  10  c.c.  of  special  solution  of  liquid  ammonia, 
prepared  by  diluting  1  volume  of  liquid  ammonia  (sp.  gr.  0*93)  with  2  volumes 
of  distilled  water. 

The  titration  with  cyanide  is  conducted  as  described  on  p.  195. 

In  the  case  of  such  ores  as  yield  over  6  per  cent,  of  copper,  and  when  a  double 
quantity  of  nitric  acid  has  consequently  been  used,  the  solution  is  diluted  with 
water,  and  made  to  occupy  a  bulk  of  100  c.c.  ;  this  bulk  is  then  exactly  divided 
into  two  portions  of  50  c.c.  each,  and  each  of  these  separately  mixed  with  10  c.c. 
of  ammonia,  and  the  copper  therein  volu  metrically  determined.  The  deep  blue 
coloured  solution  only  contains,  in  addition  to  the  copper  compound,  ammonium 
nitrate  ;  any  lead  which  might  have  been  dissolved  having  been  precipitated  as 
hydrated  oxide,  which  does  not  interfere  with  the  titration  with  cyanide.  The 
solution  of  the  last-named  salt  is  so  arranged  that  1  c.c.  thereof  indicates  exactly 


COPPER.  203 

O005  gm.  of  copper  (about  21  gm.  of  the  pure  salt  per  litre).  Since,  for  every 
assay,  5  gm.  of  ore  have  been  taken,  1  c.c.  of  the  titration  fluid  is  equal  to  0-1 
per  cent,  of  copper,  it  hence  follows  that,  by  multiplying  the  number  of  c.c.  of 
cyanide  solution  used  to  make  the  blue  colour  of  the  copper  solution  disappear 
by  0*1,  the  percentage  of  copper  contained  in  the  ore  is  immediately  ascertained. 

Steinbeck  tested  this  method  specially,  in  order  to  see  what 
influence  is  exercised  thereupon  by  (1)  ammonium  nitrate,  (2)  caustic 
ammonia,  (3)  lead.  The  copper  used  for  the  experiments  for  this 
purpose  was  pure  metal,  obtained  by  galvanic  action,  and  was  ignited 
to  destroy  any  organic  matter  which  might  accidentally  adhere  to  it, 
and  next  cleaned  by  placing  it  in  dilute  nitric  acid.  5  gm.  of  this 
metal  were  placed  in  a  litre  flask,  and  dissolved  in  266'6  c.c.  of 
special  nitric  acid,  the  flask  gently  heated,  and,  after  cooling,  the 
contents  diluted  with  water,  and  thus  brought  to  a  bulk  of  1000  c.c. 
30  c.c.  of  this  solution  were  always  applied  to  titrate  one  and  the 
same  solution  of  cyanide  under  all  circumstances.  When  5  gm.  of 
ore,  containing  on  an  average  3  per  cent,  of  copper,  are  taken  for 
assay,  that  quantity  of  copper  is  exactly  equal  to  0*150  gm.  of  the 
chemically  pure  copper.  The  quantity  of  nitric  acid  taken  to 
dissolve  5  gm.  of  pure  copper  (266- 6  c.c.)  was  purposely  taken,  so 
as  to  correspond  with  the  quantity  of  8  c.c.  of  special  nitric  acid 
which  is  applied  in  the  assay  of  the  copper  obtained  from  the  ore, 
and  this  quantity  of  acid  is  exactly  met  with  in  30  c.c.  of  the  solution 
of  pure  copper. 

The  influence  of  double  quantities  of  ammonium  nitrate  and  free 
caustic  ammonia  (the  quantity  of  copper  remaining  the  same)  is 
shown  as  follows  : — 

(a)  30  c.c.  of  the  normal  solution  of  copper,  containing  exactly  0*150  gm.  of 
copper,  were  rendered  alkaline  with  10  c.c.  of  special  ammonia,  and  were  found 
to  require,  for  entire  decoloration,  29'8  c.c.  of  cyanide.  A  second  experiment, 
again  with  30  c.c.  of  copper  solution,  and  otherwise  under  identically  the  same 
conditions,  required  29*9  c.c.  of  cyanide.  The  average  is  29'85  c.c. 

(&)  When  to  30  c.c.  of  the  copper  solution  first  8  c.c.  of  special  nitric  acid 
are  added,  and  then  20  c.c.  of  special  ammonia  instead  of  only  8,  whereby  the 
quantity  of  free  ammonia  and  of  ammonium  nitrate  is  double  what  it  was  in  the 
case  of  a,  there  is  required  of  the  same  cyanide  30 '0  c.c.  to  produce  decoloration. 
A  repetition  of  the  experiment,  under  exactly  the  same  conditions,  gave  30*4  c.c. 
of  the  cyanide :  the  average  is,  therefore,  30'35  c.c.  The  difference  amounts  to 
only  0'05  per  cent,  of  copper,  which  may  be  allowed  for  in  the  final  calculation. 

When,  however,  large  quantities  of  ammoniacal  salts  are  present 
in  the  fluid  to  be  assayed  for  copper  by  means  of  cyanide,  and 
especially  when  ammonium  carbonate,  sulphate,  and,  worse  still, 
chloride  are  simultaneously  present,  these  salts  exert  a  very  dis- 
turbing influence.*  The  presence  of  lead  in  the  copper  solution 
to  be  assayed  has  the  effect  of  producing,  on  the  addition  of  10  c.c. 
of  normal  ammonia,  a  milkiness  with  the  blue  tint ;  but  this  does 
not  at  all  interfere  with  the  determination  of  the  copper  by  means  of 
the  cyanide,  provided  the  lead  be  not  in  great  excess  ;  and  a  slight 

*  I  have  retained  this  technical  process  in  its  original  form,  notwithstanding  the  use 
of  ammonia,  because  it  is  systematic,  and  the  results  obtained  by  it  are  all  comparable 
among  themselves.  Of  course  soda  or  potash  may  be  used  in  place  of  ammonia,  if  the 
cyanide  is  standardized  with  them. 


204  COPPER. 

milkiness  of  the  solution  even  promotes  the  visibility  of  the  approach- 
ing end  of  the  operation. 

Steinbeck  made  some  experiments  purposely  to  test  this  point, 
and  his  results  show  that  a  moderate  quantity  of  lead  has  no 
influence. 

Experiments  were  also  carefully  made  to  ascertain  the  influence 
of  zinc,  the  result  of  which  showed  that  up  to  5  per  cent,  of  the 
copper  present,  the  zinc  had  no  disturbing  action  ;  but  a  considerable 
variation  occurred  as  the  percentage  increased  above  that  proportion. 
Care  must,  therefore,  always  be  taken  in  washing  the  spongy  copper 
precipitated  from  the  ore  solution  by  means  of  zinc. 

The  titration  must  always  take  place  at  ordinary  temperatures, 
since  heating  the  ammoniacal  solution  while  under  titration  to  40° 
or  45°  C.  considerably  reduces  the  quantity  of  cyanide  required. 

9.     Determination  of  Copper  Colorimetrically. 

This  method  can  be  adopted  with  very  accurate  results,  as  in  the 
case  of  iron,  and  is  available  for  slags,  poor  cupreous  pyrites,  waters, 
etc.* 

The  reagent  used  is  the  same  as  in  the  case  of  iron,  viz.,  potassium 
ferrocyanide,  which  gives  a  purple-brown  colour  with  very  dilute 
solutions  of  copper.  This  reaction,  however,  is  not  so  delicate  as  it 
is  with  iron,  for  1  part  of  the  latter  in  13,000,000  parts  of  water 
can  be  detected  by  means  of  potassium  ferrocyanide  ;  while  1  part 
of  copper  in  a  neutral  solution,  containing  ammonium  nitrate,  can 
only  be  detected  in  2,500,000  parts  of  water.  Of  the  coloured 
reactions  which  copper  gives  with  different  reagents,  those  with 
sulphuretted  hydrogen  and  potassium  ferrocyanide  are  by  far  the 
most  delicate,  both  showing  their  respective  colours  in  2,500,000 
parts  of  water. 

Of  the  two  reagents  sulphuretted  hydrogen  is  the  more  delicate  ; 
but  potassium  ferrocyanide  has  a  decided  advantage  over  sulphur- 
etted hydrogen  in  the  fact  that  lead,  when  not  present  in  too  large 
quantity,  does  not  interfere  with  the  depth  of  colour  obtained, 
whereas  to  sulphuretted  hydrogen  it  is,  as  is  well  known,  very 
sensitive.f 

And  though  iron  if  present  would,  without  special  precaution 
being  taken,  prevent  the  determination  of  copper  by  means  of 
ferrocyanide  ;  yet,  by  the  method  described  below,  the  amounts  of 
these  metals  contained  together  in  a  solution  can  be  determined 
by  this  reagent. 

Ammonium  nitrate  renders  the  reaction  much  more  delicate  ; 
other  salts,  as  ammonium  chloride  and  potassium  nitrate,  have 
the  same  effect. 

The  method  of  analysis  consists  in  the  comparison  of  the  purple- 

*  Carnelly,  C.  N.  32,  308. 

t  In  colour  titrations  of  this  character  it  is  essential  that  the  comparisons  be  made 
under  the  same  circumstances  as  to  temperature,  dilution,  and  admixture  of  foreign 
substances,  otherwise  serious  errors  will  arise. 


COPPEB,  205 

brown  colours  produced  by  adding  to  a  solution   of  potassium 
ferrocyanide — first,  a  solution  of  copper  of  known  strength  ;  and, 
second,  the  solution  in  which  the  copper  is  to  be  determined. 
The  solutions  and  materials  required  are  as  follows  : — 

(1)  Standard  copper  solution. — Prepared  by  dissolving  0'393  gm. 
of  pure  CuSO4,  5H2O  in  one  litre  of  water.     1  c.c.  =0*1  mgm.  Cu. 

(2)  Solution  of  ammonium  nitrate. — Made  by  dissolving  100  gm. 
of  the  salt  in  one  litre  of  water. 

(3)  Potassium  ferrocyanide  solution — 1   :  25. 

(4)  Two  glass  cylinders  holding  rather  more  than  150  c.c.  each, 
the  point  equivalent  to  that  volume  being  marked  on  the  glass. 
They  must  both  be  of  the  same  tint,  and  as  colourless  as  possible. 

A  burette,  graduated  to  ^  c.c.  for  the  copper  solution  ;  a  5  c.c. 
pipette  for  the  ammonium  nitrate  ;  and  a  small  tube  to  deliver  the 
ferrocyanide  in  drops. 

METHOD  OF  PROCEDURE  :  Five  drops  of  the  potassium  ferrocyanide  are  placed 
in  each  cylinder,  and  then  a  measured  quantity  of  the  neutral  solution  in 
which  the  copper  is  to  be  determined  is  added  to  one  of  them,  and  both  filled 
up  to  the  mark  with  distilled  water,  5  c.c.  of  the  ammonium  nitrate  solution 
added  to  each,  and  then  the  standard  copper  solution  run  gradually  into  the 
other  till  the  colours  in  both  cylinders  are  of  the  same  depth,  the  liquid  being 
well  stirred  after  each  addition.  The  number  of  c.c.  used  is  then  read  off. 
Each  c.c.  corresponds  to  0*1  mgm.  of  copper,  from  which  the  amount  of  copper 
in  the  solution  in  question  can  be  calculated. 

The  solution  in  which  the  copper  is  to  be  determined  must  be 
neutral ;  for  if  it  contain  free  acid  the  latter  lessens  the  depth  of 
colour,  and  changes  it  from  a  purple-brown  to  an  earthy-brown. 
If  it  should  be  acid,  it  is  rendered  slightly  alkaline  with  ammonia, 
and  the  excess  of  the  latter  got  rid  of  by  boiling.  The  solution  must 
not  be  alkaline,  as  the  brown  coloration  is  soluble  in  ammonia  and 
decomposed  by  potash  or  soda  ;  if  it  be  alkaline  from  ammonia, 
this  is  remedied  as  before  by  boiling  it  off  ;  while  free  potash  or  soda, 
should  they  be  present,  are  neutralized  by  an  acid,  and  the  latter 
by  ammonia. 

Lead,  when  present  in  not  too  large  quantity,  has  little  or  no  effect 
on  the  accuracy  of  the  method.  The  precipitate  obtained  on  adding 
potassium  ferrocyanide  to  a  lead  salt  is  white ;  and  this,  except 
when  present  in  comparatively  large  quantity  with  respect  to  the 
copper,  does  not  interfere  with  the  comparison  of  the  colours. 

When  copper  is  to  be  determined  in  a  solution  containing  iron,  the 
following  method  is  adopted  : — 

A  few  drops  of  nitric  acid  are  added  to  the  solution  in  order  to  oxidize  the  iron, 
the  liquid  evaporated  to  a  small  bulk,  and  the  iron  precipitated  by  ammonia. 
Even  when  very  small  quantities  of  iron  are  present,  this  can  be  done  easily  and 
completely  if  there  be  only  a  very  small  quantity  of  fluid.  The  precipitate  of 
ferric  oxide  is  then  filtered  off,  washed  once,  dissolved  in  nitric  acid,  and  re- 
precipitated  by  ammonia,  filtered  and  washed.  The  iron  precipitate  is  now  free 
from  copper,  and  in  it  the  iron  can  be  determined  by  dissolving  in  nitric  acid, 
making  the  solution  nearly  neutral  with  ammonia,  and  determining  the  iron  by 
the  method'on  p.  238.  The  filtrate  from  the  iron  precipitate  is  boiled  till  the 
ammonia  is 'completely  driven  off,  and  the  copper  determined  in  the  solution  so 
obtained  as  already  described. 


206  COPPER. 

When  the  solution  containing  copper  is  too  dilute  to  give  any 
coloration  directly  with  ferrocyanide,  a  measured  quantity  of  it 
must  be  evaporated  to  a  small  bulk,  and  filtered  if  necessary  ;  and 
if  it  contain  iron,  also  treated  as  already  described. 

In  the  determination  of  copper  and  iron  in  water,  for  which  the 
method  is  specially  applicable,  a  measured  quantity  is  evaporated 
to  dryness  with  a  few  drops  of  nitric  acid,  ignited  to  get  rid  of  any 
organic  matter  that  might  colour  the  liquid,  dissolved  in  a  little 
boiling  water  and  a  drop  or  two  of  nitric  acid  ;  if  it  is  not  all  soluble 
it  does  not  matter.  Ammonia  is  next  added  to  precipitate  the  iron, 
the  latter  filtered  off,  washed,  re-dissolved  in  nitric  acid,  and  again 
precipitated  by  ammonia,  filtered  off,  and  washed.  The  filtrate  is 
added  to  the  one  previously  obtained,  the  iron  determined  in  the 
precipitate,  and  the  copper  in  the  united  filtrates. 

There  is  in  use  at  several  copper  works  what  is  known  as  Heine s 
"  blue  test,"  that  is  an  ammoniacal  solution  of  copper,  but  the 
difficulty  has  been  to  keep  strictly  correct  standards  for  comparison 
except  they  are  freshly  made.  G.  L.  Heath*  has  solved  this 
difficulty  by  making  the  standard  from  copper  sulphate  instead  of 
nitrate. 

METHOD  OF  PROCEDURE  :  About  0-3  gm.  of  pure  copper  is  dissolved  in  5  c.c. 
each  of  nitric  acid  (sp.  gr.  1'4)  and  sulphuric  acid  (sp.  gr.  1'84).  Evaporate 
carefully  till  fumes  of  the  latter  acid  are  given  off.  When  cold  dissolve  in  25  c.c. 
of  water  and  add  ammonia  in  sufficient  excess  to  give  a  clear  solution.  This  is 
then  diluted  with  weak  ammonia,  about  1  :  6,  and  graduated  so  that  each  c.c. 
shall  represent  0*0025  gm.  Cu.  Standards  can  then  be  made  up  so  that  200  c.c. 
diluted  with  the  weak  ammonia  shall  contain  from  0*1  to  T3  gm.  of  Cu.  The 
standards  are  kept  in  tall  well- stoppered  cylinders  of  white  glass  marked  at 
200  c.c.,  and  when  kept  cool  and  protected  from  sunlight  they  last  a  long  time. 

The  method  is  generally  in  use  for  lean  blast  furnace  slags,  such  as  contain 
a  good  deal  of  iron,  and  alumina,  and  lime.  The  method  for  these  samples  is  as 
follows  :—  2'5  gm.  of  the  finely  ground  material  are  heated  in  a  porcelain  dish  with 
15  c.c.  of  nitric  acid,  and  after  adding  5  c.c.  of  sulphuric  acid  the  evaporation  is 
continued  until  the  mass  has  become  a  thick,  but  rather  soft,  paste  ;  it  is  then 
treated  with  70  c.c.  of  water  to  dissolve  the  copper  sulphate,  and  30  c.c.  of 
ammonia  are  added.  The  liquid  is  filtered,  and  the  residue  after  being  twice 
washed  with  10  c.c.  of  dilute  ammonia  (1  :  10),  is  rinsed  back  into  the  dish,  using 
50  c.c.  of  water,  taking  care  not  to  damage  the  filter,  and  enough  sulphuric 
acid  is  added  to  re-dissolve  the  iron  and  alumina ;  25  c.c.  of  ammonia  are  again 
added,  and  the  filtrate  and  ammoniacal  washings  are  mixed  with  the  main  filtrate, 
which  is  then  transferred  to  one  of  the  tall  cylinders  of  thin,  colourless  glass,  and 
made  up  with  dilute  ammonia  to  200  c.c.  The  colour  of  the  liquid  is  compared 
with  those  of  a  series  of  copper  solutions  of  known  strength  contained  in  similar 
cylinders.  The  colour  is  best  seen  by  placing  the  sample  and  standard  cylinder 
in  front  of  a  window,  and  with  a  piece  of  white  paper  behind  them. 


10.     Determination  by  Titration  of  Copper  Ferrocyanide  with 
Potassium  Cyanide. 

Sanchezf  has  recently  devised  the  following  process,   which 
is  applicable  in  presence  of  tin,   arsenic,  antimony  and  organic 

*  J.  Am.  C.  S.  19,  21.  f  Bull  Soc.  Chim.,  1910,  7,  9. 


CYANOGEN.  207 

acids,  but  iron,  lead,  zinc,  nickel,  cobalt,  manganese,  and  ammonium 
salts  must  be  absent.  The  method  is  based  on  the  fact  that  when 
copper  ferrocyanide  is  dissolved  in  a  solution  of  potassium  cyanide 
a  very  sharp  change  of  colour  accompanies  the  reaction,  which  is 
as  follows  : — 

Cu2FeCy6  -f  6KCy  =  K4FeCy6  +  2(CuOy2,  KCy ) . 
The  solution  must  be  exactly  neutral. 

METHOD  OF  PROCEDURE  :  The  nitric  acid  solution  of  copper  is  first  evaporated 
to  dryness  with  sulphuric  acid,  any  lead  sulphate  filtered  off  after  adding  cold 
water  to  the  residue,  and  the  copper  is  precipitated  as  Cu2S  by  acidifying  with 
H2SO4  and  boiling  with  10-20  c.c.  of  a  50  %  solution  of  sodium  thiosulphate. 
The  precipitate  is  washed  with  boiling  water,  dissolved  in  nitric  acid,  the  excess 
of  acid  boiled  off,  and  the  solution  made  up  to  a  known  volume.  An  aliquot 
portion  of  this  is  titrated  with  standard  alkali,  using  methyl  orange  as  indicator 
(or  phenolphthalein  when  organic  acids  are  present),  and  the  amount  of  alkali 
found  is  added  to  another  such  portion,  potassium  ferrocyanide  added,  and  the 
titration  with  potassium  cyanide  then  proceeded  with.  The  standard  cyanide 
solution  (6 '5  gm.  KCy  per  litre)  is  standardized  against  a  solution  containing 
1  gm.  of  copper  per  litre.  To  10  c.c.  of  the  latter  solution  1  c.c.  of  a  10  %  solution 
of  potassium  ferrocyanide  is  added,  and  the  cyanide  solution  is  carefully  run-in 
from  a  burette  until  the  reddish-brown  colour  suddenly  changes  to  greenish- 
yellow. 


CYANOGEN. 

CN  =  26-01.- 

1  c.c.  N/10  silver  solution=0'005202  gm.  Cyanogen., 
,/  ,,  =0'005404  gm.  Hydrocyanic  acid. 

;,,  „  =0'013022  gm.  Potassium  cyanide. 

'-.':*-*'     N/io  iodine  solution = 0'003255  gm.  Potassium . cyanide. 


1.     By  Standard  Silver  Solution  (L  i  e  b  i  g). 

THIS  ready  and  accurate  method  of  determining  cyanogen  in 
hydrocyanic  acid,  alkali  cyanides,  etc.,  was  discovered  by  Liebig, 
and  is  fully  described  in  Anti.  der  Chem.  und  Pharm.  Ixxvii.  102. 
It  is  based  on  the  fact  that  when  a  solution  of  silver  nitrate  is 
added  to  an  alkali  solution  containing  cyanogen,  with  constant 
stirring,  no  permanent  precipitate  of  silver  cyanide  appears  until  all 
the  cyanogen  has  combined  with  the  alkali  and  the  silver  to  form 
a  soluble  double  salt  (in  the  presence  of  potash,  for  example,  KCy, 
AgCy).  If  the  slightest  excess  of  silver,  over  and  above  the  quantity 
required  to  form  this  combination,  be  added,  a  permanent  precipitate 
of  silver  cyanide  is  produced,  the  double  compound  being  destroyed. 
If,  therefore,  the  silver  solution  be  of  known  strength,  the  quantity 
of  cyanogen  present  is  easily  found  ;  1  eq.  of  silver  in  this  case  being 
equal Jto  2  eq.  cyanogen. 

So  fast  is  this  double  combination  that,  when  sodium  chloride 
is  present,  no  permanent  precipitate  of  silver  chloride  is  produced 


208  '  CYANOGEN. 

until  the  quantity  of  silver  necessary  to  form  the  compound  is 
slightly  overstepped. 

Siebold,  however,  has  pointed  out  that  this  process,  in  the  case 
of  free  hydrocyanic  acid,  is  liable  to  serious  errors  unless  the  following 
precautions  are  observed  : — 

(a)  The  solution  of  sodium  or  potassium  hydrate  should  be  placed  in  the 
beaker  first,  and  the  hydrocyanic  acid  added  to  it  from  a  burette  dipping  into  the 
alkali.  If,  instead  of  this,  the  acid  is  placed  in  the  beaker  first,  and  the  alkali 
hydrate  added  afterwards,  there  may  be  a  slight  loss  by  evaporation,  which 
becomes  appreciable  whenever  there  is  any  delay  in  the  addition  of  the  alkali. 

(6)  The  mixture  of  hydrocyanic  acid  and  alkali  should  be  largely  diluted  with 
water  before  the  silver  nitrate  is  added.  The  most  suitable  proportion  of  water  is 
from  ten  to  twenty  times  the  volume  of  the  officinal  or  of  Scheele's  acid.  With 
such  a  degree  of  dilution,  the  final  point  of  the  reaction  can  be  observed  with 
greater  precision. 

(c)  The  amount  of  alkali  used  should  be  as  exactly  as  possible  that  required 
for  the  conversion  of  the  hydrocyanic  acid  into  alkali  cyanide,  as  either  an 
insufficiency  or  an  excess  affects  the  accuracy  of  the  result.  It  is  advisable  to 
make  first  a  rough  estimation  with  excess  of  soda  as  a  guide,  then  finish  with 
a  solution  as  neutral  as  possible.  As  a  guide  to  the  neutrality,  or  rather  the  slight 
amount  of  alkalinity  of  the  solution,  a  little  indicator  C4B*  may  be  used,  which 
gives  a  red  colour  with  alkali  hydroxides,  but  is  not  acted  upon  by  HCy  or 
alkali  cyanides. 

The  volume  taken  for  titration  should  not  contain  more  than  0*1 
gm.  HCN.  For  this  quantity  use  5  c.c.  N/1  alkali  and  0'5  gm. 
sodium  bicarbonate.  Titrate  over  black  paper.  This  method  is 
useless  for  the  determination  of  cyanogen  in  double  cyanides  and 
in  mercuric  cyanide. 

For  the  determination  of  cyanide  in  presence  of  chloride,  proceed 
as  follows  : — 

Determine  the  cyanide  by  L  i  e  b  i  g '  s  method,  add  more  than  enough  standard 
silver  nitrate  solution  to  combine  with  the  chloride,  acidify  with  nitric  acid, 
make  up  to  a  definite  volume,  filter  through  a  dry  filter,  and  titrate  the  excess  of 
silver  in  an  aliquot  portion  of  the  filtrate  by  Volhard's  method. 

Caution. — In  using  the  pipette  for  measuring  hydrocyanic  acid, 
it  is  advisable  to  insert  a  plug  of  cotton  wool,  slightly  moistened 
with  silver  nitrate,  into  the  upper  end,  so  as  to  avoid  the  danger  of 
inhaling  any  of  the  highly  poisonous  acid  ;  otherwise  it  is  decidedly 
preferable  to  weigh  it. 


2.     By  Standard  Mercuric  Chloride  (H  a  n  n  a  y). 

This  convenient  method  is  fully  described  by  the  author,!  and 
is  well  adapted  for  the  technical  examination  of  commercial  cyanides, 
etc.,  giving  good  results  in  the  presence  of  cyanates,  sulphocyanates, 
alkali  salts,  and  compounds  of  ammonia  and  silver. 

The  standard  solution  of  mercury  is  made  by  dissolving  13*537  gm. 

•Seep.  46.  1J.  C.  S.  1878,  245. 


CYANOGEN.  209 

HgCl2  in  water,  and  diluting  to  a  litre.  Each  c.c.  =0*00651  gm.  of 
potassium  cyanide  or  0*002611  gm.  Cy. 

METHOD  OF  PROCEDURE  :  The  cyanide  is  dissolved  in  water,  and  the  beaker 
placed  upon  black  paper  or  velvet ;  ammonia  is  then  added  in  moderate  quantity, 
and  the  mercuric  solution  cautiously  added  with  constant  stirring  until  a  bluish- 
white  opalescence  is  permanently  produced.  With  pure  substances  the  reaction 
is  very  delicate,  but  not  so  accurate  with  impure  mixtures  occurring  in 
commerce. 

3.     By  Iodine  (F  o  r  d  o  s  and  G  e  1  i  s). 

This  process,  which  is  principally  applicable  to  alkaline  cyanides, 
depends  on  the  fact  that  when  a  solution  of  iodine  is  added  to  one 
of  potassium  cyanide  the  colour  of  the  iodine  solution  is  discharged 
so  long  as  any  undecomposed  cyanide  remains.  The  reaction  may 
be  expressed  by  the  following  equation  : — 

KCN+I2=KI+ICN. 

Therefore,  2  eq.  iodine  represent  1  eq.  cyanogen  in  combination  ;  so 
that  1  c.c.  of  N/10  iodine  expresses  the  half  of  y^-.J^  eq.  cyanogen 
or  its  compounds.  The  end  of  the  reaction  is  known  by  the  yellow 
colour  of  the  iodine  solution  becoming  permanent.  Starch  indicator 
must  not  be  used. 

Commercial  cyanides  are,  however,  generally  contaminated  with 
caustic  or  monocarbonate  alkalies,  which  would  destroy  the  colour 
of  the  iodine  like  the  cyanide  ;  consequently  these  must  be  converted 
into  bicarbonates,  which  is  best  done  by  adding  carbonic  acid  water 
(ordinary  soda  water). 

EXAMPLE  :  5  gm.  of  potassium  cyanide  were  weighed  and  dissolved  in  500  c.c. 
water ;  then  5  c.c.  (  =0'05  gm.  cyanide)  taken  with  a  pipette,  diluted  with  about 
£  litre  of  water,  100  c.c.  of  soda  water  added,  then  N/io  iodine  delivered  from  the 
burette  until  the  solution  possessed  a  slight  but  permanent  yellow  colour : 
12'75  c.c.  were  required,  which  multiplied  by  0'003255  gave  0'0415  gm.,  or  83  per 
cent,  real  cyanide.  Sulphides  must  of  course  be  absent. 


4.     By  N/10  Silver  and  Chromate  Indicator. 

Vielhaber*  has  shown  that  weak  solutions  of  prussic  acid,  such 
as  bitter-almond  water,  etc.,  may  be  readily  titrated  by  adding 
magnesium  hydrate  suspended  in  water  until  alkaline,  adding  a  drop 
or  two  of  chromate  indicator,  and^delivering  in  N/10  silver  until 
the  red  colour  appears,  as  in  the  case  of  titrating  chlorides.  1  c.c. 
silver  solution -0*002702  gm.  HCy. 

This  method  may  be  found  serviceable  in  the  examination  of 
opaque  solutions  of  hydrocyanic  acid,  such  as  solutions  of  bitter- 
almond  oil,  etc.  ;  but  of  course  the  absence  of  chlorine  must  be 
ensured,  or,  if  present,  the  amount  must  be  allowed  for. 

It  is  preferable  to  add  the  HCy  solution  to  a  mixture  of  magnesia 
and  chromate,  then  immediately  to  titrate  with  silver. 

*  Arch.  Pharm.  [3]  13,  408. 


210  CYANIDES. 

5.     Cyanides  used  in  Gold  Extraction. 

An  interesting  series  of  papers  on  this  subject  have  been  con- 
tributed by  Clennell*  and  Bettel.f  The  experiments  carried 
out  by  these  chemists  are  far  too  voluminous  to  be  reproduced  here, 
but  a  short  summary  of  the  results  may  be  acceptable  for  the 
technical  examination  of  the  original  solutions  and  their  nature  after 
partial  decomposition  and  admixture  with  zinc  and  other  impurities 
which  naturally  occur  in  the  processes  of  gold  extraction.  The 
results  obtained  by  both  chemists  point  to  the  fact  that  the  deter- 
mination of  cyanide  in  the  weak  solutions  used  in  the  MacArthur- 
Forrest  process  is  much  hampered  by  zinc  double  cyanide,  by 
thiocyanates,  also  by  ferro-  and  ferricyanides,  together  with  organic 
matters  which  occur  in  the  liquors  after  leaching  the  ores. 
According  to  Clennell  the  presence  of  ferrocyanides  gives  too 
high  a  result  when  the  silver  process  of  Liebig  is  used,  but  is  not 
of  much  consequence  unless  the  cyanide  is  relatively  small  as 
compared  with  the  ferrocyanide  ;  with  the  iodine  process  the  inter- 
ference of  ferrocyanide  is  much  less,  and  very  fair  technical  results 
may  be  obtained  in  the  presence  of  both  ferro-  and  ferri-  salts  by  this 
process.  The  silver  process  appears  to  be  fairly  serviceable  where 
the  quantity  of  ferrocyanide  is  not  too  large  ;  the  reddish  precipitate 
which  forms  at  first  from  the  ferri-  salt  is  soluble  in  the  presence  of 
excess  of  cyanide,  and  a  definite  end-reaction  can  be  obtained. 
Thiocyanates  render  the  silver  process  useless,  but  do  not  interfere 
with  the  iodine  process.  Ammonium  carbonate  interferes  with  the 
silver  process  unless  potassium  iodide  is  added  so  as  to  produce 
silver  iodide,  which  is  insoluble  in  the  ammonia  salt.  Ferrocyanides, 
in  the  absence  of  other  reducing  agents,  may  be  accurately  deter- 
mined, as  on  p.  217  ;  the  presence  of  cyanides  and  ferricyanides  does 
not  seriously  interfere.  Ferricyanides  may  be  determined  as  on 
p.  220 ;  ferrocyanides  do  not  seriously  interfere,  but  cyanides  render 
the  results  somewhat  low.  These  remarks  apply  to  solutions  not 
complicated  by  admixture  of  zinc  or  other  matters  which  naturally 
occur  in  the  cyanide  liquors  after  they  have  been  in  contact  with 
the  ore.  For  the  actual  methods  which  have  been  found  useful  in 
examining  the  usual  cyanide  liquors  the  following  processes,  devised 
by  Bet  t  el,  are  given  not  as  being  absolutely  correct,  but  sufficiently 
so  for  technical  purposes,  and  occupying  little  time  in  the  working  : — 

It  is  necessary  to  state  at  the  outset  that  the  following  remarks  have  reference 
to  the  MacArthur-Forrest  working  solutions  containing  zinc,  an  element 
which  complicates  the  analysis  in  a  truly  surprising  manner.  Before  dealing 
with  the  analysis  proper,  attention  is  drawn  to  the  peculiarities  of  a  solution  of 
the  double  cyanide  of  zinc  and  potassium,  usually  written  K2ZnCy4.  As  is  stated 
in  works  on  chemistry,  this  cyanide  is  alkaline  to  indicators.  Now  here  lies  the 
peculiarity.  To  phenolphthalein  the  alkalinity,  as  tested  by  N/io  acid,  is  equal 
to  19*5  parts  of  cyanide  of  potassium  out  of  a  possible  130*2  parts.  With 
methyl  orange  as  indicator,  the  whole  of  the  metallic  cyanide  may  be  decomposed 
by  N/io  acid,  as  under : — 

K2ZnCy4  +4HC1  =ZnCla  +2KC1  +4HCy. 
*  C.  N.  77,  227.  t  Ibid  286,  298. 


CYANIDE    SOLUTIONS.  21] 

On  titration  with  silver  nitrate  solution  the  end-reaction  is  painfully  indefinite. 
If  caustic  alkali  in  excess  (a  few  c.c.  normal  soda)  be  added  to  a  known  quantity 
of  potassium  zinc  cyanide  solution  together  with  a  few  drops  of  potassium  iodide, 
and  standard  silver  solution  added  to  opalescence,  the  reaction  will  indicate 
sharply  the  total  cyanogen  present  in  the  double  cyanide  even  in  the  presence  of 
ferrocyanides.  If  to  a  solution  of  potassium  zinc  cyanide  be  added  a  small 
quantity  of  ferrocyanide  of  potassium,  and  the  silver  solution  added,  the 
flocculent  precipitate  of  what  is  supposed  to  be  normal  zinc  ferrocyanide 
(Zn2FeCy6)  appears,  the  end-reaction  is  fairly  sharp,  and  indicates  19*5  parts  of 
potassium  cyanide  out  of  the  actual  molecular  contents  of  130'2  KCy.  If,  how- 
ever, an  excess  of  ferrocyanide  be  present,  the  flocculent  precipitate  does  not 
appear,  but  in  its  place  one  gets  an  opalescence  which  speedily  turns  to  a 
finely  granular  (sometimes  slimy)  precipitate  of  potassium  zinc  ferrocyanide 
K2Zn3Fe2Cy12.  This  introduces  a  personal  equation  into  the  analysis  of  such 
a  solution,  for  if  the  silver  solution  be  added  rapidly  the  results  are  higher  than 
if  added  drop  by  drop,  as  this  ferrocyanide  of  zinc  and  potassium  separates  out 
slowly  in  dilute  solutions  that  are  alkaline  or  neutral  to  litmus  paper. 

For  the  determination  of  free  hydrocyanic  acid  use  is  made  of  Siebold's 
ingenious  method  for  determining  alkalies  in  carbonates  and  bicarbonates,  by 
reversing  the  process,  adding  bicarbonate  of  soda,  free  from  carbonate,  to  the 
solution  to  be  titrated  for  hydrocyanic  acid  and  free  cyanide.  This  is  the  one 
instance  where  hydrocyanic  acid  replaces  carbonic  acid  in  its  combinations,  and 
as  such  is  interesting. 

2KHC03  +  AgN03  +2HCy  =KAgCy2  +KN03  +2C02  +2H20. 

The  methods  of  analysis  are  as  follows  :  — 

1.  Free  Cyanide.  —  50  c.c.  of  solution  are  taken  and  titrated  with  silver  nitrate 
to  faint  opalescence  or  first  indication  of  a  flocculent  precipitate.     This  will 
indicate  (if  sufficient  ferrocyanide  be  present  to  form  a  flocculent  precipitate 
of  zinc  ferrocyanide)  the  free  cyanide,  and   cyanide   equal   to  7*9  per  cent,  of 
the  potassium  zinc  cyanide  present. 

W.  J.  Sharwood,*  after  criticising  the  various  processes  in  use,  recommends 
the  following  scheme.  To  the  solution  containing  the  cyanogen,  5  c.c.  of  ammonia 
and  2  c.c.  of  a  5  per  cent,  solution  of  potassium  iodide  are  added,  and  then 
standard  solution  of  silver  nitrate  until  a  faint,  permanent  cloudiness  is  produced. 
If  the  solution  contains  sulphides  in  small  amount,  5-10  c.c.  of  a  solution  made 
by  dissolving  0*5  gm.  of  iodine  and  2  gm.  of  potassium  iodide  in  100  c.c.  of  water 
is  used  in  place  of  the  potassium  iodide,  but  a  special  check  should  be  made  in 
such  case.  If  the  amount  of  sulphide  is  large,  it  must  be  removed  by  means  of 
a  solution  of  -sodium  plumbite  ;  an  aliquot  part  of  the  filtrate  is  then  titrated. 
]  '  •  If  zinc  is  present,  a  large  excess  of  alkali  should  be  added  ;  in  this  case,  the 
cyanogen  found  represents,  not  only  the  potassium  cyanide,  but  also  the  double 
zinc  compound.  By  determining  the  zinc,  the  amount  of  free  potassium  cyanide 
may  be  readily  calculated,  as  *1  part  of  zinc  corresponds  to  4  parts  of  potassium 
cyanide.  A  similar  allowance  must  be  made  if  small  quantities  of  copper  are 
present.  If  calcium,  magnesium,  or  manganese  is  present,  ammonium  chloride 
must  be  added,  whilst  soda  is  used  in  presence  of  aluminium  or  lead. 

For  technical  purposes,  it  is  best  to  prepare  a  silver  nitrate  solution  containing 
1-305  gm.  of  this  salt  per  100  c.c.  ;  taking  samples  of  10  c.c.  each,"l  c.c.  of  the 
silver  represents  O'l  per  cent,  of  potassium  cyanide,  j  *jy  i^iitfij  ; 

2.  Hydrocyanic  Acid.  —  To   50   c.c.  of   the   solution  add  a  solution  of  alkali 
bicarbonate,  free  from  carbonate  or  excess  of  carbonic  acid.     Titrate  as  for  free 
cyanide.     Deduct  the  first  from  the  second  result,   =HCy. 


1  c.c.  AgN03  =  =o-00829  %  HCy. 

3.     Double   Cyanides.  —  Add    excess    of   normal    caustic    soda    to    60    c.c.    of 
*  J.  Am.  C.  S.,  1897,  400-434. 

p  2 


212  CYANIDE    SOLUTIONS. 

solution  and  a  few  drops  of  a  10  per  cent,  solution  of  KI,  titrate  to  opalescence 
with  AgNO3.  This  gives  1,  2,  and  3.  Deduct  1  and  2  =  K2ZnCy4  as  KCy  less 
7'9  per  cent. 

A  correction  is  here  introduced.  The  KCy  found  in  3  is  calculated  to  K2ZnCy4. 
Factor:  KCy  (as  K2ZnCy4)  x  0-9423  =K2ZnCy4.  Add  to  this  7'9  per  cent,  of 
total,  or  for  every  92-1  parts  of  K2ZnCy4  add  7 '9  parts.  If  this  fraction,  calculated 
back  to  KCy,  be  deducted  from  1,  the  true  free  cyanide  (calculated  to  KCy)  is 
obtained. 

4.  Ferrocyanides  and  Thiocyanates. — In  absence  of  organic  matters  it  is  found 
that  an  acidified  solution  of  a  simple  cyanide,  such  as  KCy,  or  a  double  cyanide 
(as  K2ZuCy4),  i.e.,  solution  of  HCy,  is  not  affected  by  dilute  permanganate.  On 
the  other  hand,  acidified  solutions  of  ferrocyanides  and  sulphocyanides  are  rapidly 
oxidized — the  one  to  ferrocyanide,  the  other  to  H2S04  +  HCy. 

If,  now,  the  ferrocyanogen  be  removed  as  Prussian  blue,  by  ferric  chloride  in 
an  acid  solution,  the  filtrate  will  contain  ferric  and  hydric  thiocyanate,  both  of 
which  are  oxidized  by  permanganate  as  if  iron  were  not  present ;  by  deducting 
the  smaller  from  the  larger  result,  we  get  the  permanganate  consumed  in  oxidizing 
ferrocyanide,  the  remainder  equals  the  permanganate  consumed  in  oxidizing 
thiocyanate. 

The  method  of  titration  is  as  follows  (in  presence  of  zinc) : — A  burette  is  filled 
with  the  cyanide  solution  for  analysis,  and  run  into  10  or  20  c.c.  N/ioo  K2Mn2O8 
strongly  acidified  with  H2S04  until  colour  is  just  discharged.  Result  noted  (a) 

A  solution  of  ferric  sulphate  or  chloride  is  acidified  with  H2S04  and  50  c.c. 
of  the  cyanide  solution  poured  in.  After  shaking  for  about  half  a  minute,  the 
Prussian  blue  is  separated  from  the  liquid  by  filtration,  and  the  precipitate  and 
filter-paper  washed.  The  filtrate  is  next  titrated  with  N/ioo  K2Mn208  (&). 

Let  (c)  =c.c.  permanganate  required  to  oxidize  ferrocyanide. 

Then  a  -b  =c. 


(d) 


n  a  —o  =c. 

1  c.c.  N/IOO  K2Mn208  =0-003684  gm.  K4FeCy6. 
1  c.c.  N/100  K2Mn208  =0-0001618  gm.  KCNS. 


5.  Oxidizable  Organic  Matter  in  Solution.—  In  treating  spruit  tailings,   or 
material  containing  decaying  vegetable  matter,  the  following  method  is  used  for 
testing  coloured  solution. 

(a)     Prepare  a  solution  of  a  thiocyanate  so  that  1  c.c.  =  N/ioo  K2Mn208. 

(&)  To  50  c.c.  solution  add  sulphuric  acid  in  excess,  and  then  a  large  excess 
°f  N/ioo  permanganate.  Keep  at  60-70°  C.  for  an  hour.  Then  cool  and  titrate 
back  with  the  KCNS  solution. 

Result  Oxygen  consumed  in  oxidizing  organic  matter. 

K4FeCy6. 
KCNS. 

After  determining  KCNS  and  K4FeCy6,  a  simple  calculation  gives  the  oxygen 
to  oxidize  organic  matter.  This  result  multiplied  by  9  will  give  approximately 
the  amount  of  organic  matter  present. 

In  order  to  clarify  such  organically  charged  solutions,  they  are  shaken  up  with 
powdered  quicklime  and  filtered  ;  the  solution  is  then  of  a  faint  straw  colour, 
and  is  in  a  proper  condition  for  analysis.  In  such  clarified  solution  the  oxidizable 
organic  matter  is  no  longer  present,  and  the  determinations  are  readily 
performed. 

6.  Alkalinity.  —  Potassium   cyanide   acts   as   caustic   alkali    when   neutralized 
by  an  acid  ;  the  end-reaction,  however,  is  influenced  to  some  extent  by  the 
hydrocyanic  acid  present,  and  is  therefore  not  sharp.     It  is  possible,  however, 
to  determine  by  N/io  acid  :  — 

With    116™111^^11  as  indicator. 


100  I  rf  z       K         n%'0,  }  ™*  -thy,  orange  as  indicate, 
The  K20  in  ZnK202  .  .     With  phenolphthalein  as  indicator. 


CYANIDE    SOLUTIONS.  213 

It  will  be  necessary  to  point  out  the  decompositions  which  result  from  adding 
alkali,  or  a  carbonate  of  an  alkali,  to  a  working  solution  containing  zinc. 

K2ZnCy4  +4KHO  =ZnK2O2  +4KCy. 
K2ZnCy4  +4Na2CO3  +2H20  =2KCy  +2NaCy  +ZnNa2O2  +4NaHC08. 

Bicarbonates  have  no  action  upon  potassium  or  sodium  zinc  cyanide. 
Potassium  or  sodium  zinc  oxide  (in  solution  as  hydrate)  acts  as  an  alkali  towards 
phenolphthalein  and  methyl  orange. 

ZnK2O2  +4HC1  =2KC1  +ZnCl2  +  2H20. 

Calcium  and  magnesium  hydrates  decompose  the  double  salt  of  K2ZnCy4  to 
some  extent,  but  not  completely,  so  that  it  is  possible  to  find  in  one  and  the  same 
solution  a  considerable  proportion  of  alkalinity  towards  phenolphthalein,  duetto 
calcium  hydrate  in  presence  of  K2ZnCy4.  fc  ...  4 

The  total  alkalinity  as  determined  by  N/io  acid  with  methyl  orange  as 
indicator  gives,  in  addition  to  those  before  mentioned,  the  bicarbonates.  If  to 
a  solution  containing  sodium  bicarbonate  and  potassium  zinc  cyanide  be  added 
lime,  or  lime  and  magnesia,  the  percentage  of  cyanide  will  increase,  the  zinc 
remaining  in  solution  as  zinc  sodium  oxide. 

Clennell*  gives  a  method  for  the  approximate  determination  of  alkali  hydrates 
and  carbonates  in  the  presence  of  alkali  cyanides,  as  follows : — 

(1)  Determination  of  the  cyanide  by  direct  titration  with  silver. 

(2)  Determination  of  the  hydrate  and  half  the  carbonate  of  alkali  on  adding 
phenolphthalein  to  the  previous  solution  (after  titration  with  silver)  by   N/io 
hydrochloric  acid. 

(3)  Determination  of  the  total  alkali  by  direct  titration,  in  another  portion  of 
the  solution,  with  N/IQ  hydrochloric  acid  and  methyl  orange. 

7.  Ferricyanide  Determination. — This  is  effected  by  allowing  sodium  amalgam 
to  act  for  fifteen  minutes  on  the  solution  in  a  narrow  cylinder,  then  determining 
the  ferrocyanide  formed  by  permanganate  in  an  acid  solution.     Deduct  from  the 
results  obtained  the  ferrocyanide  and  thiocyanate  previously  found,  1  c.c.  N/ioo 
permanganate  =0-003293  gm.  K6Fe2Cy12. 

8.  Sulphides. — It  ^rarely  happens   that   sulphides   are  present   in   a   cyanide 
solution ;   if  present^  however,   shake  up  with  precipitated  carbonate  of  lead, 
filter,     and    titrate    with    N/ioo    permanganate.     The    loss    over    the    previous 
determination  (of  K4FeCy6,KCNS,  etc.)  is  due  to  elimination  of  sulphides. 

1  c.c.  N/100  K2Mn208  =0-00017  gm.  H2S,  or  0-00055  gm.  K2S. 

The  hydrogen  alone  being  oxidized  by  dilute  permanganate  in  acid  solution 
where  the  permanganate  is  not  first  of  all  in  excess. 

9.  Ammonia. — If  sufficient  silver  nitrate  be  added  to  a  solution  (say  10  c.c.) 
wholly  to  precipitate  the  cyanogen  compounds  and  a  drop  or  two  of  N/i    HC1  be 
added,  the  whole  made  up  to  100  c.c.,  and  filtered  ;  then  10  c.c.  distilled  with  about 
150  c.c.  of  ammonia-free  water  and  Nesslerized  in  the  usual  way,  the  amount  of 
ammonia  may  be  ascertained. 

10.  Copper. — This  metal  appears  to  be  in  some  cases  a  serious  obstacle  in 
.the   working   of   cyanide   processes,    and   Clennell  has   devised  a  method   of 

determining  it  which  gives  good  technical  results  when  not  less  than  0-02  gm. 
of  the  metal  is  present  in  the  cyanide  solution  to  be  examined,  and  where  zinc, 
iron,  and  silver  are  not  present  sulphocyanides  and  ammonium  salts  do  not 
materially  interfere.! 

This  method  depends  upon  the  facts — 

(1)  That  cyanide  of  copper  is  precipitated  from  solutions  of  the  double  cyanides 
of  copper  by  the  addition  of  dilute  mineral  acids. 

*  C.  N.  71,  93.  t  J-  S.  C.  /.  19,  14. 


214  CYANIDE   SOLUTIONS. 

(2)  That  hydrocyanic  and  carbonic  acids  have  little  or  no  action  on  methyl  orange. 

(3)  That  when  an  acid  is  added  gradually  to  a  mixture  of  a  double  cyanide  of 
copper  with  free  alkali  cyanides  and  caustic  or  carbonated  alkalies,  no  precipitation 
of  the  copper  takes  place  until  the  whole  of  the  alkalies  and  free  cyanides  have 
been  neutralized,  the  first  appearance  of  a  permanent  white  precipitate  of  copper 
cyanide  corresponding  precisely  with  the  point  at  which  the  solution  becomes 
alkaline  to  methyl  orange. 

METHOD  OF  PROCEDURE  :  A  measured  volume  (say  from  10  to  50  c.c.)  of  the 
liquid  to  be  tested,  which  must  be  perfectly  clear  and  transparent,  is  placed 
in  a  100  c.c.  measuring  flask,  and  N/io  sulphuric  acid  is  added  drop  by  drop  from 
a  burette  with  continual  shaking  until  the  turbidity  formed  ceases  to  disappear, 
but  leaves  the  liquid  slightly  milky.  The  reading  of  the  burette  must  now  be 
carefully  noted.  This  point  is  in  general  perfectly  sharp  and  definite.  A  further 
quantity  of  sulphuric  acid  is  now  added,  more  than  sufficient  to  precipitate  the 
whole  of  the  copper.  (This  may  be  ascertained  if  necessary  by  a  preliminary 
experiment.  A  little  of  the  liquid  is  filtered  off.  If  the  filtrate  gives  110  further 
precipitation  on  addition  of  more  sulphuric  acid,  and  if  it  is  distinctly  acid  to 
methyl  orange,  the  reaction  may  be  considered  complete.) 

The  copper  being  thrown  down  as  a  white  curdy  precipitate  of  copper  cyanide, 
and  a  slight  excess  of  sulphuric  acid  being  present,  the  reading  of  the  burette  is 
again  taken. 

The  100  c.c.  flask  is  now  filled  up  to  the  mark  with  distilled  water  and  the 
contents  thoroughly  agitated.  The  precipitate  generally  settles  rapidly  in  a 
flocculent  condition.  Now  filter  off  50  c.c.  of  the  supernatant  liquid,  taking 
care  to  use  a  filter-paper  free  from  iron  or  other  substances  soluble  in  acids. 

The  filtered  liquid  is  now  titrated  with  the  addition  of  a  single  drop  of  methyl 
orange  of  0*25  per  cent,  strength,  using  N/io  sodium  carbonate,  until  the  pink 
colour  changes  to  a  scarcely  perceptible  yellowish  tinge. 

The  number  of  c.c.  of  sodium  carbonate  used,  multiplied  by  2,  gives  us  very 
approximately  the  equivalent  of  the  excess  of  sulphuric  acid  beyond  that 
required  to  precipitate  the  copper. 

The  operations  are  very  simple,  the  essential  points  being  to  note  carefully  the 
exact  point  at  which  permanent  precipitation  of  copper  cyanide  takes  place, 
the  amount  of  acid  added  beyond  this  point,  and  the  precise  amount  of 
sodium  carbonate  added.  The  end-point  with  methyl  orange  leaves  something 
to  be  desired  in  point  of  sharpness,  but  by  carrying  out  the  test  exactly  as 
described  and  arranging  matters  so  that  the  final  bulk  of  solution  is  about  60  to 
70  c.c.,  results  may  be  obtained  which  are  more  than  sufficiently  accurate  for  any 
technical  purpose.  The  actual  value  of  the  N/io  acid  on  copper  may  be 
ascertained  by  dissolving  a  known  weight  of  pure  copper  in  nitric  acid,  boiling 
to  expel  nitrous  fumes,  neutralizing  with  caustic  soda,  then  adding  cyanide  until 
a  clear  and  colourless  solution  is  obtained,  and  titrating  with  the  acid  as  above 
described. 

Where  interfering  metals  are  present  it  becomes  necessary  to  eliminate  them 
before  making  the  test,  and  this  would  seem  at  first  sight  a  serious  limitation  to 
the  usefulness  of  the  method.  Experience  with  a  large  number  of  ores  and 
tailings  has  shown,  however,  that,  owing  to  the  extraordinarily  rapid  action  of 
cyanide  on  copper  compounds  when  a  pure  dilute  solution  of  potassium  cyanide 
has  been  allowed  to  leach  through,  or  has  been  left  in  contact  for  a  short  time 
with  a  sample  of  cupriferous  ore  or  tailings,  the  liquor  drawn  off  contains 
practically  no  other  impurity  than  the  double  cyanide  of  copper  and  potassium. 
In  all  such  cases  the  method  herein  detailed  may  be  successfully  applied. 

11.  Oxygen. — The  determination  of  this  element  in  cyanide  solutions  is  con- 
sidered to  be  valuable,  but  it  is  very  difficult  to  obtain  an  easy  method.  A  process 
has  been  submitted  to  gold  workers  by  A.  F.  Cr  o  s  s  e.*  The  author's  method  for 
testing  cyanide  solutions  for  oxygen  is  an  adaptation  of  Thresh's  method, 
as  described  infra.  Before  the  method  can  be  used,  all  cyanides  and 
absorbents  of  iodine  must  be  removed.  Hence,  in  practice,  the  author  first  treats 

»  Jour.  Chem.  and  Met.  Soc.  of  S.  Africa,  1899,  107-112. 


CYANIDE   SOLUTIONS.  215 

the  solution  to  be  examined  with  zinc  sulphate.  A  bottle  capable  of  holding 
2£  litres  of  the  liquid  to  be  tested  is  carefully  filled  and  well  stoppered,  its  exact 
capacity  being  known.  100  c.c.  of  the  solution  are  withdrawn  for  a  preliminary 
test,  and  are  titrated  with  zinc  sulphate  solution  (200  gm.  per  litre),  using 
phenolphthalein  as  an  indicator,  the  zinc  solution  being  run  into  the  cyanide 
until  the  magenta  colour  of  the  latter  is  just  destroyed.  The  quantity  of  the 
standard  zinc  solution  required  for  the  bulk  of  the  cyanide  in  the  large  bottle 
is  calculated  from  this  result,  and  the  correct  amount  is  added,  without  allowing 
air  to  enter  with  it,  the  stopper  of  the  bottle  being  replaced  as  soon  as  possible. 
The  mixture  of  cyanide  and  zinc  sulphate  is  then  thoroughly  shaken  and  set 
aside  for  the  resulting  heavy  flocculent  precipitate  of  zinc  cyanide  to  settle, 
which  happens  after  some  time,  a  small  scum  usually  remaining  on  the  surface. 
The  clear  liquor  is  then  siphoned  off  without  undue  access  of  air  by  using  a  bent 
glass  tube  passing  through  one  hole  of  a  doubly  perforated  cork  fitted  into  the 
bottle  ;  the  second  hole  carries  a  short  glass  tube  (arranged  as  in  a  washing-bottle), 
through  which  air  is  blown  momentarily  to  start  the  action  of  the  siphon.  The 
end  of  the  immersed  limb  of  the  siphon  is  covered  with  a  small  bag  of  lint,  which 
niters  off  any  floating  particles  of  precipitate.  Two  or  three  (290  to  300  c.c.) 
pipettes  full  of  the  siphoned  solution  are  drawn  off  and  retained.  A  preliminary 
test  of  the  iodine-absorbing  power  of  the  solution  (due  to  unprecipitated  double 
cyanides)  is  then  made  by  adding  to  a  quantity  equal  to  that  used  in  the  test, 
0*9  c.c.  of  sulphuric  acid  (half  acid,  half  water),  and  a  few  drops  of  potassium 
iodide  and  starch.  Dilute  bromine  water  ( 1  bromine  water :  2  water)  is  added 
until  a  blue  colour  is  obtained.  Another  pipette  full  of  the  liquid  is  now  taken, 
0'9  c.c.  of  sulphuric  acid  and  the  required  amount  of  bromine  water  (found  from 
the  preliminar}'  experiment)  are  added,  the  stopper  is  put  into  the  wide-mouthed 
bottle  used  in  Thresh's  test,  and  the  pipette  is  turned  over  several  times.  1  c.c. 
of  the  potassium  iodide  and  sodium  nitrite  solution  is  then  added,  and  the  free 
iodine — freed  in  proportion  to  the  oxygen  in  the  solution — is  determined  by 
means  of  standard  sodium  thiosulphate. 

By  this  method  the  amount  of  oxygen  per  litre  in  certain  cyanide  solutions 
was  found  to  be  as  follows  : — Solution  from  S  i  e  m  e  n  s-H  a  1  s  k  e  process  before 
precipitation,  4'65  to  4*69  mgm.  ;  tap  water,  7'7  mgm.  ;  the  same  tap-water 
with  0*2  per  cent.  KCy  and  a  little  ferrocyanide,  7*6  mgm.  ;  solution  as  pumped 
on  to  a  leaching  vat,  6'3  mgm.  ;  the  same  solution  as  run  from  the  vat  thirty 
hours  later,  0*6  mgm.  ;  and  the  same  from  the  end  of  the  zinc  boxes,  0*3  mgm. 

It  was  afterwards  discovered  that  the  cyanide  solutions  contained  a  small 
quantity  of  nitrites.  The  process,  therefore,  was  altered  as  follows : — Add 
potassium  hydroxide,  and  then  zinc  sulphate ;  determine  the  thiosulphate 
required  by  Thresh's  method  with  clear  solution  decanted  from  precipitates 
formed  in  the  closed  bottle ;  make  a  qualitative  test  for  nitrites  by  acidifying 
a  little  of  the  clear  solution  with  dilute  sulphuric  acid,  and  adding  potassium 
iodide  and  starch  ;  and  finally  apply  a  correction  for  the  nitrites  and  reagents 
used.  To  make  this  correction,  pour  into  a  very  strong  350-c.c.  flask,  a  quantity 
of  solution  equal  to  that  used  in  the  experiment  (say  293  c.c.),  add  a  few  drops 
of  potassium  hydroxide,  and  close  the  flask  with  a  rubber  stopper  having  one 
perforation,  through  which  is  passed  a  glass  tube  with  a  glass  stopcock.  Boil  the 
solution  for  a  few  minutes  and  close  the  stopcock.  Cool  the  flask,  and,  when 
cold,  pour  the  liquid  into  the  pipette,  and  add  the  1  c.c.  of  iodide-and-nitrite 
solution  and  1  c.c.  of  sulphuric  acid  (1  :  1).  Then  let  it  stand  for  ten  minutes, 
and,  in  the  presence  of  coal-gas,  run  it  into  the  bottle  described  in  the  previous 
paper,  add  starch,  and  titrate  with  thiosulphate.  The  quantity  required  gives 
the  correction  for  nitrites  and  for  the  reagents,  as  the  same  amount  of  acid  and 
of  iodide  and  nitrite  solution  is  used  in  each  case. 

W.  J.  Sharwood,  chemist  to  the  Montana  Mining  Company, 
has  furnished  me  with  some  details  as  to  cyanide  solutions  communi- 
cated by  him  to  the  Engineering  and  Mining  Journal,  1898,  p.  216, 
but  the  results  are  too  voluminous  to  be  shown  here.  The  methods 
adopted  were  as  follows  : — 


216  CYANIDE  SOLUTIONS. 

Free  cyanide  was  determined  by  silver  nitrate,  using  a  few  drops  of  5  per  cent, 
ferrocyanide  solution  as  indicator.  Total  cyanogenjwas  obtained  by  continuing 
the  titration  with  silver  after  addition  of  caustic  soda  and  a  little  ammonia  and 
potassium  iodide ;  this,  however,  does  not  include  cyanogen  in  double  cyanides 
of  copper,  silver,  gold  or  mercury. 

Calcium  waa*determined  by  direct  precipitation  of  100  c.c.  of  the  solution  with 
ammonium  oxalate,  after  addition  of  ammonium  chloride  and  some  excess  of 
ammonia,  the  washed  precipitate  being  usually  dissolved  in  hot  dilute  sulphuric 
acid  and  titrated  with  permanganate;  in  some  cases  the  precipitate  was  ignited 
and  weighed  as  oxide. 

For  iron,  copper  and  zinc  100  c.c.  of  the  solution  were  twice  evaporated  with 
nitric  acid,  redissolved  in  dilute  sulphuric,  and  iron  precipitated  by  ammonia  in 
excess,  the  precipitate  being  at  once  redissolved  in  hydrochloric  acid  and  iron 
determined  colorimetrically  as  thiocyanate,  unless  the  quantity  sufficed  to  allow  of 
reduction  by  zinc  and  titration  by  permanganate.  Copper  was  approximately 
determined  by  the  colour  of  the  ammoniacal  nitrate  from  the  iron.  It  was  then 
removed  by  acidulating  with  sulphuric  acid  and  heating  with  a  strip  of  aluminium  ; 
the  metal  was,  then  washed,  redissolved  in  nitric  acid,  and  determined 
by  the  iodide  and  thiosulphate  method.  The  nitrate  after  removal  of  iron  and 
copper  was  neutralized  by  sodium  carbonate,  acidulated  with  a  fixed  amount  of 
hydrochloric  acid,  diluted  to  200  c.c.,  heated,  and  zinc  determined  in  it  by 
ferrocyanide  with  uranium  indicator. 

,.  Thiocyanate  was  determined  by  acidulating  10  or  20  c.c.  with  hydrochloric  acid, 
adding  ferric  chloride,  and  comparing  the  colour-  with  standard  thiocyanate 
under  the  same  conditions  ;  in  some  cases  ferrocyanides  precipitated  and  required 
to.  be  filtered  off.  Ferrocyanide  was  calculated  from  the  iron  found  above.  The 
methods  for  determination  of  ferrocyanides  and  thiocyanates,  based  upon  oxidation 
by  permanganate,  were  found  to  be  totally  unreliable  when  tested  experimentally 
upon  solutions  containing  known  quantities  in  presence  of  the  substances 
accompanying  them  in  cyanide  solutions.  The  colorimetric  methods  give  fairly 
approximate  results. 

Sulphate  was  weighed  as  barium  sulphate,  precipitated  by  adding  barium 
chloride  to  100  c.c.  of  solution,  after  first  adding  some  excess  of  hydrochloric  acid, 
heating  till  odour  disappeared,  and  filtering  off  any  zinc  and  copper  ferrocyanides, 
Prussian  blue,  or  silver  chloride  that  separated  out. 

The  solid  residue  was  obtained  by  evaporating  20  to  50  c.c.  in  a  nickel  or 
platinum  dish ;  the  former  appears  to  be  the  less  attacked  by  cyanide  solutions 
and  fused  residues. 

•  Alkalinity  toward  methyl  orange  was  determined  (a)  by  direct  titration  of 
25  or  50  c.c.  with  •decinormal  acid,  (6)  by  adding  the  standard  acid  in  considerable 
excess,  heating  till  all  odour  disappeared,  and  titrating  back  with  standard  alkali ; 
the  results  were  rendered  somewhat  uncertain  by  the  precipitation  of  zinc  com- 
pounds and  ferrocyanides. 

The  same  authority  states  that  although  the  method  given  in  the 
first  part  of  these  gold  cyanide  processes  give  fair  results  with 
tolerably  pure  substances,  theyjbecome  much  less  accurate  when  the 
solutions  are  muc'h  worked  and  old,'  owing  to  their  containing 
organic  matters,  and  various  decomposition  products  of  KCN. 


FERRO-    AND    FERRI-CYANIDES. 

Potassium  Ferrocyanide. 
K4FeCy6  +  3H20  =  422-36. 

Metallic  iron  x  7  -563  =  Crystallized  Potassium  ferrocyanide. 
Double  iron  salt  x  1-080  = 


FERROCYANIDES.  217 

Oxidation  to  Ferricyanide  by  Permanganate  (D  e  H  a  e  n). 

Ferrocyanide  may  be  determined  by  potassium  permanganate, 
by  which  it  is  converted  into  ferricyanide.  The  process  is  easy  of 
application,  and  the  results  accurate.  A  standard  solution  of  pure 
ferrocyanide  should  be  used  as  the  basis  upon  which  to  work,  but 
may  be  dispensed  with,  if  the  operator  chooses  to  calculate  the 
strength  of  his  permanganate  from  iron  or  its  compounds.  If  the 
permanganate  is  decinormal,  there  is  of  course  very  little  need  for 
calculation  (1  eq.  =422-36  must  be  used  as  the  systematic  number, 
and  therefore  1  c.c.  of  N/10  permanganate  is  equal  to  O042236  gm. 
of  yellow  prussiate).  The  standard  solution  of  pure  ferrocyanide 
contains  20  gm.  in  the  litre  :  each  c.c.  will  therefore  contain  0-02  gm. 

METHOD  or  PROCEDURE  :*  This  method  is  found  to  give  accurate  results  when 
carried  out  as  follows :  20  c.c.  of  sulphuric  acid  (1  mol.  of  acid  to  4  mols.  of 
water)  are  added  to  150-200  c.c.  of  the  solution  (containing  about  1  grm.  of 
ferrocyanide),  and  the  mixture  is  titrated  with  N/ao  permanganate,  until  the 
colour  changes  from  yellowish-green  to  yellowish-red.  Hydroferricyanic  acid 
may  also  be  determined  by  permanganate,  after  reduction  with  ferrous  sulphate  ; 
but  this  acid  is  preferably  determined  by  Mohr's  method  (see  below),  which  is 
accurate  when  carried  out  in  the  following  manner : — 0'7  grm.  of  ferricyanide  is 
dissolved  in  about  50  c.c.  of  water,  and  to  the  neutral  solution  are  added  3  grrns. 
of  potassium  iodide  and  T5  grms,  of  zinc  sulphate  (free  from  iron)  ;  the  solution 
is  then  shaken  and  titrated  with  N/so  thiosulphate.  The  determination  of 
hydroferrocyanic  acid  by  oxidation  with  iodine  and  titration  with  thiosulphate 
in  the  presence  of  alkali  bicarbonate  (R  u  p  p  and  S  c  h  i  e  d  t ;  J.  8.  C.  I.,  1902,  1099), 
is  found  to  be  inaccurate. 

Commercial  Ferrocyanides. 

Potassium  ferrocyanide,  K4Fe  (CN)6,  3H2O  =  422*36. 
Sodium  „  Na4Fe  (CN)6,  10H20 =484-07. 

Calcium  „  Ca2Fe  (CN)6,  12H2O  =  508-28. 

All  of  the  above  are  obtained  as  by-products  in  the  coal-gas 
industry  from  the  hydrocyanic  acid  present  in  the  crude  gas.  The 
crude  products  in  each  case^may  consist  of  the  soluble  potassium, 
sodium,  or  calcium  salt,  butB  usually  there  are  also  present  insoluble 
ferrocyanides  in  the  form  of  "double  ferrocyanides  of  iron  with  these 
metals  or  ammonium,  the  amount  of  which  has  to  be  included  in 
the  analytical  results.  The  products  obtained  from  crude  coal-gas 
frequently  contain,  in  addition,  appreciable  quantities  of  the  very 
soluble  carbonyl  ferrocyanide,  Na3FeCO(CN)5,  or  sodium  ferro- 
cyanide (Fe(CN)2,  4NaCN)  in  which  one  molecule  of  NaCN  has 
been  replaced  by  the  radicle  CO.  In  other  cases,  such  as  the  spent 
oxide  from  gas-works,  the  whole  of  the  ferrocyanide  present  is  in 
an  insoluble  form  and,  before  analysis,  must  be  converted  into 
soluble  salts.  £  This  is  done  as  follows  : — 

The  samplers  ground,  thoroughly  mixed,  and  a  weighed  portion 
(30-40  gramsfput  into  a  mortar,  caustic  soda  in  excess  and  about 

*E.  Miiller  and  O.  Dief enthaler,  Z.  anorg.  Chem.  1910,  67,  418-436;  also, 
Mecklenburg,  Z.  a.  C.,  1910,  322,  and  J.  S.  C-  /.,  29,  946, 


218  FERROCYANIDES. 

100  c.c.  of  water  added,  and  allowed  to  stand  for  several  hours, 
with  frequent  trituration  with  the  pestle.  A  few  crystals  of  ferrous 
sulphate  may  also  be  added,  to  bring  about  the  conversion  of  any 
cyanide  present  into  ferrocyanide  and  the  reduction  of  any  ferri- 
cyanide  to  ferrocyanide.  Heat  should  not  be  employed,  as  the 
products  mostly  contain  both  ammonia  and  sulphides  or  free  sulphur, 
the  latter  yielding  sodium  sulphide  by  the  action  of  the  NaOH,  and 
in  hot  solution  the  alkali  sulphides,  in  presence  of  ammonia,  effect 
a  partial  conversion  of  the  ferrocyanide  into  sulphocyanide,  thus 
tending  to  make  the  ferrocyanide  results  low.  In  absence  of 
ammonia,  however,  boiling  sodium  sulphide  has  but  little  action 
on  ferrocyanide.  The  mixture,  when  decomposition  is  complete, 
is  either  filtered  and  made  up  to  1  litre  or  it  may  be  at  once  placed 
in  a  litre  flask,  made  up  to  the  mark,  and  a  further  addition  of  water 
made  equal  to  the  volume  of  the  insoluble  matter  present  :  the 
whole  is  then  well  shaken,  allowed  to  settle,  and  the  clear  solution 
taken  for  analysis. 

Owing  to  the  frequent  presence  of  sulphocyanides  and  other 
oxygen-consuming  bodies  in  the  above  products  the  old  method  of 
titration  by  standard  permanganate  is  inadmissible  and  the  two 
following  methods  are  now  recommended  : — 

1.  Determination  of   the  amount  of   hydrocyanic  acid  present 
(Feld). 

2.  Direct  determination  of  the  ferrocyanide  present  by  titration 
with  standard  zinc  or  copper  sulphate  solution  (Knublauch). 

The  first  method  is  described  by  Dr.  H.  G.  Colman*  as  the  one 
giving  the  most  accurate  results,  and  is  carried  out  as  follows  : — 

A  quantity  of  the  solution,  prepared  as  described  above,  and 
containing  the  equivalent  of  0'3— 0*5  gm.  of  K4FeCy6,  is  diluted 
with  water  in  a  large  flask,  mixed  with  10  c.c.  *%  caustic  soda, 
and  heated  to  boiling  :  to  this  15  c.c.  of  hot  magnesium  chloride 
solution  (610  gm.,  MgCl2,  6H20  per  litre)  are  added  slowly  with 
continual  shaking  in  order  to  get  a  milky  precipitate  of  magnesium 
hydrate,  and  the  boiling  continued  for  5  minutes  and  no  more — 
the  object  being  to  convert  any  free  alkali  into  magnesium  hydroxide. 
(If  free  cyanide  is  also  present,  the  HCN  in  this  form  is  then  evolved, 
and  may  be  condensed  and  collected  for  analysis,  the  boiling  in 
that  case  being  continued  for  a  longer  period.)  100  c.c.  of  boiling 
mercuric  chloride  solution  (27'1  gm.  HgCl2  per  litre)  are  then 
added,  and  the  boiling  continued  for  a  further  ten  minutes,  all 
ferrocyanide  being  thus  converted  into  mercuric  cyanide.  The 
flask  is  then  connected  to  a  condenser,  30  c.c.  of  4N  sulphuric 
acid  added  by  means  of  a  stoppered  funnel,  and  distillation  continued 
for  20-30  minutes,  the  end  of  the  condenser  dipping  under  the 
surface  of  25  c.c.  of  N/l  NaOH  placed  in  the  receiver.  The  solution 
of  sodium  cyanide  thus  obtained  is  now  diluted  to  about  400  c.c., 
a  crystal  of  KI  added,  and  N/10  silver  nitrate  run  in  from  a  burette 
until  a  permanent  yellow  precipitate  of  Agl  is  obtained. 

*  The  Analyst,  1908,  33,  261 :  1910,  35,  295. 


FERROCYANIDES.  219 

1  c.c.  N/10  AgNO3= 0-005404  gram  HCN 

ind  from  this  the  amount  of  ferrocyanide  is  readily  calculated. 

Knublauch's  method  consists  in  titrating  the  ferrocyanide 
solution,  acidified  with  sulphuric  acid,  with  a  standard  solution  of 
copper  or  zinc  sulphate,  using  a  ferric  salt  as  indicator.  If  the 
solution  contains  sulphide,  this  is  first  removed  by  shaking  with 
lead  carbonate  and  filtering  off  the  lead  sulphide  and  the  excess 
of  lead  carbonate.  In  order  to  obtain  accurate  results,  it  has  been 
stated  to  be  necessary  to  standardize  the  zinc  or  copper  solution 
with  the  same  salt  of  ferrocyanic  acid  as  is  used  for  the  titration, 
owing  to  the  influence  of  the  soluble  sulphate  formed  during  the 
process.  Dr.  Colman*  has  shown,  however,  that,  if  the  ferro- 
cyanide present  is  not  potassium  ferrocyanide,  by  adding  50  c.c. 
of  a  cold  saturated  solution  of  potassium  sulphate  before  titration 
(and  the  same  quantity  to  the  pure  potassium  ferrocyanide  solution 
when  standardizing  the  copper  or  zinc  solution)  excellent  results 
are  obtained.  He  uses  standard  solutions  containing  10  grams 
crystallized  copper  sulphate  or  (its  exact  equivalent)  11*51  gm.  of 
crystallized  zinc  sulphate  per  litre.  A  standard  solution  of  4 
grams  crystallized  potassium  ferrocyanide  per  litre,  equal  to  2'044 
gm.  ferrocyanic  acid  H4Fe  (CN)6,  may  be  used,  50  c.c.  being  taken 
for  standardizing.  The  end-point  of  the  titration  is  determined 
either  by  filtering  small  portions  of  the  liquid  through  Swedish 
filter-paper  and  adding  ferric  solution  to  the  filtrate,  or  placing 
a  drop  of  the  liquid  on  a  Schleicher  and  Schull  drop  re- 
action filter  paper,  followed  by  a  drop  of  ferric  chloride  so  placed 
as  to  come  in  contact  with  the  clear  wetted  portion  only  of  the 
first  spot.  The  titration  should  be  carried  out  in  a  good  light. 
The  amount  of  dilution  and  degree  of  acidity,  within  reasonable 
limits,  affect  the  result  but  little. 

Dr.  Skirrowf  recommends  stronger  solutions  than  those  given 
above.  He  uses  solutions  of  50  gm.  crystallized  sodium  ferrocyanide 
and  46' 5  gm.  of  zinc  sulphate  per  litre,  and  in  making  a  test  always 
acidifies  with  2  c.c.  of  dilute  H2SO4  (1  :  2).  He  states  that  with 
the  more  concentrated  solutions  a  sharper  end-point  is  obtained 
and  that  the  effect  of  the  presence  of  excess  of  alkali  sulphate  is 
minimized. 

The  presence  of  carbonyl  ferrocyanide  interferes  with  the  analysis 
in  both  of  the  methods  described  above,  as  it  is  precipitated  along 
with  the  ferrocyanide  by  both  zinc  and  copper  solutions,  and  the 
hydrocyanic  acid  from  it  is  determined  along  with  the  rest  by  the 
Feld  method.  Dr.  Colman  uses  a  simple  method  of  separation 
based  on  the  fact  that  the  carbonyl  ferrocyanides  are  readily 
soluble  in  dilute  alcohol  whilst  ferrocyanides  are  insoluble.  To 
the  solution  of  ferrocyanides,  which  must  be  alkaline  or  neutral, 
4  or  5  times  its  volume  of  methylated  spirit  (industrial  alcohol)  is 
added,  the  mixture  is  allowed  to  stand  for  several  hours,  and 

*  Loc.  tit.  t  J.  S.  C.  /.  1910,  29,  319. 


220  FERRICYANIDES. 

filtered.  The  precipitate  is  washed  with  a  little  methylated  spirit 
and  dried  in  the  water-oven.  The  filter  paper  and  precipitate  are 
then  transferred  to  the  titrating  basin,  water  added,  the  liquid 
acidified,  and  then  proceeded  with  as  described  above,  or  simply 
dissolved  in  water  and  treated  by  the  Feld  process. 

Potassium   Ferricyanide. 
K6Fe2Cy12  =  658-42. 

Metallic  iron  x     5-895     =  Potassium  ferricyanide. 

Double  iron  salt       x     1-684     =  „  „ 

N/10  Thiosulphate     x     0-03292  = 


By  Iodine  and  Thiosulphate. 

This  salt  can  be  determined  either  by  reduction  to  ferrocyanide 
and  titration  with  permanganate  or  dichromate,  or  by 
Lenssen's  method,  which  is  based  upon  the  fact  that  when 
potassium  iodide  and  ferricyanide  are  mixed  with  tolerably  concen- 
trated hydrochloric  acid  iodine  is  set  free. 

K6Fe2Cy12 +2KI =2K4FeCy6 +I2 

the  quantity  of  which  can  be  determined  by  N/10  thiosulphate  and 
starch.  This  method  does  not,  however,  give  the  most  satisfactory 
results,  owing  to  the  variation  produced  by  working  with  dilute 
or  concentrated  solutions.  The  modification  given  under  Zinc, 
is,  however,  more  accurate,  and  is  as  follows  : — The  ferricyanide  is 
dissolved  in  a  convenient  quantity  of  water,  potassium  iodide  in 
crystals  added,  together  with  hydrochloric  acid  in  tolerable  quantity, 
then  a  solution  of  pure  zinc  sulphate  in  excess  ;  after  standing 
a  few  minutes  to  allow  of  complete  decomposition,  the  excess  of 
acid  is  slightly  over-neutralized  by  addition  of  sodium^carbonate. 

At  this  stage  all  the  zinc  ferricyanide  first  formed  is  converted 
into  the  ferrocyanide  of  that  metal,  and  an  equivalent  quantity  of 
iodine  set  free,  which  can  at  once  be  titrated  with  N/10  thiosulphate 
and  starch,  and  with  very  great  exactness.  1  c.c.  N/10  thiosulphate 
=0-03292  gm.  potassium  ferricyanide. 

Another  method  consists  in  boiling  with  excess  of  potash,  then 
cooling,  and  adding  H2O2  till  the  colour  becomes  yellow.  The 
excess  of  the  peroxide  is  then  boiled  off,  H2SO4  added,  and  the 
solution  titrated  with  permanganate. 

Reduction   of  Ferri-  to  Ferro-cyanide. 

This  process  is,  of  course,  necessary  when  the  determination  by 
permanganate  has  to  be  made,  and  is  best  effected  by  boiling  the 
weighed  ferricyanide  with  an  excess  of  potash  or  soda,  and  adding 
small  quantities  of  concentrated  solution  of  ferrous  sulphate  until 
the  precipitate  which  is  formed  possesses  a  blackish  colour  (signifying 
that  the  magnetic  oxide  is  formed).  The  solution  is  then  diluted 


THIOCYANATES.  221 

to  a  convenient  quantity,  say  300  c.c.,  well  mixed,  and  filtered 
through  a  dry  filter  ;  50  or  100  c.c.  may  then  be  taken,  sulphuric 
acid 'added,  and  titrated  with  permanganate  as  before  described. 

Kassner*  suggests  the  use  of  sodium  peroxide  for  the  reduction 
of  ferri-  to  ferro-cyanide  as  being  rapid  and  complete.  About  0'5 
gm.  in  100  c.c.  water  requires  about  0*06  gm.  of  the  peroxide  ; 
the  mixture  is  heated  till  all  effervescence  is  over,  acidified  with 
sulphuric  acid,  cooled,  and  titrated  with  permanganate  in  the  usual 
way. 


THIOCYANATES. 

Volhard's  method  is  described  on  p.    145. 

For  the  determination  of  thiocyanic  acid  in  combination  with 
the  alkali  or  earthy  bases,  Barnes  and  Liddlef  have  devised 
a  method  which  is  easy  of  application,  and  gives  good  technical 
results.  It  is  not,  however,  available  for  gas  liquors. 

The  method  depends  upon  the  fact  that  when  a  solution  of  a  cupric 
salt  is  added  to  a  solution  of  a  thiocyanate  in  presence  of  a  reducing 
agent,  as  sodium  bisulphite,  the  insoluble  cuprous  salt  of  thiocyanic 
acid  is  precipitated,  the  end  of  the  reaction  being  ascertained  by 
a  drop  of  the  solution  in  the  flask  giving  a  brown  colouration  when 
brought  in  contact  with  a  drop  of  ferrocyanide.  The  following 
reactions  take  place  : — 

2CuS04 +2KSCN  +Na2S03  +H20  =  2CuSCN  +K2SO4  +2NaHS04 
and 
2CuS04+Ba(SCN)2+Na2SO3+H2O=2CuSCN  +  BaS04+2NaHS04 

The  following  solutions  are  required  : — 

1.     A  standard  solution  of  copper  sulphate  containing  6'2375  gm. 
per  litre,  1  c.c.  of  which  is  equivalent  to  0*00145  gm.  SON. 
i  :  2.     A  solution  of  sodium  bisulphite  of  specific  gravity  T3. 
/  3.     A  solution  of  potassium  ferrocyanide  (1  :  20). 

METHOD  OP  PROCEDURE  :  About  3  gm.  of  the  sample  are  weighed  from 
a  stoppered  tube  into  a  litre  flask,  dissolved  in  water,  and  made  up  to  the  mark. 
After  well  mixing,  25  c.c.  are  measured  into  a  flask,  about  3  c.c.  of  the  bisulphite 
added,  and  the  whole  boiled.  Whilst  this  is  heating  a  burette  is  filled  with  the 
copper  solution,  and  a  white  porcelain  slab  is  spotted  over  with  the  ferrocyanide. 
When  the  liquid  in  the  flask  has  reached  the  boiling  point,  20  c.c.  of  the  copper 
solution  are  run  in,  well  shaken,  the  precipitate  allowed  to  settle  for  about  a  minute, 
a  drop  is  taken  out  by  means  of  a  glass  rod,  and  brought  in  contact  with  a  drop 
of  ferrocyanide,  and  should  no  brown  colouration  appear,  more  of  the  copper 
solution  is  run  in,  say  1  c.c.  at  a  time,  and  again  tested.  This  is  continued  until 
a  drop  gives  a  colour  immediately.  By  this  means  an  approximation  to\the 
truth  is  obtained.  It  will  be  observed  during  a  titration  that  the  mixed  drops, 
after  standing  for  a  minute,  or  even  less,  produce  a  brown  tint.  It  is  of  the  utmost 
importance  that  the  colouration  be  immediate. 

A  second  25  c.c.  of  the  thiocyanate  solution  are  run  into  a  clean  flask,  the 
bisulphite  added,  and  boiled  as  before. 

Suppose  that  in  the  first  experiment,  after  an  addition  of  27  c.c.  of  copper 

*  Arch.  Pharm.  232,  226.  t  J-  S.  C.  I.  2,  122. 


222  THIOCYANATES. 

solution,  no  colour  was  formed  with  ferrocyanide,  but  that  28  c.c.  gave  an 
immediate  colour  ;  then  in  the  second  experiment  27  c.c.  are  run  in  at  once,  and 
the  liquid  is  again  tested,  when  no  colour  should  appear.  The  copper  solution 
is  then  run  in  drop  by  drop  until  there  is  a  slight  excess  of  copper,  as  proved  by 
the  delicate  reaction  with  the  ferrocyanide.  The  second  experiment  is  thus 
rendered  more  exact  by  the  experience  gained  in  the  first. 

According  to  Schroder,*  thiocyanates  can  be  determined  by 
means  of  permanganate,  the  reaction  being  approximately 
quantitative  according  to  the  equation  : 


When  N/10  solutions  are  employed  and  when  the  manganese  is 
precipitated  by  sodium  carbonate,  redissolved  in  HC1  and  again 
titrated.  A  correct  determination  of  thiocyanic  acid  may,  how- 
ever, be  made  by  adding  a  definite  volume  of  the  thiocyanate 
solution  to  a  known  excess  of  warmed  permanganate  solution 
acidified  with  sulphuric  and  phosphoric  acids,  decomposing  the 
excess  of  permanganate  by  the  addition  of  an  excess  of  either 
standardized  oxalic  acid  solution  or  hydrogen  peroxide,  and 
titrating  the  latter  with  permanganate. 

For  the  determination  of  thiocyanates  and  ferrocyanides,  etc., 
in  cyanide  solutions  containing  copper,  see  Green.f 


GOLD. 

Au  =  197-2. 
1  c.c.  of  normal  oxalic  acid =0*0657  gm.  Gold. 

THE  technical  assay  of  gold  for  coining  purposes  is  invariably 
performed  by  cupellation.  Trichloride  of  gold  is,  however,  largely 
used  in  photography  and  electro-gilding,  and  therefore  it  may  be 
necessary  sometimes  to  ascertain  the  strength  of  a  solution  of  the 
chloride,  or  its  value  as  it  occurs  in  commerce. 

If  to  a  solution  of  gold  in  the  form  of  chloride  (free  from  nitric 
acid  .and  the  free  hydrochloric  acid  nearly  neutralized  by  ammonia) 
an  excess  of  oxalic  acid  be  added,  in  the  course  of  from  eighteen  to 
twenty-four  hours  all  the  gold  will  be  precipitated  in  the  metallic 
form,  while  the  corresponding  quantity  of  oxalic  acid  has  been 
dissipated  in  the  form  of  carbonic  acid  ;  if,  therefore,  the  quantity 
of  oxalic  acid  originally  added  be  known,  and  the  excess,  after 
complete  precipitation  of  the  gold,  be  found  by  permanganate, 
the  amount  of  gold  can  be  ascertained. 

A  more  rapid  method  consists  in  boiling  the  neutral  gold  solution 
with  an  excess  of  standard  solution  of  potassium  oxalate  containing 
8-3  gm.  of  the  pure  salt  per  litre,  and  titrating  back  with  a  perman- 
ganate solution  which  has  the  same  working  strength  as  the  oxalate. 
Each  c.c.  of  oxalate  solution  decomposed  represents  0*00657  gm.  Au. 

*  J.  S.  C.  I.  1909,  28,  1066.  f  J.  S.  C.  I.  1908,^27,  1085. 


GOLD.  223 

The  determination  of  small  proportions  of  gold  in  solution  can 
be  dene  by  iodine  and  thiosulphate  as  shown  by  Peter  sen*,  and 
the  method  has  been  verified  by  F.  A.  Gooch  and  F.  H.  Morley.f 
These  chemists  found  that  the  reduction  of  the  auric  salt  with  the 
consequent  liberation  of  iodine  was  somewhat  influenced  by  the 
volume  of  the  solution,  the  amount  of  iodine  present,  and  the 
time  of  action.  Their  experiments  showed  that  the  best  effects 
were  obtained  in  a  solution  of  pure  gold  chloride  of  about  0'8  gm. 
of  the  salt  to  the  litre  by  using  O'l  gm.  KI  to  volumes  of  the 
chloride  ranging  between  25  and  50  c.c.  The  iodine  and  thio- 
sulphate solutions  used  were  about  N/10o  strength  and  verified 
against  each  other.  The  solution  of  KI  contained  10  gm.  per  litre. 

METHOD  OF  PROCEDURE  :  The  gold  solution  is  measured  from  a  burette  and 
the  potassium  iodide  added  in  the  proportion  above  mentioned  ;  there  must 
always  be  enough  of  this  to  more  than  redissolve  the  aurous  iodide  precipitated 
at  first.  A  clear  solution  of  starch  is  then  added,  and  the  blue  colour  produced 
by  it  is  just  removed  by  thiosulphate.  The  standard  iodine  is  then  added  until 
the  liquid  assumes  a  faint  rose  colour,  and  the  amount  of  gold  is  obtained.  Of 
course  the  gold  value  of  the  standard  solutions  must  be  ascertained  by  experiment 
upon  a  pure  gold  solution  of  known  strength.  For  very  small  quantities  of  gold 
N/iooo  solutions  of  iodine  and  thiosulphate  may  be  used  with  good  effect,  but  in 
this  case  a  correction  of  0*1  c.c.  for  the  iodine  must  be  allowed  for  volumes  not 
exceeding  30  c.c.  of  the  gold,  because  that  is  the  amount  required  to  bring  out 
the  rose  colour  in  a  blank  experiment.  In  the  practical  use  of  this  process  for 
the  determination  of  metallic  gold,  the  metal  can  of  course  be  got  into  solution  by 
chlorine  water  or  aqua  regia,  but  in  the  removal  of  the  excess  of  the  oxidizer  by 
evaporation  it  is  difficult  to  prevent  the  formation  of  aurous  chloride.  Gooch 
and  M  o  r  1  e  y,  however,  found  that  by  adding  ammonia  in  excess  to  the  solution, 
boiling  gently,  acidifying  with  HC1,  and  heating  if  necessary  to  redissolve  the 
precipitate  by  ammonia,  again  treating  with  ammonia  and  heating,  and  once 
more  acidifying,  the  ammonium  chloride  so  formed  acts  apparently  in  producing 
a  clear  solution  ready  for  titration. 

Colorimetric  Determination  of  Gold  in  Ores. — This  method  is 
mentioned  in  Rose's  Gold  Metallurgy  as  being  of  service. 

METHOD  OF  PROCEDURE  :  Take  100  gm.  of  the  ore,  or  less  if  more  than  a  trace  of 
gold,  and  heat  it  in  a  stoppered  bottle  for  some  hours  with  10  c.c.  of  bromine  and 
100  c.c.  of  water.  Then  filter  off  the  liquid,  and  wash  the  residue  several  times 
with  water.  Evaporate  the  filtrate  till  it  no  longer  smells  of  bromine.  Make  it 
up  to  100.  c.c.  and  raise  it  to  boiling.  Place  5-10  c.c.  of  a  fresh  saturated  solution 
of  stannous  chloride  in  a  beaker  and  rapidly  pour  upon  it  the  boiling  extract. 
A  precipitate  will  form  and  sink  to  the  bottom.  If  no  gold  be  present  the 
precipitate  has  a  slight  bluish  tint.  Gold  causes  it  to  be  purplish  red  to  blackish 
purple,  according  to  the  quantity  present.  The  gold  is  determined  by  taking  small 
quantities  of  standard  gold  solution,  making  up  to  100  c.c.,  boiling  and  pouring 
into  stannous  chloride,  exactly  as  was  done  with  the  ore  extract.  In  this  way  the 
gold  can  be  approximately  determined.  The  gold  present  should  be  between 
0-0001  and  0*00002  gm.  If  there  be  more  than  O'OOOl  gm.  a  more  dilute  extract 
of  ore  should  be  prepared.  If  less  than  0'00002  gm.  be  present  a  larger  quantity 
of  ore  should  be  used. 

Determination  of  Gold  in  dilute  Cyanide  Solutions. — J.  M  o  i  r  J  gives  the  following 

*  Zeit.  /.  Anorg.  Chem.  19,  63.  t  Amer.  Jour.  Sci.,  October,  1899. 

t  J.  S.  C.  /.  abstr.  22,  1257. 


224  GOLD. 

rapid  method : — 100  c.c.  of  the  solution  are  boiled  for  two  minutes  with  about 
1  gm.  of  sodium  peroxide,  to  destroy  cyanides.  Next,  two  drops  of  10  per  cent, 
lead  acetate  solution  are  added,  and  about  0'2  gm.  of  aluminium  powder  is  stirred  in. 
Metallic  lead  is  thus  precipitated,  and  the  gold  is  also  extracted  by  the  galvanic 
action.  The  whole  is  filtered  when  the  aluminium  has  dissolved  (the  filtrate  being 
free  from  cyanide  as  well  as  gold).  The  black  precipitate  is  dissolved  in  10  c.c. 
of  boiling  60  per  cent,  aqua  regia,  and  treated  carefully  with  stannous  chloride 
solution  until  the  yellow  colour  is  bleached,  whereupon  the  purple  tint  (purple  of 
Cassius)  develops,  and  is  constant  after  a  minute.  It  is  then  compared  with  a 
set  of  artificial  standards,  after  making  the  liquid  up  to  15  c.c.  in  a  tube  of  fixed 
diameter.  The  standard  tubes  are  filled  with  a  permanent  imitation  of  "  purple 
of  Cassius,"  made  by  mixing  copper  and  cobalt  salts  in  the  required  proportion. 
They  are  standardized  empirically.  The  shade  is  easily  visible  with  solutions 
carrying  1  part  of  gold  per  million,  and  by  looking  down  the  tubes  (as  in 
Nesslerizing),  2  grains  per  ton  (1  in  seven  millions)  can  be  detected.  Of  course, 
even  less  than  this  can  be  recognised,  if  more  than  100  c.c.  of  solution  be  used 
at  the  beginning. 

« 
IODINE. 

1  =  126-92. 
1.     By  Distillation. 

FREE  iodine  is  of  course  very  readily  determined  by  solution  in 
potassium  iodide,  and  titration  with  starch  and  N/10  thiosulphate, 
as  described  on  p.  128  et  seq. 

Combined  iodine  in  haloid  salts,  such  as  the  alkali  iodides,  must 
be  subjected  to  distillation  with  hydrochloric  acid  and  some  other 
substance  capable  of  assisting  in  the  liberation  of  iodine,  which 
is  received  into  a  solution  of  potassium  iodide,  and  then  titrated 
with  N/10  thiosulphate  in  the  ordinary  way.  Such  a  substance 
presents  itself  best  in  the  form  of  ferric  oxide,  or  some  of  its  combi- 
nations. If,  therefore,  hydriodic  acid,  or,  what  amounts  to  the 
same  thing,  an  alkali  iodide,  be  mixed  with  an  excess  of  ferric 
oxide  or  chloride  and  distilled  in  the  apparatus  shown  in  fig.  38 
or  39,  the  following  reaction  occurs  : — 

Fe203 +2HI  =  2FeO  +H2O  +I2. 

The  best  form  in  which  to  use  the  ferric  oxide  is  iron  alum. 

The  iodide  and  iron  alum  having  been  brought  into  the  little 
flask  (fig.  39),  sulphuric  acid  of  about  1'3  sp.  gr.  is  added,  and  the 
cork  carrying  the  distillation  tube  inserted.  This  tube  is  not 
carried  into  the  solution  of  potassium  iodide  in  this  special  case, 
but  within  a  short  distance  of  it ;  and  the  end  must  not  be  drawn 
out  to  a  fine  point,  as  there  represented,  but  cut  off  straight.  The 
reason  for  this  arrangement  is  that  it  is  not  a  chlorine  distillation 
for  the  purpose  of  setting  iodine  free  from  the  iodide  solution, 
as  is  usually  the  case,  but  an  actual  distillation  of  iodine,  which  would 
speedily  choke  up  the  narrow  point  of  the  tube  and  so  prevent 
the  further  progress  of  the  operation. 

As  the  distillation  goes  on,  the  steam  washes  the  condensed 
iodine  out  of  the  tube  into  the  solution  of  iodide,  which  must  be 


IODINE. 


225 


present  in  sufficient  quantity  to  absorb  it  all.  When  no  more 
violet  vapours  are  to  be  seen  in  the  flask,  the  operation  is  ended  ; 
but  to  make  sure  it  is  well  to  empty  the  solution  of  iodine  out  of 
the  condensing  tube  into  a  beaker,  and  put  in  a  little  fresh  iodide 
solution  with  starch,  then  heat  the  flask  again ;  the  slightest  traces 
of  iodine  may  then  be  detected  by  the  production  of  the  blue  colour 
when  cooled.  When  this  takes  place  the  distillation  is  continued 
a  little  while,  then  both  liquids  are  mixed,  and  titrated  with  N/10 
thiosulphate  as  usual. 

It  has  been  previously  stated  that  the  rubber  joints  to  the  special 
apparatus  of  F  r  e  s  e  n  i  u  s,  B  u  n  s  e  n ,  or  M  o  h  r  f or  iodine  distillations 
are  objectionable.  Topf*  avoids  this  by  fitting  his  apparatus 
together  so  that,  although  rubber  is  used  the  reagents  do  not  come 
in  contact  with  it. 


Fig.  43. 

Another  form  of  apparatus  designed  by  Stortenbekerfis 
•shown  in  fig.  43,  in  which  rubber  joints  are  entirely  dispensed 
with,  and  glass  connections  alone  used.  The  connection  between 
the  distilling  tube  and  the  absorbing  apparatus  is  a  water  joint, 
the  tube  resting  in  a  socket  kept  wet  with  water,  the  chloride  of 
calcium  tube  is  filled  with  glass  beads,  moistened  with  concentrated 
solution  of  potassium  iodide,  and  the  connection  with  the  absorbing 
apparatus  is  ground  in  like  an  ordinary  stopper.  The  absorbing 
bulbs  are  immersed  in  water  to  the  middle  of  the  bulbs,  and  the 
immersed  portion  partially  filled  with  iodide  solution. 

Ferric  chloride  may  be  used  instead  of  the  iron  alum,  but  it  must 
be  free  from  nitric  acid  or  active  chlorine  (best  prepared  from  dry 
Fe2O3  and  HC1). 

The  iodides  of  silver,  mercury,  and  copper  cannot  be  accurately 
analysed  in  this  way,  but  must  be  specially  treated.  They  should 
be  dissolved  in  the  least  possible  quantity  of  sodium  thiosulphate 


Z.  a.  C.  26,  293. 


t  Z.a.  C.  29,  273. 


Q 


226  IODINE. 

solution,  and  precipitated  boiling  with  sodium  sulphide,  then 
filtered  ;  the  filtrate  contains  the  whole  of  the  iodine  free  from 
metal.  The  filtrate  is  evaporated  to  dryness  and  ignited,  then 
dissolved  in  water,  and  distilled  with  a  good  excess  of  ferric  salt.* 


2.     Mixtures  of  Iodides,  Bromides,  and  Chlorides. 

Donathf  has  shown  that  iodine  may  be  accurately  determined 
by  distillation  in  the  presence  of  other  halogen  salts  by  means  of 
a  solution  containing  about  2  to  3  per  cent,  of  chromic  acid  free 
from  sulphuric  acid. 

In  the  case  of  a  mixture  of  iodides  and  chlorides  the  action  is 
perfectly  regular,  and  the  whole  of  the  iodine  may  be  received  into 
potassium  iodide  without  any  interference  from  the  chlorine. 

In  the  case  of  bromides  being  present,  the  chromic  solution  must 
be  rather  more  dilute,  and  the  distillation  must  not  be  continued 
for  more  than  two  or  three  minutes  after  ebullition  has  com- 
menced, otherwise  a  small  amount  of  bromide  is  decomposed. 

The  reaction  in  the  case  of  potassium  iodide  may  be  expressed 
thus  : 

6KI  +  8Cr03  =  3I2  +  O2O3 + 3K2Cr  2O7 

The  distillation  may  be  made  in  Mohr's  apparatus  (fig.  39), 
using  about  50  c.c.  of  chromic  solution  for  about  0*3  gm.  I. 

The  titration  is  made  with  thiosulphate  in  the  usual  way. 

A  much  less  troublesome  method  of  determining  iodine  in  the 
presence  of  bromides  or  chlorides  has  been  worked  out  by  Cook,J 
and  depends  on  the  fact  that  hydrogen  peroxide  liberates  iodine 
completely  from  an  alkali  base  in  the  presence  of  excess  of  acetic 
acid,  while  neither  bromine  nor  chlorine  is  affected. 

Hydrogen  peroxide  alone  will  only  partially  liberate  iodine  from 
potassium  iodide,  but  with  excess  of  a  weak  organic  acid  to  combine 
with  the  alkali-  hydroxide,  the  liberation  is  complete.  Strong 
mineral  acids  must  not  be  used,  as  bromine  and  chlorine,  if  present, 
would  then  be  set  free  also. 

METHOD  OF  PROCEDURE  :  The  solution  is  strongly  acidified  with  acetic  acid, 
and  sufficient  hydrogen  peroxide  added  to  liberate  the  iodine  (5  c.c.  will  suffice 
for  1  gni.  KI).  The  mixture  is  allowed  to  stand  from  half  an  hour  to  an  hour  ; 
the  whole  of  the  iodine  separates,  some  being  in  the  solid  state  if  the  quantity  is 
considerable.  Chloroform  is  now  added  in  sufficient  volume  to  dissolve  the 
iodine,  the  solution  siphoned  off,  and  the  globule  repeatedly  washed  with  small 
quantities  of  water  to  remove  excess  of  peroxide,  then  titrated  with  thiosulphate. 
with  or  without  starch,  in  the  usual  way.  If  the  peroxide  is  not  completely 
removed  by  washing,  it  will  decompose  the  sodium  iodide  produced  in  the  titration, 
and  so  liberate  traces  of  iodine. 

The  results  obtained  by  Cook  in  mixtures  of  bromides,  iodides, 
and  chlorides  showed  about  99  per  cent,  of  the  iodine  present. 
Gooch    and    Browning|j    publish    a    method    of    determining 

«Mensel,  Z.  a.  C.  12,  137.  f  Z.  a.  C.  19,  19.  %  J.  C.  S.,  1885,  471. 

II  Amer.  Jour.  Science  39,  March,  1890,  also  C.  N.  61,  279. 


IODINE,  227 

iodine  in  halogen  salts  of  the  alkalies  which  gives  excellent  results, 
and  which  is  based  on  the  fact  that  arsenic  acid  in  strongly  acid 
solution  liberates  iodine,  becoming  itself  reduced  to  arsenious  acid, 
according  to  the  equation. 

H3  AsO4  +  2HI = H3AsO3 + H2O  + 12. 

A  series  of  very  careful  experiments  are  detailed  in  the  original 
paper,  the  outcome  of  the  whole  being  summarized  in  the  following 
process  : — 

METHOD  OF  PROCEDURE  :  The  substance  (which  should  not  contain  of  chloride 
more  than  an  amount  corresponding  to  0'5  gm.  of  sodium  chloride,  nor  of  bromide 
more  than  corresponds  to  0*5  gm.  of  potassium  bromide,  nor  of  iodide  much 
more  than  the  equivalent  of  0'5  gm.  of  potassium  iodide)  is  dissolved  in  water 
in  a  conical  beaker  of  300  c.c.  capacity,  and  to  the  solution  are  added  2  gm.  of 
potassium  binarsenate  dissolved  in  water,  20  c.c.  of  a  mixture  of  sulphuric 
acid  and  water  in  equal  volumes,  and  enough  water  to  increase  the  total  volume 
to  100  c.c.  or  a  little  more.  A  platinum  spiral  is  introduced,  a  trap  made  of 
a  straight  two-bulb  drying  tube,  cut  off  short,  is  hung  with  the  larger  end  down- 
ward in  the  neck  of  the  flask,  and  the  liquid  is  boiled  until  the  level  reaches  a  mark 
put  upon  the  flask  to  indicate  a  volume  of  35  c.c.  Great  care  should  be  taken 
not  to  press  the  concentration  beyond  this  point  on  account  of  the  double  danger 
of  losing  arsenious  chloride  and  setting  up  reduction  of  the  arsenate  by  the  bromide. 
On  the  other  hand,  though  35  c.c.  is  the  ideal  volume  to  be  attained,  failure  to 
concentrate  below  40  c.c.  introduces  no  appreciable  error.  The  liquid  remaining 
is  cooled  and  nearly  neutralized  by  sodium  hydrate  (ammonia  is  not  equally  good), 
neutralization  is  completed  by  potassium  bicarbonate,  an  excess  of  20  c.c.  of  the 
saturated  solution  of  the  latter  is  added,  and  the  arsenious  oxide  in  solution  is 
titrated  by  standard  iodine  in  the  presence  of  starch. 

With  ordinary  care  the  method  is  rapid,  reliable,  and  easily 
executed,  and  the  error  is  small.  In  analyses  requiring  extreme 
accuracy,  all  but  accidental  errors  may  be  eliminated  from  the 
results  by  applying  the  following  corrections.  These  corrections 
are  based  on  a  long  series  of  experiments,  which  cannot  well  be 
given  here,  but  the  results  may  be  stated  shortly  as  follows  : — 

When  no  chloride  or  bromide  is  present  the  iodine  may  be 
estimated  with  a  mean  error  of  0*2  mgm.  in  O5  gm.  or  so  of  the 
alkali  iodide.  When  sodium  chloride  is  present,  there  is  a  slight 
deficiency  in  iodine  which  is  proportional  to  the  amount  of  iodide 
decomposed.  For  about  0*56  gm.  of  potassium  iodide  and  O5  gm. 
of  sodium  chloride  the  deficiency  in  iodine  amounted  to  O0011  gm. 
When  the  iodide  is  decreased,  say  to  one-tenth  or  less,  the  deficiency 
falls  to  0*0002  gm.  The  presence  of  potassium  bromide  liberates 
traces  of  bromine,  .and  consequently  increases  the  AsO3,  and  gives 
apparent  excess  of  iodine,  the  mean  error  being  0*0008  gm.  for  0'5 
gm.  of  bromide. 

The  simultaneous  action  of  the  chloride  and  bromide  tends  of 
course  to  neutralize  the  error  due  to  each.  Thus,  in  a  mixture 
weighing  about  1*5  gm.  and  consisting  of  sodium  chloride,  potassium 
bromide,  and  potassium  iodide  in  equal  parts,  the  mean  error 
amounts  to  —0*0003  gm.  The  largest  error  in  the  series  is +0*0016 
gm.,  when  the  bromide  was  at  its  maximum,  and  no  chloride  was 

Q  2 


228  IODINE. 

present  ;  and  the  next  largest  was  —  0*0013  gm.,  when  the  chloride 
was  at  its  maximum  and  no  bromide  was  present. 

From  a  series  of  experiments  detailed  in  the  original  paper  it 
was  deduced  that  the  amount  of  iodine  to  be  added,  in  each  case, 
may  be  obtained  by  multiplying  the  product  of  the  weights  in 
grams  of  sodium  chloride  and  potassium  iodide  by  the  constant 
0*004 ;  and  the  amount  to  be  subtracted,  by  multiplying  the 
weight  in  grams  of  potassium  bromide  by  0*0016  ;  but  in  order  to 
make  use  of  these  corrections  the  approximate  amounts  of  these 
salts  present  must  be  known. 

3.     Titration  with  ^/i0  Silver  and  Thiocyanate. 

The  thiocyanate  and  silver  solutions  are  described  on  p.  145, 
et  seq. 

The  iodide  is  dissolved  in  300  or  400  times  its  weight  of  water 
in  a  well-stoppered  flask,  and  N/10  silver  delivered  in  from  the 
burette  with  constant  shaking  until  the  precipitate  coagulates, 
showing  that  silver  is  in  excess.  Ferric  indicator  and  nitric  acid 
are  then* added  in  proper  proportion,  and  the  excess  of  silver 
determined  by  thiocyanate  as  described  on  p.  145. 

4.     Oxidation  of  combined  Iodine  by  Chlorine  (G  o  1  f  i  e  r 
Besseyre    and    Dupre). 

This  wonderfully  sharp  method  of  determining  iodine  depends 
upon  its  conversion  into  iodic  acid  by  free  chlorine.  When  a  solution 
of  potassium  iodide  is  treated  with  successive  quantities  of  chlorine 
water,  iodine  is  liberated  at  first,  then  chloride  of  iodine  (IC1) 
formed.  If  starch,  chloroform,  benzole,  or  bisulphide  of  carbon 
be  added,  the  first  will  be  turned  blue,  while  any  of  the  others 
will  be  coloured  intense  violet.  A  further  addition  of  chlorine,  in 
sufficient  quantity,  produces  pentachloride  of  iodine  (IC15),  or 
rather,  as  water  is  present,  iodic  acid  (IO3H).  No  colouration  of 
the  above  substances  is  produced  by  these  compounds,  and  the 
accuracy  with  which  the  reaction  takes  place  has  been  made  use 
of  by  Golfier  Besseyre  and  Dupre,  independently  of  each 
other,  for  the  purpose  of  determining  iodine.  The  former 
suggested  the  use  of  starch  ;  the  latter  chloroform  or  benzole, 
with  very  dilute  chlorine  water.  Dupre's  method  is  preferable 
on  many  accounts. 

EXAMPLE  :  30  c.c.  of  weak  chlorine  water  were  put  into  a  beaker  with  potassium 
iodide  and  starch,  and  then  titrated  with  N/ioo  thiosulphate,  of  which  17  c.c. 
were  required. 

10  c.c.  of  solution  of  potassium  iodide  containing  O'OIO  gin.  of  iodine  wen- 
put  into  a  stoppered  bottle,  chloroform  added,  and  the  same  chlorine  water  as 
above  delivered  in  from  the  burette,  with  constant  shaking,  until  the  red  colour 
of  the  chloroform  had  disappeared  :  the  quantity  used  was  85'8  c.c.  The  excess 
of  chlorine  was  then  ascertained  by  adding  sodium  bicarbonate,  potassium  iodide 
and  starch.  A  slight  blue  colour  was  produced  ;  this  was  removed  by  N/ioo 
thiosulphate,  of  which  1'2  c.c.  was  used.  Now,  as  30  c.c.  of  the  chlorine  solution 
required  17  c.c.,  the  85'8  c.c.  required  48'6  c.c.  of  thiosulphate.  From  this, 


IODINE.  .229 

however,  must  be  deducted  the  1'2  c.c.  in  excess,  leaving  47*4  C.C.N/IQO  =4*74  c.c. 
°f  N/io  solution,  which  multiplied  by  0-00211,  the  one-sixth  of  TZJ&OZT  e(l-  U  e(lt 
of  iodio  acid  liberating  6  cq.  iodine),  gave  O'OIOO  gm.  iodine. 

Mo hr  suggests  a  modification  of  this  method  which  dispenses 
with  the  use  of  chloroform  or  other  similar  agent. 

The  weighed  iodine  compound  is  brought  into  a  stoppered  flask,  and  chlorine 
water  delivered  from  a  large  burette  until  all  yellow  colour  has  disappeared.  A 
drop  of  the  mixture  brought  in  contact  with  a  drop  of  starch  must  produce  no 
blue  colour  ;  sodium  bicarbonate  is  then  added  till  the  mixture  is  neutral  or 
slightly  alkaline,  together  with  potassium  iodide  and  starch ;  the  blue  colour  is 
then  removed  by  N/io  thiosulphate.  The  strength  of  the  chlorine  water  being 
known,  the  calculation  presents  no  difficulty. 

Mohr  obtained  by  this  means  T0101  gm.  iodine,  instead  of 
I -01  gm. 


5.     Oxidation  by  Permanganate  (R  e  i  n  i  g  e). 

This  process  for  determining  iodine  in  presence  of  bromides  and 
chlorides  gives  satisfactory  results. 

When  potassium  iodide  and  permanganate  are  mixed,  the  rose 
colour  of  the  latter  disappears,  a  brown  precipitate  of  manganic 
peroxide  results,  and  potassium  hydroxide  with  potassium  iodide 
remains  in  solution.  1  eq.  I(  =  126*92)  reacts  with  1  eq.  K9Mn2O8 
(  =  316-06),  thus— 

KI  +  K2Mn2O8 + H2O  =  KI03 + 2KOH  +  2Mn02. 

Heat  accelerates  the  reaction,  and  it  is  advisable,  especially  with 
weak  solutions,  to  add  a  small  quantity  of  potassium  carbonate  to 
increase  the  alkalinity.  No  organic  matter  must  be  present. 

The  permanganate  and  thiosulphate  solutions  required  in  the 
process  may  conveniently  be  of  N/10  strength,  but  their  reaction 
upon  each  other  must  be  definitely  fixed  by  experiment  as  follows  :— 
'2  c.c.  of  permanganate  solution  are  freely  diluted  with  water, 
a  few  drops  of  sodium  carbonate  added,  and  the  thiosulphate  added 
in  very  small  portions  until  the  rose  colour  is  just  discharged.  The 
slight  turbidity  produced  by  the  precipitation  of  hydra  ted  manganic 
oxide  need  not  interfere  with  the  observation  of  the  exact  point, 

METHOD  OF  PROCEDURE  :  The  iodine  compound  being  dissolved  in  water, 
and  always  existing  only  in  combination  with  alkali  or  alkaline  earthy  bases,  is 
heated  to  gentle  boiling,  rendered  alkaline  with  sodium  or  potassium  carbonate,  and 
permanganate  added  till  in  distinct  excess,  best  known  by  removing  the  liquid 
from  the  source  of  heat  for  a  minute,  when  the  precipitate  will  subside,  leaving  the 
upper  liquid  rose-coloured  ;  the  whole  may  then  l>e  poured  into  a  500-c.c.  flask, 
cooled,  diluted  to  the  mark,  and  100  c.c.  taken  out  for  titration  with  thiosulphate. 
The  amount  so  used,  being  multiplied  by  5,  will  give  the  proportion  required  for 
the  whole  liquid,  whence  can  be  calculated  the  amount  of  iodine.  To  prove  the 
accuracy  of  the  process  in  a  mixture  of  iodides,  bromides,  and  chlorides,  with 
excess  of  alkali,  the  following  experiment  was  made.  7  gm.  commercial  potassium 
bromide,  the  same  quantity  of  sodium  chloride,  with  1  gm.  each  of  potassium 
hydrate  and  carbonate,  were  dissolved  in  a  convenient  quantity  of  water,  and  heated 
to  boiling ;  permanganate  was  then  added  cautiously  to  destroy  the  traces  of  iodine 


230  IODINE. 

and  other  impurities  affecting  the  permanganate  so  long  as  decolouration  took 
place ;  the  slightest  excess  showed  a  green  colour  (manganatc).  To  the  mixture 
was  then  added  0'1246  gm.  pure  iodine,  and  the  titration  continued  as  des- 
cribed :  the  amount  found  was  0'125  gm.  I. 

With  systematic  solutions  of  permanganate  and  thiosulphate  the 
calculation  is  as  follows  : — 

1  c.c.  N/10  solution -0-012692  gm.  I. 

6.     By  Nitrous  Acid  and  Carbon  Bisulphide  (Fresenius), 

This  process  requires  the  following  standard  solutions  :— 

(a)  Potassium  iodide,  about  5  gm.  per  litre. 

(b)  Sodium   thiosulphate,    -£0   normal,    12 -4   gm.    per   litre,    or 
thereabouts. 

(c)  Nitrous  acid,  prepared  by  passing  the  gas  into  tolerably 
strong  sulphuric  acid  until  saturated. 

(d)  Pure  carbon  disulphide. 

(e)  Solution  of  sodium  bicarbonate,  made  by  dissolving  5  gm. 
of  the  salt  in  1  litre  of  water,  and  adding  1  c.c.  of  hydrochloric 
acid. 

METHOD  OF  PROCEDURE  :  The  strength  of  the  sodium  thiosulphate  in  relation 
to  iodine  is  first  ascertained  by  measuring  50  c.c.  of  the  iodide  solution  into 
a  500  c.c.  stoppered  flask,  then  about  150  c.c.  water,  20  c.c.  carbon  disulphide, 
then  dilute  sulphuric  acid,  and  lastly,  10  drops  of  the  nitrous  solution.  The 
stopper  is  then  replaced,  and  the  whole  well  shaken,  set  aside  to  allow  the  carbon 
disulphide  to  settle,  and  the  supernatant  liquid  poured  into  another  clean  flask. 
The  carbon  disulphide  is  then  treated  three  or  four  times  successively  with  water 
in  the  same  way  till  the  free  acid  is  mostly  removed,  the  washings  being  all  mixed 
in  one  flask  ;  10  c.c.  of  disulphide  are  then  added  to  the  washings,  well  shaken. 
and  if  at  all  coloured,  the  same  process  of  washing  is  carried  on.  Finally,  the 
two  quantities  of  disulphide  are  brought  upon  a  moistened  filter,  washed  till  free 
from  acid,  a  hole  made  in  the  filter,  and  the  disulphide  which  now  contains  all 
the  iodine  in  solution  allowed  to  run  into  a  clean  small  flask,  30  c.c.  of  the  sodium 
bicarbonate  solution  added,  then  brought  under  the  thiosulphate  burette,  and  the 
solution  allowed  to  flow  into  the  mixture  while  shaking  until  the  violet  colour  is 
entirely  discharged.  The  quantity  so  used  represents  the  weight  of  iodine  con- 
tained in  50  c.c.  of  the  standard  potassium  iodide,  and  may  be  used  on  that  basis 
to  ascertain  any  unknown  weight  contained  in  a  similar  solution. 

When  very  small  quantities  of  iodine  are  to  be  titrated,  weaker  solutions  and 
smaller  vessels  may  be  used. 

This  process  is  especially  useful  in  determining  small  amounts 
of  iodine  in  the  presence  of  chlorine  and  bromine,  as  in  mnjej^t 
waters. 

7.     By  N/10  Silver  Solution  and  Starch  Iodide  (Pisani). 

The  details  of  this  process  are  given  under  the  head  of  silver 
assay  and  are  of  course  simply  a  reversal  of  the  method  there  given. 
This  method  is  exceedingly  serviceable  for  determining  small 
quantities  of  combined  iodine  in  the  presence  of  chlorides  and 
bromides,  inasmuch  as  the  silver  solution  does  not  react  upon  these 
bodies  until  the  blue  colour  is  destroyed. 


IRON. 

IRON. 

Fe  =  55-85. 
Factors. 

N/io  permanganate,  dichromate. 


231 


1  c.c. 

or  thiosulphate 


=0-005585  Fe 
=0-007185  FeO 
=0-007985  Fe2O3 


DETERMINATION  IN  THE  FERROUS  STATE. 

1.     Verification  of  the  standard  solutions  of 
Permanganate  or  Dichromate. 

THE  determination  of  iron  in  the  ferrous  state  has  already  been 
incidentally  described  under  Analysis  by  Oxidation  and  Reduction, 
pp.  122  and  126.  The  present  and  following  sections  are  an 
amplification  of  the  methods  there  given,  as  applied  more 
distinctly  to  ores  and  products  of  iron  manufacture  ;  but  before 
applying  the  permanganate  or  dichromate  process  to  these 
substances,  and  since  many  operators  prefer,  with  reason,  to 
standardize  such  solutions  upon  metallic  iron,  especially  for  use 
in  iron  analysis,  the  best  method  is  given  on  p.  122.  The 
apparatus  used  is  shown  in  fig.  44. 

Instead  of  the  two  flasks,  many  operators  use  a  single  flask,  fitted 
with  caoutchouc  stopper,  through  which  a  straight  glass  tube  is 
passed,  fitted  with  an  india-rubber  slit  valve  (known  as  Bun  sen's 
valve),  which  allows  gas  or  vapour  to  pass  out,  but  closes  by 
atmospheric  pressure  when  the  evolution  ceases. 


Fig.  44. 


A  large  number  of  technical  operators  do  not  trouble  themselves 
to  arrange  any  apparatus  of  the  kind  described,  but  simply  dissolve 
a  weighed  quantity  of  wire  of  known  composition  in  a  conical 
beaker  covered  with  a  clock  glass  or  in  a  flask  covered  with  a  glass 
marble.  If  kept  from  draughts  of  cold  air  while  dissolving  so  as 
to  avoid  convection,  it  is  said  that  practically  no  oxidation  takes 
place.  The  following  plan  answers  well  :— 

Put  into  a  220  c.c.  flask  25  c.c.  of  water  and  heat  on  a  hot  plate 


232  IRON. 

till  boiling.  Then  drop  in  the  weighed  portion  (about  O25  gm.) 
of  iron  wire  and  immediately  add  15  c.c.  of  strong  HC1  and  cover 
the  flask  with  a  glass  marble.  In  4-6  minutes  the  wire  will  have 
dissolved  to  a  perfectly  colourless  solution,  which  should  then  be 
rinsed  out  into  a  basin  and  the  titration  with  dichromate  at  once 
proceeded  with. 

The  double  iron  salt  (p.  123)  is  a  very  convenient  material  for 
adjusting  standard  solutions,  but  it  must  be  most  carefully  made 
from  pure  materials,  dried  perfectly  in  the  granular  form,  and  kept 
from  the  light  in  small  dry  bottles,  well  closed. 

It  should  be  borne  in  mind  that  ferrous  compounds  are  much 
more  stable  in  sulphuric  than  in  hydrochloric  acid  solution,  and 
Avhenever  possible  sulphuric  acid  should  be  used  as  the  solvent. 
When  hydrochloric  acid  must  be  used,  and  permanganate  is  em- 
ployed, some  manganous  or  ammonium  sulphate  should  ^e  added 
unless  the  solution  is  very  dilute. 

Friend*  finds  that  accuracy  is  attained,  provided  that  the 
manganous  sulphate  present  is  not  less  than  2  gm.  in  200  c.c., 
if  (1)  the  titration  is  performed  slowly  with  constant  shaking, 
(2)  the  concentration  of  the  HC1  does  not  exceed  N/4. 

Jones  and  Jefferyf  recommend  a  modified  Zimmermann- 
Reinhardt  process  of  titration  in  which  the  following  reagents  are 
employed  : — 

MANGANESE  SOLUTION  :  Prepared,  according  to  Reinhardt's  formula,  by 
dissolving  200  gm.  of  crystallized  manganese  sulphate  in  1000  c.c.  of  water,  and 
adding  to  this  a  cooled  mixture  of  400  c.c.  of  concentrated  sulphuric  acid,  600  c.cv 
of  water,  and  1000  c.c.  of  phosphoric  acid  of  sp.  gr.  1*3.  STANNOUS  CHLORIDE  : 
50  gm.  of  the  crystallized  salt  and  100  c.c.  of  concentrated  hydrochloric  acid  made- 
up  to  1000  c.c.  with  water. 

MERCURIC  CHLORIDE  :  A  cold,  saturated  solution.  HYDROCHLORIC  ACID  : 
Acid  of  sp.  gr.  I'l.  The  slight  oxidizing  power  sometimes  possessed  by  the 
manganese  solution,  when  freshly  prepared,  may  be  easily  determined  and 
allowed  for  if  necessary  ;  but  it  has  been  found  that  this  oxidizing  effect  is-' 
immeasurable  when  the  solution  is  a  weak  old.  In  the  presence  of  hydro- 
chloric acid,  permanganate  titrations  give  results  which  are  a  little  too  highr 
even  when  the  manganese  solution  is  employed  ;  the  authors  find  that  the 
small  error  so  introduced  is  a  constant  one,  independent  of  the  amount  present,, 
and  therefore  easily  allowed  for.  The  titration  is  carried  out  as  follows : — 

The  iron  solution  (in  hydrochloric  acid  of  I'l  sp.  gr.)  is  reduced  with  the  smallest 
possible  excess  of  stannous  chloride,  and  10  c.c.  of  mercuric  chloride  are  added  to 
the  cold  solution,  the  mixture  being  then  allowed  to  stand  for  ten  minutes  to 
ensure  the  complete  conversion  of  the  stannous  salt  to  the  stannic  condition,  no 
appreciable  oxidation  of  the  reduced  iron  occurring  in  the  meantime.  A  quantity 
of  water,  which  may  vary  between  400  and  1000  c.c.  without  appreciable  effect  on 
the  titration,  is  tken  mixed,  in  a  capacious  bowl,  with  a  volume  of  the  manganese 
solution  equal  to  that  of  the  hydrochloric  acid  present  in  the  assay,  and  the 
mixture  tinted  with  permanganate,  of  which  one  drop  should  suffice.  The- 
ferrous  solution  is  then  transferred  to  the  bowl,  together  with  the  rinsings  of  its 
containing  vessel,  and  the  permanganate  added,  drop  by  drop,  with  constant 
stirring,  iintil  the  end  point  is  reached.  O'l  c.c.  is  then  deducted  from  the  burette 
reading,  and  the  amount  of  iron  present  is  calculated  by  reference  to  the  known 
titre  of  the  permanganate,  as  determined  by  titration  against  a  ferrous  solution! 
free  from  hydrochloric  acid. 

*  Chem.  Soc.  Trans.,  1909,  95,  1228. 
t  Analyst,  1909,  34,  306.    See  also  note  p.  12 1. 


IRON.  233- 

2.     Reduction  of  Ferric  Compounds  to  the  Ferrous  State. 

This  may  be  accomplished  by  metallic  zinc  or  magnesium,  for  use  with 
permanganate,  or  by  stannous  chloride  or  an  alkali  sulphite  for  dichromate- 
solution.  The  magnesium  method  is  elegant  and  rapid,  but  costly.  In  the- 
case  of  zinc  being  used,  the  metal  must  either  be  free  from  iron  or,  if  it  contain 
any,  the  exact  quantity  must  be  known  and  allowed  for  ;  and  further,  the  pieces 
of  zinc  used  must  be  entirely  dissolved  before  the  solution  is  titrated.*  The 
solution  to  be  reduced  by  zinc  should  not  contain  more  than  0'15  gm.  Fe  per 
250  c.c.,  and  for  this  quantity  about  10  gm.  of  Zn  and  25  c.c.  H2S04  are  advisable  ; 
when  the  zinc  is  all  dissolved,  the  whole  should  be  boiled  with  exclusion  of  air,, 
then  cooled  rapidly  before  titration  with  the  permanganate.  In  the  case  of 
stannous  chloride  the  solution  must  be  clear,  and  is  best  made  to  contain  10  to- 
15  gm.  per  litre,  as  directed  011  p.  128.  The  point  of  exact  reduction  in  the  boiling 
hot  and  somewhat  concentrated  acid  liquid  may  be  known  very  closely  by  the 
discharge  of  colour  in  the  ferric  solution  ;  but  may  be  made  sure  by  the  use  of 
a  saturated  aqueous  solution  of  mercuric  chloride  as  mentioned  on  p.  127. 

It  is  exceedingly  difficult  to  hit  the  exact  point  of  reduction  so 
that  there  shall  be  neither  excess  of  tin  nor  unreduced  iron,  and 
technical  iron  analysts  now  almost  universally  use  mercuric  chloride- 
as  a  precaution  against  excess  of  tin  solution.  The  general  method 
of  procedure  is  to  dissolve  the  material  in  diluted  hydrochloric  acid 
(1  acid  2  water)  in  a  conical  beaker  moderately  heated  over  a  rose 
burner  ;  when  solution  is  complete  the  sides  of  the  vessel  are  washed 
clown  with  hot  water,  the  liquid  brought  to  gentle  boiling,  and 
strong  tin  solution  added  from  a  dropping  bottle  until  the  colour 
of  the  iron  solution  is  nearly  discharged,  a  dilute  tin  solution  i& 
then  dropped  in  until  all  colour  has  disappeared,  and  there  is  a 
decided  slight  excess  of  tin.  Cold  air-free  water  is  then  washed 
over  the  sides  of  the  beaker,  the  vessel  covered  with  a  clock  glass 
placed  in  a  bowl  of  cold  water  and  allowed  to  cool,  and  a  slight 
excess  of  the  mercuric  solution  is  then  added,  and  the  titration 
with  dichromate  is  at  once  completed  in  the  usual  way  (see  pp. 
126-128). 

Some  technical  operators  prefer  to  use  sodium  sulphite  or 
ammonium  bisulphite  for  the  reduction.  If  sodium  sulphite  be 
used,  the  solution  of  iron  must  not  be  too  acid  and  should  be  dilute, 
say  a  volume  of  half  a  litre  for  |  gm.  of  Fe.  The  sulphite  is  added 
and  the  flask  gently  heated  till  the  liquid  is  colourless.  It  is  then 
boiled  briskly  till  all  SO2  is  dissipated ;  when  cooled  it  is  ready  for 
titration  with  dichromate.  In  the  case  of  ores  containing  titanium 
it  is  preferable  to  avoid  the  use  of  zinc  for  reduction,  as  it  also- 
reduces  the  titanium  more  or  less  ;  alkali  sulphite  does  not. 

The  procedure  with  ammonium  bisulphite  is  as  follows  : — f  To  the  acid  solution- 

*Beebe  (C.  A".  53,  2fi9)  suggests  the  following  convenient  arrangement: — A  strip 
of  thin  platinum  foil,  1  in.  square,  is  perforated  all  over  with  pin  holes,  then  bent  into 
a  (J  form,  and  the  ends  connected  with  platinum  wire  so  as  to  form  a  basket.  In  this 
is  placed  a  piece  of  amalgamated  zinc  and  the  whole  suspended  by  a  stout  platinum 
wire  in  the  reducing  flask.  When  lowered  into  the  solution,  another  strip  of  platinum 
foil,  2  in.  square,  is  dropped  in  and  leaned  against  the  wire  carrying  the  basket;  a  very 
free  evolution  of  hydrogen  is  then  obtained  from  the  foil.  When  the  reduction  is  com- 
plete, the  basket  is  lifted  out  and  well  washed  into  the  beaker  containing  the  liqiiid  to- 
be  treated.  C.  Jones's  redactor  is  often  used,  as  described  on  page  234.  Another 
method  with  zinc  dust  is  shown  on  the  next  page. 

t  Austen,  C.  A'.  46,  287. 


234  IRON. 

of  the  ore  or  metal,  diluted  and  filtered,  ammonia  is  added  until  a  faint  precipitate 
of  ferrie  hydroxide  is  produced.  This  is  re-dissolved  with  a  few  drops  of  HC1,  and 
some  strong  solution  of  bisulphite  added,  in  the  proportion  of  about  1  c.c.  for 
each  O'l  gm.  of  ore,  or  O'Oo  gm.  Fe.  The  mixture  is  well  stirred,  boiling  water 
added,  then  acidified  with  dilute  sulphuric  acid,  and  boiled  for  half  an  hour  :  it 
is  then  ready  for  titration. 

D.  J.  Carnegie*  points  out  the  value  of  zinc  dust  for  the  rapid  reduction  of 
ferric  solutions,  and  suggests  the  following  method  of  carrying  it  out. 

The  bottom  of  a  dry  and  narrow  beaker  is  covered  with  zinc  dust  sifted  through 
muslin.  A  known  volume  of  ferric  solution,  previously  nearly  neutralized  with 
ammonia,  is  placed  in  the  beaker  and  shaken  with  the  zinc  dust ;  then  a  known 
volume  of  dilute  sulphuric  acid  is  added  and  shaken  for  a  few  moments.  The 
reduction  is  much  more  rapid  in  neutral  than  in  acid  solutions,  but  of  course 
acid  in  this  case  must  be  present  in  excess  to  keep  the  iron  in  solution. 
Carnegie  withdraws  a  portion  of  the  reduced  solution  from  the  undissolved 
zinc  by  help  of  a  filter,  such  as  is  described  on  p.  19,  and,  as  measured  volumes 
have  been  used,  an  aliquot  part  taken  with  a  pipette  may  be  at  once  titrated,  and 
the  amount  of  iron  found. t 

Clemens  JonesJ  in  a  paper  read  before  the  American  Institute  of  Mining 
Engineers,  adopts  the  plan  suggested  by  Carnegie,  and  has  designed  a  special 
apparatus  for  filtering  the  ferric  solution  through  a  column  of  zinc  dust.  This 
arrangement  gives  complete  reduction  in  a  very  short  period  of  time,  and  is  service- 
able where  a  large  number  of  titrations  have  to  be  carried  oiv 


DETERMINATION  OF  IRON  IN  THE  FERRIC  STATE. 

1.     Direct  Titration  of  Iron  by  Stannous  Chloride  (see  p.  127). 
2.     Direct  Titration  of  Iron  by  Titanous  Chloride. 

Titanous  chloride  (TiCl3),  which  was  employed  in  the  first  place 
as  a  reducing  agent  for  the  determination  of  ferric  iron,  has  lately 
received  much  wider  application  in  the  domain  of  volumetric 
analysis,  it  being  now  successfully  used  for  the  quantitative 
determination  of  nitro-compounds,  the  azo-dyes  and  dyestuffs,  etc.jj 

The  readily  oxidizable  solution  of  titanous  chloride  is  prepared 
and  stored  as  follows  : — 

50  c.c.  commercial  titanous  chloride  (20  %  solution)  and  100  c.c. 
cone.  HC1  are  boiled  together  in  a  small  flask  for  about  a  minute, 
the  mixture  is  cooled,  made  up  to  about  2J-  litres,  thoroughly 
mixed  by  shaking,  and  the  storage  bottle  A  (see  fig.  45)  filled  up  to 
the  neck  with  the  solution. 

*  J.  C.  S.  53,  468. 

t  Commercial  zinc  dust  is  probably  a  by-product  in  ziuc  manufacture,  and  cannot 
therefore  be  obtained  pure.  Samples  examined  by  myself,  and  apparently  by  others 
also,  do  not,  however,  contain  much  iron,  btit  a  good  deal  of  zinc  oxide,  with  traces  of 
cadmium  and  lead.  Carnegie  states  that  the  oxide  may  be  removed  by  repeatedly 
digesting  with  weak  acid,  or  better  still,  by  treatment  with  ammonium  chloride  and 
ammonia,  the  well-washed  dust  being  finally  dried  on  porous  tiles  in  a  vacuum.  I  find 
that  by  washing  once  with  strong  alcohol  after  the  water  and  finally  with  other,  the 
dust  may  be  rapidly  dried  in  good  condition,  and  when  a  quantity  of  such  purified 
dust  is  obtained,  the  amount  of  iron  in  it  may  easily  be  determined  once  for  all,  and 
allowed  for  in  titration.  Good  zinc  dust  is  undowbtedlj-  a  valuable  reagent  in  a 
laboratory  for  other  purposes  besides  iron  titrations. 

J  C.  N.  80,  93. 

'i  See  Knecht  and  Hibbert  "New  Reduction  Methods  in  Volumetric  Analysis," 
1910. 


IRON. 


233 


B    E 


Fig.  45. 


The  mode  of  connection  of 
this  bottle  with  the  burette 
(B)  will  readily  be  seen  by 
the  fig.  45.  Rubber  stoppers 
are  used  and  at  Dx  and  D2 
are  valves  constructed  of 
glass  rod  and  rubber  tubing 
as  described  on  p.  13.  C  is 
a  hydrogen  generator  con- 
sisting of  a  glass  cylinder 
half  filled  with  dilute  HC1 
(1  :  1)  into  which  dips  an 
inner  tube  contracted  at  the 
bottom  to  a  narrow  aperture 
and  filled  with  ordinary 
granulated  zinc.  The  hydro- 
gen generated  when  D2  is( 
opened  passes  into  the 
storage  bottle  and  burette. 
By  opening  the  valve  Dt  the 
titanous  chloride  flows  into 
the  burette  and  rises  into 
the  tube  above  it,  then  by 
opening  D2  the  solution  is 
entirely  run  out  and  the 
hydrogen  allowed  to  escape 
for  some  minutes.  The 
burette  can  now  be  filled  up 
to  the  zero  mark  and  the 
apparatus  is  ready  for  use. 

Standardization  of  the 
Solution.  Dissolve  3-511  gm. 
of  the  purest  ferrous 
ammonium  sulphate  in 
water,  add  about  100  c.c. 
5  N.H2SO4,  and  make  up 
the  solution  to  250  c.c.  in 
a  measuring  flask.  Pipette 
25  c.c.  ( =0-05  gm.  Fe)  of  the 
well  mixed  solution  into  a 
flask  and  carefully  oxidize 
with  permanganate  of  about 
N/50  strength  until  a  faint 
pink  tinge  is  obtained.  Then 
add  a  large  excess*  of 
potassium  sulphocyanate  and 
run  in  the  titanous  chloride 
from  the  burette  until  the 


*  A  large  excess  renders  the  enc -reaction  much  sharper. 


236  IRON. 

red  colour,  due  to  ferric  sulphocyanate,  has  entirely  disappeared. 
Suppose  for  example  that  25  c.c.  of  the  iron  solution,  after 
oxidation,  require  26'3  c.c.  titanous  chloride  to  reduce  it,  then 

1  c.c.  Ti  C13 =0*0019  gm.  Fe. 

Solutions  of  ferric  iron,  to  which  an  excess  of  potassium 
sulphocyanate  is  added  as  indicator,  can  be  titrated  directly  with 
the  titanous  solution.  It  is  immaterial  whether  the  iron  is  present 
in  sulphuric  or  hydrochloric  acid  solution,  but  the  presence  of 
some  mineral  acid  is  essential,  as  otherwise  the  indicator  is  not 
sensitive.  Ferrous  iron  must  first  be  oxidized  to  the  ferric  state. 
This  is  generally  done  as  follows  : — 

To  a  measured  volume  of  the  solution  add  ammonia  and  hydrogen 
peroxide,  and  remove  the  excess  of  oxygen  by  boiling  the  liquid 
for  5  to  10  minutes.  Then  add  HC1  in  quantity  more  than  sufficient 
to  dissolve  the  ferric  hydrate  ;  or  : 

To  the  ferrous  solution  add  a  crystal  of  KC1O3  and  some  HC1, 
boil  down  to  a  small  bulk,  add  water  or  HC1,  again  evaporate 
down,  and  when  the  excess  of  chlorine  has  thus  been  removed  the 
solution  is  ready  for  titration. 

The  reaction  which  takes  place  is 

FeCl3  +  TiCl3  =  FeCl2  +  TiCl4. 


3.     Titration  by  Sodium  Thiosulphate. 

Scherer  first  suggested  the  direct  titration  of  iron  with 
thiosulphate,  which  latter  was  added  to  a  solution  of  ferric 
chloride  until  no  further  violet  colour  was  produced.  This 
was  found  by  many  to  be  inexact;  but  Kremer*  has  made  a 
series  of  practical  experiments,  the  result  of  which  is  that  the 
following  modified  method  can  be  recommended. 

The  reaction  which  takes  place  is  such  as  to  produce  ferrous 
chloride,  sodium  tetrathionate,  and  chloride.  2Na2S2O3+Fe2Cl6  + 
2HCl=H2S4O6  +  4NaCl  +  2FeCl2.  The  thiosulphate,  which  may 
conveniently  be  of  N/10  strength,  is  added  in  excess,  and  the  excess 
determined  by  iodine  and  starch. 

METHOD  OF  PROCEDURE  :  The  iron  solution,  containing  not  more  than  1  per 
cent,  of  metal,  which  must  exist  in  the  ferric  state  without  any  excess  of  oxidizing 
material  (best  obtained  by  adding  excess  of  hydrogen  peroxide,  then  boiling  till 
the  excess  is  expelled),  is  moderately  acidified  with  hydrochloric  acid,  sodium 
acetate  added  till  the  mixture  is  red,  then  dilute  hydrochloric  acid  until  the  red 
colour  disappears  ;  then  diluted  till  the  iron  amounts  to  1  or  J  per  cent.,  and 
N/io  thiosulphate  added  in  excess,  best  known  by  throwing  in  a  particle  of 
potassium  sulphocyanide  after  the  violet  colour  produced  has  disappeared  ;  if  any 
red  colour  appears,  more  thiosulphate  must  be  added.  Starch  and  N/1O  iodine 
are  then  used  to  ascertain  the  excess.  A  mean  of  several  experiments  gave 
1 00  -Ofi  Fe,  instead  of  100. 

J.  T.  Nor  ton  f  has  carefully  experimented  on  this  method  and 

*  Journ.  f.  Pract-  CJicm.  84,  339.  |  J-  Am.  C.  S.  1899,  p.  25. 


IRON.  237 

found  that  it  needs  careful  management  to  ensure  accurate  results. 
The  volume  of  dilution  and  amount  of  acid  must  be  carefully 
regulated,  so  also  must  the  amount  of  thiosulphate  used  in  excess. 
There  should  always  be  at  least  15  c.c.  of  thiosulphate  in  excess 
with  O'l  gm.  of  ferric  oxide  and  1  c.c.  of  strong  hydrochloric  acid 
in  not  less  than  400  or  500  c.c.  of  freshly  boiled  water  used  for 
dilution.  The  time  occupied  in  reduction  should  be  as  short  as 
possible. 

METHOD  OF  PROCEDURE  :  In  treating  ferric  oxide,  the  following  method  is 
recommended  : — Dissolve  an  amount  not  exceeding  0'2  gm.  of  the  oxide  in  2  c.c. 
of  hydrochloric  acid,  evaporate  to  a  pasty  mass,  dilute  to  about  800  c.c.  with 
freshly-boiled  water,  add  a  drop  of  potassium  sulphocyanide,  and  into  this 
solution  run  50  c.c.  of  N/io  sodium  thiosulphate  ;  allow  the  liquid  to  stand  until 
perfectly  colourless,  and  determine  the  excess  of  thiosulphate  by  N/io  iodine  and 
starch.  For  quantities  of  iron  oxide  up  to  0*2  gm.  this  process  is  quick  and  most 
accurate  ;  when  care  is  taken  to  preserve  the  relations  of  acidity  and  dilution, 
twice,  the  amount  of  ferric  oxide  mentioned  above  may  be  handled. 

The  accuracy  of  the  process  is  not  interfered  with  by  the  presence 
of  salts  of  the  alkalies,  strontia,  lime,  magnesia,  alumina,  or 
manganous  oxide  ;  neither  do  salts  of  nickel,  cobalt,  or  copper, 
unless  their  quantity  is  such  as  to  give  colour  to  the  solution. 

The  process  is  a  rapid  one,  and  with  care  gives  very  satisfactory 
results. 


4.     Determination  with  Iodine  and  Sodium  Thiosulphate. 

When  ferric  chloride  is  digested  with  potassium  iodide  in  excess, 
iodine  is  liberated,  which  dissolves  in  the  free  potassium  iodide  — 


METHOD  OF  PROCEDURE  :  The  hydrochloric  acid  solution,  which  must 
contain  no  free  chlorine  or  nitric  acid,  and  all  the  iron  in  the  ferric  state,  is  nearly 
neutralized  with  caustic  potash  or  soda,  transferred  to  a  well-stoppered  flask, 
and  an  excess  of  strong  solution  of  potassium  iodide  added  ;  it  is  then  heated 
to  50°  or  60°  C.  on  the  water  bath,  closely  stoppered,  for  about  twenty  minutes  ; 
the  flask  is  then  cooled,  starch  added,  and  titrated  with  thiosulphate  till  the 
blue  colour  disappears.  This  process  gives  very  satisfactory  results  in  the  absence 
of  all  substances  liable  to  affect  the  potassium  iodide,  such  as  free  chlorine  or 
nitric  acid,  and  is  particularly  serviceable  for  determining  small  quantities  of  iron. 

Instead  of  starch  another  indicator  is  suggested  by  H  a  s  w  e  1  1  .  *  To  an  aliquot 
portion  of  the  solution  of  the  ferric  salt  a  little  cupric  sulphate  and  salicylic  acid 
are  added  as  an  indicator,  and  then  from  a  burette  standard  thiosulphate  solution 
is  run  in  until  the  violet  coloration  is  entirely  destroyed.  The  liquid  is  then  titrated 
back  with  the  solution  of  the  ferric  salt,  until  the  colour  just  reappears. 

Josephf  has  shown  that  good  results  may  be  obtained  by  the 
following  simplified  procedure  :— 

To  the  ferric  solution,  acidified  with  HC1  (the  quantity  used  seems  to  be  of  no 
practical  importance),  add  a  few  grams  of  KI,  and  at  once  titrate  the  iodine 
liberated  with  N/io  thiosulphate.  In  those  frequent  cases  where  the  substance 

*  Zeits.  /.  angew.  Chem.  49,  1265.  t  J-  S.  C.  I.,  1910,  £9,  187. 


238  IRON. 

must  be  dissolved  in  HC1  and  potassium  chlorate,  on  account  of  the  presence  of 
ferrous  iron,  the  liquid  is  boiled  almost  to  dryness  before  titration. 

1  c.c.  N/io  thiosulphate  =0'005585  gm.  Fe. 

The  solution  is  standardized  either  with  iodine  or  with  iron  alum.     Excellent 
results  are  recorded. 


5.     Determination  of  Iron  by  Colorimetric  Methods. 

These  methods,  which  approach  in  delicacy  the  Nessler  test  for 
ammonia,  are  applicable  for  very  minute  quantities  of  iron,  such  as 
may  occur  in  the  ash  of  bread  when  testing  for  alum,  water 
residues,  alloys,  and  similar  cases. 

It  is  first  necessary  to  have  a  standard  solution  of  iron  in  the  ferric  state,  which 
can  be  made  by  dissolving  1-004  gm.  of  iron  wire  in  nitro-hydrochloric  acid, 
precipitating  with  ammonia,  washing  and  re-dissolving  the  ferric  oxide  in  a  little 
hydrochloric  acid,  then  diluting  to  one  litre.  1  c.c.  of  this  solution  contains 
1  milligram  of  pure  iron  in  the  form  of  ferric  chloride.  It  may  be  further  diluted, 
when  required,  so  as  to  contain  ^  milligram  in  a  c.c.,  and  this  is  the  best 
standard  to  use.*  The  solution  for  producing  the  colour  is  either  potassium 
ferrocyanide  or  thiocyanate  dissolved  in  water  (1  :  20). 

PROCEDURE  WITH  FERROCYANIDE  :  The  material  containing  a  minute 
unknown  quantity  of  iron,  say  a  water  residue,  is  dissolved  in  hydrochloric  acid, 
and  diluted  to  100  c.c.,  or  any  other  convenient  measure.  10  c.c.  are  measured 
into  a  white  glass  cylinder  marked  at  100  c.c.,  1  c.c.  of  concentrated  nitric  acid 
added  (the  presence  of  free  acid  is  always  necessary  in  this  process),  then  diluted 
to  the  mark  with  distilled  water,  and  well  stirred. 

1  c.c.  of  ferrocyanide  solution  is  then  added,  well  mixed,  and  allowed  to  stand 
at  rest  a  few  minutes  to  develop  the  colour. 

A  similar  cylinder  is  then  filled  with  a  mixture  of  say,  1  c.c.  of  standard  iron 
solution,  1  c.c.  nitric  acid  and  distilled  water,  and  1  c.c.  ferrocyanide  added  ;  if 
this  does  not  approach  the  colour  of  the  first  mixture,  other  quantities  of  iron 
are  tried  until  an  exact  similarity  of  colour  occurs.  The  final  adjustment  is 
made  by  pouring  out  a  little  of  the  stronger  solution  into  a  measuring  cylinder 
until  the  tints  are  exactly  equal.  The  calculation  is  made  exactly  as  in 
Nesslerization.  The  exact  strength  of  the  iron  solution  being  known,  it  is  easy 
to  arrive  at  the  quantity  of  pure  iron  present  in  the  substance  examined,  and  to 
convert  it  into  its  state  of  combination  by  calculation. 

Carter  Bellf  adopts  the  following  plan  in  the  case  of  waters: — 
70  c.c.  of  the  water  are  evaporated  to  dryness  in  a  platinum  dish, 
and  gently  ignited  to  burn  off  organic  matters.  1  c.c.  of  dilute 
nitric  acid  (50  c.c.  of  strong  acid  in  a  litre)  is  then  poured  over  the 
residue  from  a  pipette,  and  evaporated  to  dryness  on  the  water 
bath  ;  the  residue  is  then  dissolved  in  1  c.c.  of  a  10  per  cent. 
hydrochloric  acid,  5  or  10  c.c.  of  distilled  water  added,  the  solution 
filtered  through  a  small  filter,  washed,  made  up  to  50  c.c.  in 
a  Nessler  glass,  and  finally  mixed  with  1  c.c.  each  of  ferrocyanide 
solution  and  nitric  acid. 

*  A  solution  of  this  strength  can  also  be  made  by  weighing  0-7022  gm.  of  pure 
ferrous  ammonium-sulphate  (p.  123),  dissolving  in  water,  acidifying  with  sulphuric 
acid,  adding  sufficient  permanganate  solution  to  convert  the  iron  exactly  into  ferric 
salt,  then  diluting  to  1  litre.  Hydrogen  peroxide  may  also  be  used  in  place  of 
permanganate,  taking  care  to  dissipate  the  excess  by  boiling. 

t  J.  S.  C.  I.  8,  175. 


IRON    ORES.  239 


With  Thiocyanate. — Thomson*  recommends  this  method  as 
being  specially  available  in  the  presence  of  other  ordinary  metals 
and  organic  matters,  silver,  copper,  and  cobalt  being  the  only 
interfering  substances.  The  delicacy  is  said  to  be  such  that  1  part 
of  iron  can  be  recognised  in  50  million  parts  of  water.  The  presence 
of  free  mineral  acids  adds  greatly  to  the  sensitiveness.  The 
standard  ferric  solution  may  be  the  same  as  for  ferrocyanide  ; 
and  in  preparing  the  material  for  titration  the  weighed  quantity  is 
dissolved  in  an  appropriate  acid,  evaporated  nearly  to  dryness, 
taken  up  with  water,  converted  into  the  ferric  state  by  cautious 
addition  of  permanganate,  then  diluted  with  water  to  a 
measured  volume,  and  an  aliquot  portion  taken  for  titration. 

The  standard  iron  solution  used  by  Thomson=T1Q  mgm.  Fe 
per  c.c.  (G'7022  gm.  double  iron  salt  [oxidized]  per  litre.) 

EXAMPLE  :  Into  two  colourless  glass  cylinders  marked  at  100  c.c.  pour  5  c.c. 
of  nitric  or  hydrochloric  acid  (1  :  5),  together  with  15  c.c.  of  dilute  thiocyanate, 
and  to  one  glass  a  measured  volume  of  the  solution  to  be  tested  ;  fill  up  both 
glasses  to  the  mark  with  pure  water.  If  iron  be  present,  a  blood  red  colour 
more  or  less  intense  will  be  produced.  Standard  iron  is  then  cautiously  added 
from  a  burette  to  the  other  glass  till  the  colour  agrees.  The  quantity  of  Fe  taken 
should  not  require  more  than  2  or  3  c.c.  of  the  standard  to  equal  it,  or  the  colour 
will  be  too  deep  for  comparison. 

If  other  metals  are  present  which  forms  two  sets  of  salts,  they 
must  be  in  the  higher  state  of  oxidation,  or  the  colour  is  destroyed. 
Oxalic  acid  also  destroys  it.  Examples  in  the  presence  of  a  great 
variety  of  metals  show  very  good  results. 


IRON    ORES,    IRON    AND    STEEL. 

THE  great  desideratum  in  the  analysis  of  iron  ores  is  to  get  them 
into  the  finest  possible  state  of  division,  and  ten  minutes'  hard 
work  with  the  agate  mortar  will  often  save  hours  of  treatment  of 
the  material  with  acids.  The  operator  of  experience  can  generally 
tell  if  the  ore  to  be  examined  will  dissolve  in  acids.  Some  clay 
ironstones  and  brown  haematites  contain  organic  matters,  and 
they  are  best  first  roasted  in  an  open  platinum  crucible,  gradually 
raising  the  heat  to  redness  ;  this  course  is  advisable  also  when  an 
ore  contains  pyrites  ;  this  latter  is  easily  converted  to  Fe2O3  by 
roasting.  The  proportion  in  iron  ores  is  generally  under  half 
a  per  cent.  Some  ores  give  a  residue  in  any  case  by  treatment 
with  HC1  ;  this  should  be  separated  by  filtration  and  fused  with 
fusion  mixture,  which  will  render  all  the  iron  in  a  soluble  state. 
In  the  analysis  of  iron  ores  it  is  very  often  necessary  to  determine 
not  only  the  total  amount  of  iron,  but  also  the  state  in  which  it 
exists  ;  for  instance,  magnetic  iron  ore  consists  of  a  mixture  of  the 
two  oxides  in  tolerably  definite  proportions,  and  it  is  sometimes 
advisable  to  know  the  quantities  of  each. 

In  order  to  prevent,  therefore,  in  such  cases,  the  further  oxidation 

*./.  C.  S.  1885,  493. 


240  IRON    ORES. 

of  the  ferrous  oxide,  the  little  flask  apparatus  (fig.  46)  adopted  by 
Mohr  is  recommended,  or  that  shown  in  fig.  44  is  equally  service- 
able. 

The  left-hand  flask  contains  the  weighed  ore  in  a  finely  powdered  state,  to 
which  tolerably  strong  hydrochloric  acid  is  added ;  the  other  flask  contains 
•distilled  water  only,  the  tube  from  the  first  flask  reaching  to  the  bottom  of  the 
second.  When  the  ore  is  ready  in  the  flask  and  the  tubes  fitted,  hydrochloric 
acid  is  poured  in,  and  a  few  grains  of  sodium  bicarbonate  added  to  produce  an 
evolution  of  CO2.  The  air  of  the  flask  is  thus  expelled,  and  as  the  acid  dissolves 
the  ore,  the  gases  evolved  drive  out  in  turn  the  C02,  which  is  partly  absorbed  by 
the  water  in  the  second  flask.  When  the  ore  is  all  dissolved,  the  lamp  is  removed, 
.and  the  water  immediately  rushes  out  of  the  second  flask  into  the  first,  diluting 
and  cooling  the  solution  of  ore,  so  that,  in  the  majority  of  cases;  it  is  ready  for 
immediate  titration.  If  not  sufficiently  cool  or  dilute*,  a  sufficient  quantity  of 
boiled  and  cooled  distilled  water  is  added. 


Fig.  46. 

When  the  total  amount  of  iron  present  in  any  sample  of  ore  has 
to  be  determined,  it  is  necessary  to  reduce  any  peroxide  present  to 
the  state  of  protoxide  previous  to  titration. 

Reduction  to  the  Ferrous  state  may  be  effected  by  sodium 
sulphite  in  dilute  solution,  but  not  so  with  stannous  chloride  ;  the 
latter  must  be  used  in  a  boiling  and  concentrated  solution  strongly 
-acidified  with  hydrochloric  acid.  Most  technical  operators  now 
use  the  tin  method,  which,  by  the  help  of  mercuric  chloride  as 
described  on  p.  127,  is  rendered  both  rapid  and  trustworthy. 
With  both  the  sulphite  and  the  tin  methods,  dichromate  is  the 
titrating  solution  invariably  used.  When  permanganate  is  to  be 
used  for  titration,  the  reduction  is  always  best  made  with  zinc  or 
magnesium  in  sulphuric  or  very  weak  hydrochloric  acid  solution. 
With  dichromate,  the  best  reducing  agent  is  either  pure  sodium 
.sulphite,  ammonium  bisulphite,  or  stannous  chloride. 


IRON    ORES.  241 

1.  Red     and     Brown     Haematites. — Red     haematite     consists 
generally  of  ferric  oxide  accompanied  with  matters  insoluble  in 
acids.     Sometimes,  however,  it  contains  phosphoric  acid,  manganese, 
and  earthy  carbonates. 

Brown  haematite  contains  hydrated  ferric  oxide,  often  accom- 
panied by  small  quantities  of  ferrous  oxide,  manganese,  and 
alumina  ;  sometimes  traces  of  copper,  zinc,  nickel,  cobalt,  with 
lime,  magnesia,  and  silica  ;  occasionally  also  organic  matters. 

In  cases  where  the  total  iron  only  has  to  be  determined,  it  is 
-advisable  to  ignite  gently  (about  0*5  gm.)  to  destroy  organic 
matters,  then  treat  with  strong  hydrochloric  acid  at  near  boiling 
heat  till  all  iron  is  dissolved,  and  in  case  ferrous  oxide  is  present 
add  small  quantities  of  potassium  chlorate,  afterwards  evaporating 
to  dryness  to  dissipate  free  chlorine  ;  then  dissolve  the  iron  with 
hot  dilute  hydrochloric  acid,  filter,  and  make  up  to  a  given  measure 
for  reduction  and  titration. 

In  some  instances  the  insoluble  residue  persistently  retains  some 
iron  in  an  insoluble  form  ;  when  this  occurs,  resort  must  be  had  to 
fusing  the  residue  with  fusion  mixture,  followed  by  solution  in 
hydrochloric  acid. 

2.  Magnetic  Iron  Ore. — The  ferrous  oxide  is  determined  first 
by  means  of  the  apparatus  fig.  44  or  46.     The  ore  (about  1  gm.)  is 
put  into  the  vessel  in  a  state  of  very  fine  powder,  strong  hydro- 
chloric acid  added,  together  with  a  few  grains  of  sodium  bicarbonate, 
and  heat  applied  gently  until  the  ore  is  dissolved,  then  diluted  if 
necessary,     and     titrated     with     dichromate     or     permanganate. 
Technical  operators  generally  use  only  a  covered  beaker  or  a  flask 
closed  with  a  glass  marble. 

3.  Spathose  Iron  Ore. — The  total  amount  of  ferrous  oxide  in 
this  carbonate  is  ascertained  directly  by  solution  in  hydrochloric 
acid  ;  as  the  carbonic  acid  evolved  is  generally  sufficient  to  expel 
all  air,  the  tube  dipping  under  water  may  be  dispensed  with.     If 
the  ore  contains  pyrites  it  should  be  first  roasted,  but  this  of  course 
converts  the  ferrous  carbonate  into  Fe2O3. 

As  the  ore  contains,  in  most  cases,  the  carbonates  of  manganese, 
lime,  and  magnesia,  these  may  all  be  determined,  together  with  the 
iron,  as  follows  : — 

A  weighed  portion  of  ore  (about  1  gm.)  is  brought  into  solution  in  hydrochloric 
acid,  after  ignition  if  pyrites  is  present,  and  filtered,  if  necessary,  to  separate 
insoluble  silicious  matter. 

The  solution  is  then  boiled  with  a  few  drops  of  nitric  acid  to  peroxidize  the 
iron,  diluted,  nearly  neutralized  with  ammonia,  and  a  solution  of  ammonium 
acetate  added,  then  boiled  for  two  minutes  and  allowed  to  settle.  The  precipitate 
is  collected  on  a  filter  and  washed  with  boiling  water  containing  a  little 
ammonium  acetate.  It  is  then  dissolved  off  the  filter  in  HC1,  which  also 
dissolves  any  A1203  or  P203  which  may  be  present.  The  liquid  is  then 
•evaporated,  reduced,  and  titrated  as  usual. 

The  filtrate  from  the  above  is  concentrated  by  evaporation,  cooled,  3  or  4  c.c. 
of  bromine  added,  and  well  mixed  by  shaking  ;  when  most  of  the  bromine  is 


242  IRON. 

dissolved  the  liquid  is  rendered  alkaline  by  ammonia,  and  gently  warmed  till  the 
Mn  separates  in  large  flocks  as  hydrated  oxide,  which  is  collected  and  titrated  by 
one  of  the  methods  given  under  "  Manganese." 

The  filtrate  from  the  last  is  mixed  with  ammonium  oxalate  to  precipitate  the 
lime,  which  is  determined  by  permanganate,  as  on  p.  172. 

The  filtrate  from  the  lime  contains  the  magnesia,  which  may  be  precipitated 
with  sodium  phosphate  and  ammonia,  and  the  precipitate  weighed  as  usual,  or 
titrated  with  uranium  solution. 

4.  Determination  of  Iron  in  Silicates. — Wilbur,  and  Whittle- 
sey*  give  a  series  of  determinations  of  iron  existing  in  various 
silicates,  either  as  mixtures  of  ferric  and  ferrous  salts  or  of  either 
separately,  which  appear  very  satisfactory. 

The  very  finely  powdered  silicate  is  mixed  with  rather  more  than  its  own  weight 
of  powdered  fluor-spar  or  cryolite  (free  from  iron)  in  a  platinum  crucible,  covered 
with  hydrochloric  acid,  and  heated  on  the  water-bath  until  the  silicate  is  all 
dissolved.  During  the  digestion  either  carbonic  acid  gas  or  coal  gas  free  from  H28 
is  supplied  over  the  surface  of  the  liquid  so  as  to  prevent  access  of  air.  AVhen 
decomposition  is  complete  (the  time  varying  with  the  nature  of  the  material), 
the  mixture  is  diluted  and  titrated  with  permanganate  in  the  usual  way  for  ferrous 
oxide  ;  the  ferric  oxide  can  then  be  reduced  by  zinc  and  its  proportion  found. 

By  Hydrofluoric  Acid. — Silicates  may  also  be  decomposed  by 
hydrofluoric  acid,  about  2  gm.  being  treated  with  40  c.c.  of  the  acid 
(containing  about  20  per  cent.  HF)  in  a  deep  platinum  crucible. 
In  this  case  Leedsf  recommends  that  the  finely  powdered  silicate 
be  mixed  with  a  suitable  quantity  of  dilute  sulphuric  acid,  and  air 
excluded  by  CO2  during  the  action  of  the  hydrofluoric  acid,  which 
should  be  aided  by  heat.  When  decomposition  is  complete,  the 
crucible  and  its  contents  are  quickly  cooled,  diluted  with  recently 
boiled  water,  and  the  ferrous  salt  determined  with  permanganate 
or  dichromate  as  usual. 

If  the  hydrofluoric  acid  has  been  prepared  in  leaden  vessels,  it 
invariably  contains  SO2  ;  in  such  cases  it  is  necessan^  to  add  to  it, 
previous  to  use,  some  hydrogen  peroxide  (avoiding  excess)  so  as  to 
oxidize  the  SO2. 

The  process  is  a  rapid  and  satisfactory  one,  yielding  much  more 
accurate  results  than  the  method  of  fusion  with  alkali  carbonates 
or  acid  potassium  sulphate. 

5.  Colorimetric  determination  of  Carbon  in  Steel  and  Iron.— 

The  method  devised  byEggertz,  and  largely  adopted  by  chemist* 
for  determination  of  combined  carbon,  is  well  known,  but  is  open 
to  the  objection  that  minute  quantities  of  carbon  cannot  be  dis- 
criminated by  it,  owing  to  the  colour  of  the  ferric  nitrate  present. 
Stead  J  in  order  to  overcome  this  difficulty  has  devised  a  method 
described  as  follows  : — 

In  some  careful  investigations  on  the  nature  of  the  colouring 
matter  which  is  produced  by  the  action  of  dilute  nitric  acid  upon 
white  iron  and  steel,  it  was  found  it  had  the  property  of  being 

*  C.  Ar.  22,  2.  t  Z.  a  C.  16,  323.  J  C.  N.  47,  285. 


IIICXN    AND    STEEL.  243 

soluble  in  potash  and  soda  solutions,  and  that  the  alkaline  solution 
had  about  two  and  a  half  times  the  depth  of  colour  possessed  by 
the  acid  solution.  This  being  so,  it  was  clear  that  the  colouring 
matter  might  readily  be  separated  from  the  iron,  and  be  obtained 
in  an  alkaline  solution,  by  simply  adding  an  excess  of  sodiuin 
hydrate  to  the  nitric  acid  solution  of  iron,  and  that  the  colouring 
solution  thus  'obtairied  might  be  used;  as  a  mean's  of  determining 
the  amount  of  carbon  present.  Upon  trial  this  was  found  to  be 
the  case,  and  that  as  small  a  quantity  as  0'03  per  cent,  of  carbon 
could  readily  be  determined. 

The  solutions  required  are  : — 

Nitric  acid,  1*20  sp.  gr. 

Sodium  hydroxide,  solution  1.-27  sp.  gr. 

METHOD  or  PROCEDURE  :  .One  gra.  of  the  steel  or  iron  to  be  tested  is  weighed 
and  placed  in  a  200  c.c.  beaker,  and  after  covering  with  a  watch-glass,  12  c.c. 
of  standard  nitric  acid  are  added.  The  beaker  and  contents  are  then  placed  on 
a  warm  plate,  heated  to  about  90°  to  100°  C.,  and  there  allowed  to  remain  until 
dissolved,  which  does  not  usually  take  more  than  ten  minutes.  At  the  same  time 
a  standard  iron  containing  a  known  quantity  of  carbon  is  treated  in  exactly  the 
same  way,  and  "when  both  "are  dissolved  30  c.c.  of  hot  water  are  added  to  each 
and  13  c.c.  soda  solution. 

The  contents  are  now  to  be  well  shaken,  and  then  poured  into  a  glass 
measuring  jar  and  diluted  till  they  occupy  a  bulk  of  60  c.c.  After  again  well 
mixing  and  allowing  to  stand  for  ten  minutes  in  a  warm  place,  they  are  filtered 
through  dry  filters,  and  the  nitrates,  only  a  portion  of  which  is  used,  are 
compared.  This  may  be  done  by  pouring  the  two  liquids  into  two  separate 
measuring  tubes  in  such  quantity  or  proportion  that  upon  looking  down  the 
tubes  the  colours  appears  to  be  equal. 

Thus  if  50  measures  of  the  standard  solution  are  poured  into  one  tube,  and  if 
the  steel  to  be  tested  contains  say  half  as  much  as  the  standard,  there  will  be  100 
measures  of  its  colour  solution  required  to  give  the  same  tint.  The  carbon  is 
therefore  inversely  proportional  to  the  bulk  compared  with  the  standard,  and  in 
the  above  assumed  case,  if  the  standard  steel  contained  0'05  per  cent,  carbon,  the 
following  simple  equation  would  give  the  carbon  in  the  sample  tested: — 

0-05x50     , 

— Ynn — =0'02D  per  cent. 
lu'J 

The  proportions  here  given  must  be  strictly  adhered  to  in  order  to  ensure 
exactness.  The  colours  from  low  carbon  irons  differ  in  tint  from  those  in  high 
carbon  steels,  and  therefore  a  low  standard  specimen  must  be  used  for  comparison. 
It  is  evident  that  the  safest  plan  to  ensure  absolute  comparison  is  to  weigh  and 
dissolve  a  known  standard  steel  or  iron  for  each  batch  of  tests. 

Stead  has  devised  a  special  colorimeter  for  the  process,  but  it  is 
evident  that  any  of  the  usual  instruments  may  be  used. 

Arnold*  gives  the  following  conditions  as  necessary  for  the 
accurate  working  of  the  Eggertz  test  :— 

(a)  The  standard  steel  should  have  been  made  by  the  same  process  as  the 
sample. 

(6)  The  standard  should  be  in  the  same  physical  condition,  as  far  as  this  can 
be  secured  by  mechanical  means. 

(c)    The  standard  should  not  differ  greatly  in  the  percentage  of  carbon. 

»  Steel  Works  Analysis,  p.  46. 

R    2 


244  IRON   AND    STEEL. 

(d)  The  solution  of  the  standard  and  the  samples  should  be  made  at  the  same 
time,  and  under  identical  conditions,  and  the  comparisons  should  be  made  without 
delay. 

(e)  Above  all,  the  standard  should  be  above  suspicion,  its  carbon  contents 
having  been  settled  on  the  mean  of  several  concordant  combustions  made  on 
different  weights  of  steel  from  a  homogeneous  bar. 

6.  Determination  of  Phosphorus  in  Iron   and  Steel. — Dudley 
and  Pease*  adopt  the  following  method  : — 

l-gm.  of  the  sample  is  dissolved  in  an  Erlenmeyer  flask,  in  75  c.c.  of  nitric 
acid  of  sp.  gr.  1*15:  when  dissolved,  it  is  boiled  for  a  minute  and  mixed  with 
10  c.c.  of  a  solution  of  potassium  permanganate,  and  then  again  boiled  until 
manganese  dioxide  begins  to  separate.  The  liquid  is  now  cleared  by  the  cautious 
addition  of  pure  ferrous  sulphate  ;  heated  to  85°  C.,  and  mixed  with  75  c.c.  of 
ammonium  molybdate  solution  at  27°  C.  After  shaking  for  five  minutes  in  a 
whirling  apparatus,  the  precipitate  is  washed  with  solution  of  ammonium  sulphate 
until  the  washings  give  no  colouration  with  ammonium  sulphide,  and  then  dissolved 
in  a'mixture  of  5  c.c.  of  ammonia  and  25  c.c.  of  water.  The  solution  is  now  mixed 
with  10  c.c.  of  strong  sulphuric  acid,  diluted  to  200  c.c.  and  reduced  with  zinc. 
The  solution  is  then  titrated  with  permanganate.  The  volume  of  the  latter 
which  represents  I  gm.  of  Fe  equals  0*01724  gm.  of  P. 

7.  Determination   of    Sulphur  in  Iron  and    Steel. — J.   Thillf 
gives  the  following  method  : — 

METHOD  or  PROCEDURE  :  By  attacking  the  metal  with  hydrochloric  acid  in 
the  well-known  apparatus  used  for  this  purpose,  the  sulphur  is  liberated  as 
sulphuretted  hydrogen.  The  gas  is  received  in  25  c.c.  of  a  decinormal  solution 
of  arsenious  acid,  to  which  has  been  added  50  c.c.  of  a  cold  saturated  solution  of 
bicarbonate  of  soda.  Care  should  be  taken  that  the  gas  is  not  given  off  too 
rapidly. 

When  the  attack  is  finished,  the  gas  which  still  fills  the  apparatus  is  driven  out 
by  means  of  a  current  of  carbonic  acid,  and  the  passage  of  this  current  of  gas  is 
continued  until  the  hydrochloric  acid  carried  with  it  has  neutralized  the  alkaline 
solution,  and  precipitated  almost  the  whole  of  the  trisulphide  of  arsenic.  This 
operation  takes  eight  or  ten  minutes. 

A  few  c.c.  of  hydrochloric  acid  are  added,  and  the  volume  made  up  to  500  c.c. 
and  filtered.  The  filtrate  is  collected  in  a  dry  beaker,  and  100  c.c.  are  taken,  and 
titrated  with  N/iO  iodine,  after  adding  starch  and  a  sufficient  quantity  of 
ammonium  carbonate  to  render  the  solution  alkaline. 

From  the  number  of  c.c.  of  iodine  used  is  subtracted  the  number  of  c.c.  required 
by  25  c.c.  of  a  decinormal  solution  of  arsenious  acid.  The  difference  corresponds 
to  the  sulphuretted  hydrogen  given  off.  This  result,  multiplied  by  0'0024  gives 
the  amount  of  sulphur  contained  in  the  sample. 

At  the  National  Physical  Laboratory,  Bushy  House,  Teddington, 
the  volumetric  determination  of  sulphur  is  carried  out  thus : — J 

The  steel  drillings  are  dissolved  in  an  evolution  flask  in  hydrochloric  acid  of 
1*10  sp.  gr.  the  operation  being  aided  by  heat,  although  boiling  the  acid  should 
be  avoided.  Prior  to  the  commencement  of  the  operation,  the  evolution  flask 
and  entire  apparatus  are  filled  with  an  atmosphere  of  carbon  dioxide,  obtained  by 
passing  a  stream  of  this  gas,  derived  from  a  cylinder  of  liquid  carbonic  acid,  through 
the  entire  apparatus.  The  evolved  gases,  aided,  to  wards  the  end  of  the  operation 
by  a  further  stream  of  C02  are  bubbled  through  an  absorption  flask  containing 
a  solution  of  cadmium  acetate  strongly  acidified  with  acetic  acid  (25  gm.  pure 
cadmium  acetate  and  100  gm.  glacial  acetic  acid  per  litre).  After  passing  this  flask 

*  J.  Anal.  Chem.  7,  108.  t  Z  a  C.  38,  342. 

}  Rosenhain,  Iron  and  Steel  Institute  Journal,  Vol.  I.,  1908. 


LEAD.  245 

the  gases  pass  through  a  narrow-bore  tube  of  vitreous  silica  heated  to  redness 
by  a  B  u  n  se  n  burner  with  a  flat  flame,  the  gases  passing  finally  through  a  second 
cadmium  acetate  absorption  flask  and  then  away  to  the  fume  chamber.  When 
the  steel  has  completely  dissolved,  the  contents  of  the  two  absorption  flasks  are 
mixed  and  the  yellow  cadmium  sulphide  is  filtered  off  ;  this  is  a  rapid  operation 
since  the  flask  need  not  be  washed  carefully, — the  operation  is  merely  intended 
to  separate  the  sulphide  from  the  bulk  of  the  absorption  liquid.  As  soon  as 
this  has  been  done  the  precipitate  is  washed  from  the  filter  back  into  the  original 
flask,  and  there  dissolved  in  10  c.c.  of  standard  iodine  solution,*  the  action 
being  aided  by  the  addition  of  a  small  quantity  of  HC1.  The  excess  of  iodine  is  then 
titrated  by  means  of  sodium  thiosulphate  and  starch.  It  is  to  be  observed  that 
while  this  titration  can  be  carried  out  in  the  liquid  of  the  absorption  flasks  without 
filtration,  it  has  been  found  that  this  occasionally  leads  to  discrepancies  in 
the  results.  Apparently,  particularly  in  the  case  of  high  carbon  steel,  the  evolved 
gases  carry  into  the  absorption  flask  something  which  is  capable  of  absorbing 
iodine,  but  which  is  not  sulphur ;  this  disturbing  substance  can  be  eliminated  by 
the  filtration  described  above.  A  table  is  given  showing  the  comparative  results 
obtained  by  this  process  and  by  the  gravimetric  (oxidation)  method,  the  agreement 
in  all  cases  being  exceedingly  close. 

LEAD. 

Pb- 207-1. 

F  THE  accurate  determination  of  lead  is  in  most  cases  better 
effected  by  weight  than  by  measure  ;  there  are,  however,  instances 
in  which  the  latter  may  be  used  with  advantage.  The 
precipitation  as  oxalate  or  carbonate  is  only  of  use  where  the  lead 
exists  in  the  form  of  a  tolerably  pure  salt  or  metal. 

1.  As  Oxalate  (H  e  m  p  e  1). — The  acetic  lead  solution,  which  must  contain  no 
other  body  precipitable  by  oxalic  acid,  is  put  into  a  300  c.c.  flask,  and  a  measured 
quantity  of  normal  oxalic  acid  added  in  excess,  the  flask  filled  to  the  mark  with 
water,  shaken,  and  put  aside  to  settle  ;  100  c.c.  of  the  clear  liquid  may  then  be 
taken,  acidified,  with  sulphuric  acid,  and  titrated  with  permanganate  for  the 
excess  of  oxalic  acid.     The  amount  so  found  multiplied  by  3,  and  deducted  from 
that  originally  added,  will  give  the  quantity  combined  with  the  lead. 

2.  Alkalimetric  Method  (Mohr). — The  lead  is  precipitated  as  carbonate  by 
means  of  a  slight  excess  of  ammonium  carbonate,  together  with  free  ammonia  : 
the  precipitate  well  washed,  and  dissolved  in  a  measured  excess  of  normal  nitric 
acid  :  neutral  solution  of  sodium  sulphate  is  then  added  to  precipitate  the  lead 
as  sulphate.     Without  filtering,  the  excess  of  nitric  acid  is  then  determined  by 
normal  alkali,  each  c.c.  combined  being  equal  to  0*10355  gm.  of  lead. 

3.  Bichromate  Method. — 1  c.c.  N/IO  dichromate  = -010355  gm.  Pb. 

The  following  process  for  carbonate  ores,  pig  lead,  and  specially 
for  red  and  white  leads  and  litharge,  has  been  worked  out  by  J.  H. 
Wainwrightf.  The  necessary  solutions  are:  potassium  dichrom- 
ate,  of  such  strength  that  1  c.c.  represents  about  O'Ol  gm.  of 
metallic  lead,  not  much  more  or  less,  standardized  either  upon 
pure  metal,  or  on  white  lead  which  has  been  accurately  analysed  for 
actual  lead  by  weight :  silver  nitrate  solution  as  outside  indicator, 
not  exceeding  2  or  3  per  cent,  in  strength. 

*2  gm.  of  re -sublimed  iodine  are  dissolved  in  50  c.c.  of  water  containing  4  gm.of  KI, 
and  diluted  to  1  litre. 

t  /.  Am.  C.  S.  19,  380. 


246  LEAD. 

•  METHOD  OF  PROCEDURE  :  Fronvl  to  1'5  gm.  of  ore-,  litharge,  etc.,  is  dissolved 
in  10  to  15  c.c.  of  nitric  acid  (sp.  gr.  1*20),  the  solution  made  slightly  alkaline 
with  ammonia,  and  a  considerable  excess  of  acetic  acid  added.  It  is  then  boiled, 
and  potassium  dichromate  solution,  in  sufficient  quantity  to  precipitate  nearly  all 
the  lead,  is  run  in  from  a  burette.  The  liquid  is  boiled  until  the  precipitate  becomes 
orange-coloured,  after  which  the  titration  is  finished,  the  final  point  being  indicated 
by  contact  with  silver  nitrate  as  an  outside  indicator  on  a  white  plate. 

The  precautions  to  be  observed  are  : — 

The  solution  of  the  lead  salt  must  be  as  concentrated  as  possible,  and  decidedly 
acid  with  acetic  acid.  There  must  be  absence  of  other  metals,  especially  such  as 
can  exist  in  lower  forms  of  oxidation.  Antimony  and  tin,  unless  thoroughly 
oxidized,  and  bismuth  are  particularly  to  be  avoided.  Ihiring  titration  the 
solution  should  be  kept  as  near  the  boiling-point  as  possible.  The  strength  of 
the  dichromate  solution  should  not  vary  much  from  that  given  above,  nor  should 
the  solution  of  silver  nitrate. 

In  the  case  of  dealing  with  ores  containing  small  quantities  of  silver,  it  is 
desirable  to  precipitate  this  before  filtration  with  a  little  solution  of  sodium  chloride. 
In  this  case  it  is  well  to  employ  larger  drops  of  the  silver  nitrate  used  as  indicator. 

The  method  is  specially  suitable  for  such  substances  as  white  lead,  red  lead, 
litharge,  etc.  Red  lead  is  dissolved  by  treating  it  with  nitric  acid,  and  adding  a 
dilute  solution  of  oxalic  acid  drop  by  drop  until  the  lead  oxide  is  completely 
dissolved.  If  organic  matter  is  present  the  solution  should  be  filtered  before 
titration.  White  lead  can  be  dissolved  directly  in  acetic  acid,  and  the  solution 
titrated  without  filtration.  In  th^case  of  white  lead  ground  in  oil,  the  sample 
should  be  dissolved  in  dilute  nitric  acid,  the  solution  boiled,  filtered,  ammonia 
added  in  excess,  and  then  an  excess  of  acetic  acid.  The  method  can  also  be 
used  with  ingot  lead,  and  alloys  containing  tin  and  antimony,  but  the  sample  must 
be  thoroughly  oxidized  by  repeated  -evaporation  with  fuming  nitric  acid,  and 
the  solution  filtered  before  titration. 

The  lead  in  solution,  after  addition  of  ammonium  or  sodium 
acetate,  may  be  precipitated  by  excess  of  N/i0  dichromate  solution. 
After  boiling  for  a  minute  or  two  the  precipitate  is  quickly  filtered 
off,  well  washed,  and  the  excess  of  dichromate  in  the  cooled  filtrate 
titrated  with  standardized  ferrous  ammonium  sulphate  solution 
and  potassium  ferricyanide  as  external  indicator.  Or  the  excess 
of  dichromate  in  the  filtrate  may  be  determined  iodimetrically  by 
addition  of  potassium  iodide  and  titration  with  sodium 
thiosulphate. 

Lead  in  various  Ores. — An  investigation  of  many  methods  of 
determining  this  metal  has  been  carried  out  by  J.  C.  Bull.*  The 
best  results,  including  the  foregoing  dichromate  process,  were 
obtained  by  the  molybdate  and  the  ferrocyanide  methods.  The 
initial  procedure  in  all  the  trials  was  to  separate  the  lead  as  sulphate, 
which  contained  also  gangue  and  other  insoluble  sulphates,  by 
treating  the  ore  with  nitric  or  nitro-hydrochloric  acid,  evaporating 
with  sulphuric  acid  and  filtering  off  from  soluble  sulphates  after 
being  diluted. 

The  Molybdate  Method. — The  mixture  of  lead  sulphate  and  impurities,  obtained 
as  above,  is  boiled  for  at  least  ten  minutes  with  ammonium  acetate  solution  ; 
the  solution  is  then  acidified  with  acetic  acid,  diluted  to  200  c.c.,  again  boiled, 
and  a  standard  ammonium  molybdate  solution  added  until  all  the  lead  is  precipi- 
tated. The  end-point  is  ascertained  by  shaking  the  solution  vigorously,  allowing 
it  to  stand  for  a  few  minutes,  and  testing  1  drop  of  the  clear  liquid  with  1  drop  of  a 

*  Analyst,  28,  15. 


LEAD.  2.47 

solution  of  1  part  tannin  in  300  parts  water  and  1  drop  of  a  lead  solution  ;  the 
appearance  of  a  yellow  colour  indicates  the  presence  of  ammonium  molybdate  in 
excess.  The  molybdate  solution  is  prepared  by  dissolving  9  grammes  of  the  salt 
in  1  litre  of  water  and  standardizing  against  lead  sulphate.  Since  the  indicator  is  not 
very  sensitive,  requiring  about  0*8  c.c.  of  molybdate  to  affect  it,  a  blank  must 
be  made  to  ascertain  the  correction  due  to  this.  This  method  gave  very  good  results 
when  tried  on  the  ores  ;  the  presence  of  antimony,  bismuth,  and  calcium  had  no 
effect  on  it ;  but  in  the  presence  of  barium  and,  to  a  lesser  extent,  strontium,  it 
gave  low  results. 

The  Ferrocyanide  Method. — The  mixture  containing  the  lead  sulphate  is  gently 
heated  to  boiling  with  10  c.c.  of  a  saturated  solution  of  ammonium  carbonate. 
Alter  cooling,  the  precipitate  is  transferred  to  a  filter,  thoroughly  washed,  and  then 
placed  with  the  filter-paper  in  a  flask  containing  a  hot  mixture  of  5  c.c.  glacial 
acetic  acid  and  25  c.c.  water.  This  is  boiled  until  the  lead  carbonate  has  dissolved, 
diluted  to  150  c.c.,  heated  to  60°  C.,  and  titrated  with  standard  1  per  cent,  potassium 
ferrocyanide  solution,  drops  of  uranium  acetate  solution  placed  on  a  white  tile 
being  used  as  indicator.  Here  also  the  correction  due  to  the  indicator  must  be 
determined.  This  method  also  gave  very  good  results  when  no  interfering  metals 
were  present. 

4.  Lead  in  Citric  and  Tartaric  Acids,  and  in  Cream  of  Tartar. — 
War  ing  ton*  has  worked  out  the  best  method  of  ascertaining  the 
proportions  of  lead  in  these  commercial  acids,  and  shows  that 
ammonium  sulphydrate  is  to  be  preferred  to  sulphuretted  hydrogen 
for  the  process,  inasmuch  as  the  tint  produced  is  much  more 
uniform  throughout  a  long  scale,  and  very  free  from  turbidity. 
Waring  ton's  description  of  the  method  is  as  follows  : — 

The  depth  of  tint  produced  for  the  same  quantity  of  lead  present  is  far  greater 
in  an  ammoniacal  tartrate  or  citrate  solution  than  in  the  same  volume  of  water ; 
it  is  quite  essential,  therefore,  if  equality  of  tint  is  to  be  interpreted  as  equality 
•of  lead,  that  all  comparisons  should  be  between  two  citrate  and  tartrate  solutions, 
and  not  between  one  of  these  and  water. 

To  carry  out  the  method  it  is  necessary  to  have  solutions  of  lead -free  tartaric 
and  citric  acids  supersaturated  with  pure  ammonia ;  these  solutions  should  develop 
no  colour  when  tieated  with  ammonium  sulphydrate.  A  convenient  strength  is 
100  gm.  of  acid  in  300  c.c.  of  final  solution. 

The  standard  lead  solutions  are  made  by  dissolving  T6  gm.  of  crystallized  lead 
nitrate  dried  over  sulphuric  acid  in  a  litre  of  water,  each  c.c.  =0-001  gm.  Pb. 
A  weaker  solution  is  also  made  by  diluting  100  c.c.  of  this  to  a  litre. 

Of  the  tartaric  or  citric  acid  to  be  examined,  40  gm.  are  taken  and  dissolved  in 
a  little  water ;  warm  water  is  most  convenient  for  crystal  and  cold  for  powder  ; 
the  solution  is  best  prepared  in  a  flask.  To  the  cold  solution  pure  strong 
ammonia  is  gradually  added  till  it  is  in  slight  excess  ;  the  final  point  is  indicated 
in  the  case  of  tartaric  acid  by  the  solution  of  the  acid  ammonium  tartrate  first 
formed  ;  in  the  case  of  citric  acid  it  is  conveniently  shown  by  a  fragment  of 
turmeric  paper  floating  in  the  liquid.  When  an  excess  of  ammon;a  is  reached 
the  liquid  is  cooled,  diluted  to  120  c.c.,  and  filtered. 

As  a  preliminary  experiment  10  c.c.  are  taken,  diluted  to  50  c.c.  in  the  measuring 
cylinder,  and  placed  in  a  Nesslerizing  glass,  one  drop  of  ammonium  sulphydrate 
solution  added,  and  the  whole  well  stirred  ;  the  colour  developed  indicates  what 
volume  of  solution  should  be  taken  for  the  determination, — this  volume  may 
range  from  5  c.c.  to  50  c.c.  If  less  than  50  c.c.  are  taken  the  volume  is  brought 
to  50  c.c.  with  water,  and  one  drop  of  ammonium  sulphydrate  is  then  added. 

The  tint  thus  produced  has  now  to  be  matched  with  the  pure  solutions.  A 
volume  of  the  pure  ammoniacal  tartrate  or  citrate,  identical  with  that  taken  of 
the  acid  under  examination,  receives  a  measured  quantity  of  lead  solution  from 

*  J.  S.  C.I.  1893,12,97,  222. 


248  LEAD. 

the  burette,  the  volume  is  brought  to  50  c.c.,  it  is  placed  in  a  Nesslerizing  glass, 
and  receives  one  drop  of  ammonium  sulphydrate  ;  the  experiment  is  repeated 
till  a  match  is  obtained.  As  in  the  previous  method,  the  best  comparison  of 
tints  is  obtained  by  making  finally  three  simultaneous  experiments,  one  with  the 
acid  under  examination,  the  other  two  with  pure  acid  containing  slightly  varying 
amounts  of  lead,  the  aim  being  that  the  tint  given  by  the  acid  to  be  analysed 
shall  lie  within  this  narrow  scale.  In  following  this  method,  considerable  us& 
has  to  be  made  of  the  weaker  of  the  two  lead  solutions  already  mentioned. 

The  whole  time  required  for  a  determination  of  lead  by  this  method  now  given, 
is  about  1£  hours  :  this  time  will  be  somewhat  shortened  as  the  operator  becomes 
familiar  with  the  tints  produced  by  varying  proportions  of  lead.  If  traces  of 
copper  or  iron  are  present,  any  interference  on  their  part  may  be  removed  by 
adding  to  the  alkaline  solution  a  few  drops  of  potassium  cyanide  solution. 

T  a  1 1  o  c  k  and  Thomson*  proceed  as  follows  : — 

(1)  Cream  of  Tartar. — Treat  10  grams  with  50  c.c.  of  water  and  40  c.c.  of  2N 
ammonia  solution,  agitating  till  dissolved,  then  making  up  to  100  c.c.  with  water, 
mixing  well  and  filtering  through  a  dry  filter. 

(2)  Tartaric    acid. — Take  10  grams  and  use  81  c.c.  of  2X  ammonia  solution 
and  9  c.c.  of  water,  etc.,  as  in  (1). 

(3)  Citric  acid. — Take  10  grams  and  use  85  c.c.  of  2N  ammonia  solution  and 
5  c.c.  of  water,  etc.,  as  in  (1). 

Of  the  100  c.c.  of  filtered  liquid  obtained  as  above,  50  c.c.  are  taken  and  to  them 
are  now  added  O'l  gm.  of  KCy  and  1  c.c.  of  a  colourless  or  almost  colourless  strong 
solution  of  ammonium  sulphide,  and  comparison  then  made  with  a  standard 
solution  of  lead  as  described  above.  It  is  important  to  notice,  however,  that  the 
amount  of  lead  present  in  50  c.c.  of  the  standard  solution,  and  also  in  the  quantity 
of  sample  used,  should  not  exceed  0'2  mgm.  and  that  no  lead  should  be  added 
to  make  up  deficiency  after  the  addition  of  ammonium  sulphide,  but  that  a  fresh 
standard  should  always  be  prepared.  The  reagents  should  invariably  be  added 
in  the  order  mentioned. 

Dr.  Mac  Fad  den  in  a  report  to  the  Local  Government  Board  f  recommends 
the  adoption  of  a  limit  of  0-002  per  cent,  (approximately  l/7th  grain  per  Ib.)  of 
lead  as  impurity  in  tartar ic  and  citric  acids  and  cream  of  tartait 

5.  Colorimetric  determination  for  Waters. — When  there  is  no 
other  metal  than  lead  present,  simple  addition  of  freshly  made 
sulphuretted  hydrogen  water  in  the  presence  of  weak  acetic  acid  as 
suggested  by  Miller  gives  good  results,   comparison  being  made 
with  a  standard  solution  of  lead  acetate  containing  0-1831  gm. 
per    litre.     Each    c.c.  =00001    gm.    lead.     The    determination    is 
made  in  colourless  glass  cylinders  in  the  same  way  as  described  for 
copper  (p.  204),  or  iron  (p.  238),  taking  care  that  the  comparative 
tests  are  made  under  precisely  the  same  conditions. 

6.  Colorimetric  determination  of  lead  in  the  presence  of    iron 
(for  testing  chemicals  generally).     J.  M.   WilkieJ  recommends 
the  following  procedure  : — 

A  weighed  portion  of  the  substance  is  dissolved  in  water  and 
the  solution  made  distinctly  acid  by  adding  a  few  drops  of  acetic 
or  other  acid.  1,  c.c.  of  10  %  potassium  cyanide  solution  is  added, 
then  a  considerable  excess  of  ammonia.  If  the  solution  is  now 
colourless,  it  only  remains  to  add  a  few  drops  of  sodium  sulphide 

*  The  Analyst,  1908,  33,  173. 

t  Report  (No.  2)  on  Lead  and  Arsenic  in  Tartaric  acid,  Citric  acid,  and  Cream  of 
Tartar,  1907. 

%  J.  S.  C.  I.  1909,  28,  636,  and, 1910,  7. 


MAGNESIUM.  240 

solution  and  compare  the  colour  produced  with  that  developed  by 
a  solution  containing  a  known  amount  of  lead  and  treated  precisely 
as  detailed  above.  If  the  ammoniacal  liquid  is  coloured,  iron  is 
present,  and,  if  originally  present  in  the  ferrous  state,  it  is  only 
necessary  to  heat  the  solution  to  boiling,  when  the  colour  disappears, 
and  on  cooling  to  add  the  sulphide.  When,  however,  ferric  iron  is 
present  another  portion  of  the  substance  must  be  dissolved,  acidified, 
and  a  few  drops  of  N/10  sodium  thiosulphate  added — the  exact  amount 
required  depending  on  the  amount  of  iron  present.  The  solution 
is  heated  slowly  to  incipient  boiling,  allowed  to  stand  until  the 
colour  suddenly  disappears,  and  then  potassium  cyanide,  etc., 
added  as  usual. 


MAGNESIUM. 

Mg  =  24-32. 

AN  alkalimetric  process  for  the  determination  of  this  substance 
has  been  adopted  by  Stolba,  a  reference  to  which  is  made  under 
Phosphoric  Acid,  p.  114,  but  the  time  and  trouble  required  to 
wash  out  the  ammonia  by  alcohol  renders  the  method  too  difficult 
for  general  purposes.  A  much  shorter  procedure  has  been  devised 
by  R.  K.  Meade.* 

The  method  is  based  on  the  same  principles  as  W  i  1 1  i  a  m  s  o  n '  s  process  described 
under  "  Arsenic,"  p.  156.  He  found  that  when  a  solution  of  arsenic  acid  contained 
sufficient  sulphuric  or  hydrochloric  acid  the  arsenic  is  quickly  reduced  to  arsenious 
acid  even  in  the  cold.  For  every  molecule  of  arsenic  acid  so  reduced  there  corre- 
sponds two  atoms  of  magnesium,  and  two  molecules  or  four  atoms  of  iodine  are 
liberated.  This  latter  is  titrated  with  sodium  thiosulphate,  and  from  the  volume 
of  standard  solution  required  the  magnesium  is  calculated. 

The  standard  solutions  are  conveniently  made  as  follows : — 

Standard  sodium  arsenate  is  prepared  by  dissolving  12-29  gm.  of  pure  arsenious 
acid  in  nitric  acid,  evaporating  on  a  water-bath  to  dryness,  neutralizing  with 
sodium  carbonate  in  solution,  and  when  dissolved  made  up  to  a  litre  with  distilled 
water.  Each  c.c.  =CK)05  gm.  of  Mg. 

The  standard  solution  of  sodium  thiosulphate  is  made  to  correspond  to  this 
either  by  direct  titration,  or  by  making  it  equal  to  a  standard  iodine  solution 
made  by  dissolving  52J24  gm.  of  pure  iodine,  and  75  gm.  of  potassium  iodide  in 
about  200  c.c.  of  water,  and  making  up  to  one  litre.  Each  c.c.  =0'005  gm.  Mg. 

METHOD  OF  PROCEDURE  :  Pour  the  magnesia  solution,  which  should  not 
contain  too  great  an  excess  of  ammonium  chloride  or  oxalate,  into  a  conical  flask 
or  a  gas-bottle  of  sufficient  size.  Add  one-third  the  volume  of  the  solution  of 
strong  ammonia  and  50  c.c.  of  sodium  arsenate.  Cork  up  tightly  and  shake- 
vigorously  for  ten  minutes.  Allow  the  precipitate  to  settle  somewhat,  then  filter 
and  wash  with  a  mixture  of  water  and  strong  ammonia  (3:1)  until  the  washings 
cease  to  react  for  arsenic  ;  avoid,  however,  using  an  excess  of  the  washing  fluid. 
Dissolve  the  precipitate  in  dilute  hydrochloric  acid  (1  :  1),  allowing  the  acid 
solution  to  run  into  the  flask  in  which  the  precipitation  was  made,  and  wash  the- 
filter-paper  with  the  dilute  acid,  until  the  washings  and  solution  measure  80  or 
100  c.c.  Cool,  and  add  from  3  to  5  gm.  of  potassium  iodide,  free  from  iodate; 
allow  the  solution  to  stand  for  a  few  minutes,  and  then  run  in  the  standard 
thiosulphate  until  the  colour  of  the  liberated  iodine  fades  to  a  pale  straw  colour. 
Add  starch,  and  titrate  until  the  blue  colour  of  the  iodide  of  starch  is  discharged^ 

*  J.  Am.  C.  S.  21,  746. 


250  MAGNESIUM. 

If  preferred,  an  excess  of  thiosulphate  may  be  added,  then  starch  and  standard 
iodine  until  the  blue  colour  is  produced.  On  adding  the  iodide  of  potassium  to 
the  acid  solution,  a  brown  precipitate  forms,  which,  however,  dissolves  when  the 
thiosulphate  is  added. 

Experience  has  proved  that  the  whole  process  can  be  done  within  an  hour,  and 
the  results  in  the  case  of  dolomite,  limestone,  slag  and  cement  are  very  near  those 
given  by  gravimetric  methods. 

Frankfort  er  and  Cohen*  state  that  a  much  sharper  end-reaction  is  obtained 
if  starch  indicator  is  not  used.  They  apply  the  process  to  the  determination  of 
magnesium  in  water,  thus  :— 

500  c.c.  of  the  water,  after  removal  of  iron  and  calcium  as  usual,  are  acidified, 
evaporated  to  100  c.c.,  33  c.c.  of  strong  ammonia  and  25  c.c.  of  a  10  %  solution  of 
sodium  arsenate  added,  and  the  flask  shaken  vigorously  for  10  minutes.  The 
'precipitate  is  filtered  off,  washed  with  the  least  possible  amount  of  dilute  ammonia, 
dissolved  in  25  c.c.  dilute  sulphuric  acid  (1:4)  into  the  original  flask,  the  filter 
washed  with  50  c.c.  hot  water,  and  10  c.c.  sulphuric  acid  (1  :  1)  added.  After 
cooling,  3 — 5  gm.  of  potassium  iodide  are  added,  the  solution  allowed  to  stand  for 
5  minutes,  then  the  liberated  iodine  -titrated  with  thiosulphate  without  starch 
indicator. 


MANGANESE. 

Mn  =  54-93,  MnO  =  70-93,  MnO2  =  86-93. 
Factors. 

Metallic  iron  x  0*6350 -MnO. 
„      x  0-4918 =Mn. 
„      x  0-7783  =MnO2. 
Double  iron  salt  x  0-0907  =MnO. 
Cryst.  oxalic  acid  x  0-6896  =MnO2. 
Double  iron  salt  xO-1112=MnO2. 

1    c.c.  N/10  solution -0-003547  gm.  MnO  or =0-004347  gm.  Mn02. 

ALL  the  oxides  of  manganese,  with  the  exception  of  the  first  or 
protoxide,  when  boiled  with  hydrochloric  acid,  yield  chlorine  in 
the  following  ratios  : — 


Mn2O3  =  l  eq.  O  =  2  eq.  Cl. 
Mn3O4  =  l  eq.  O  =  2  eq.  Cl. 
Mn  O2  =  l  eq.  O-  2  eq.  Cl. 
Mn  O3  =  2  eq.  O=  4  eq.  Cl. 
Mn2O7  =  5  eq.  O-10  eq.  Cl. 


The  chlorine  so  produced  can  be  allowed  to  react  upon  a  known 
weight  of  ferrous  salt  ;  and  when  the  reaction  is  completed,  the 
unchanged  amount  of  iron  salt  is  found  by  permanganate  or 
dichromate. 

Or,  the  chlorine  may  be  led  by  a  suitable  arrangement  into 
a  solution  of  potassium  iodide,  there  setting  free  an  equivalent 
quantity  of  iodine,  which  is  found  by  the  aid  of  sodium  thiosulphate. 

Or,  in  the  case  of  manganese  ores,  the  reaction  may  take  place 
with  oxalic  acid,  resulting  in  the  production  of  carbonic  acid, 
which  can  be  weighed  as  in  Fresenius'  and  Wills'  method,  or 

*  J.  Am.  Chan.  Soc.  1907,  1464 


MANGANESE.  251 

'"the  amount  of  unchanged  acid  remaining  after  the  action 'can  be 
found  by  permanganate. 

The  largely  increased  use  of  manganese  in  the  manufacture  of 
steel  has  now  rendered  it  imperative  that  some  rapid  and  trust- 
worthy methods  of  determination  should  be  devised,  and  this  has 
been  done  by  well-known  chemists.  The  first  method  described 
appears  to  have  been  simultaneously  suggested  by  Pattinson  and 
Kessler;  both  have  succeeded  in  finding  a  method  of  separating 
manganese  as  dioxide  of  perfectly  definite  composition.  Pattinson* 
found  that  the  regular  precipitation  was  secured  by  ferric  chloride, 
and  Kessler  by  zinc  chloride.  Wright  and  Me  like  have 
experimented  on  both  processes  with  equally  satisfactory  results, 
but  give  a  slight  preference  to  zinc.  Pattinson  titrates  the 
resulting  MnO2  with  standard  dichromate,  and  Kessler  with 
permanganate. 


1.     Precipitation  as  Mn02  and  Titration  with  Dichromate 
(Pattinson). 

The  author's  own  description  of  the  method  is  as  follows  : — 
This  method  depends  upon  the  whole  of  the  manganese  being 
precipitated  as  hydrated  dioxide  by  calcium  carbonate  when  chlorine 
or  bromine  is  added  to  a  solution  of  manganous  salt  containing 
also  a  persalt  of  iron  or  a  salt  of  zinc,  and  under  certain  conditions 
of  temperature,  etc.  This  method  is  now  adopted  by  many 
chemists  both  in  private  laboratories  and  in  the  laboratories  of 
steel  works  ;  and  it  is  therefore  thought  that  the  following  description 
of  it  in  its  slightly  modified  form,  as  now  used  for  determining 
manganese  in  manganiferous  iron  ores,  manganese  ores,  spiegeleisen, 
ferromanganese,  etc.,  will  not  be  out  of  place. 

METHOD  OF  PROCEDURE  :  A  quantity  of  the  sample  to  be  analysed,  containing 
not  more  than  about  0'25  gm.  of  manganese,  is  dissolved  in  hydrochloric  acid. 
In  the  case  of  spiegeleisen  and  ferromanganese,  about  3 — 4  c.c.  of  nitric  acid 
are  afterwards  added  to  oxidize  the  iron.  In  the  case  of  manganese  ores, 
ferromanganese,  and  manganese  slags,  which  do  not  contain  as  much  iron  as 
manganese,  there  is  added  to  the  solution  as  much  iron,  in  the  form  of  ferric 
chloride,  as  will  make  the  quantities  of  iron  and  manganese  in  the  solution  about 
equal.  An  excess  of  iron  is  no  drawback,  except  that  a  larger  precipitate  has 
afterwards  to  be  filtered  and  washed. 

The  excess  of  acid  in  the  solution  is  then  neutralized  by  the  addition  of  calcium 
carbonate,  which  is  added  until  a  slight  reddening  of  the  solution  is  produced. 
The  solution  is  then  rendered  very  slightly  acid  by  dropping  into  it  just  enough 
hydrochloric  acid  to  remove  the  red  colour. 

Then  add  in  all  cases  30  c.c.  of  a  solution  of  zinc  chloride  containing  0'5  gm. 
of  metallic  zinc.  The  liquid  is  then  brought  to  the  boiling  point,  and  diluted 
with  boiling  water  to  about  300  c.c. 

60  c.c.  of  a  solution  of  calcium  hypochlorite,  made  by  dissolving  about  33  gm. 
of  bleaching  powder  per  litre  and  filtering,  are  then  poured  into  the  manganese 
solution  ;  but  to  the  hypochlorite  solution,  before  pouring  it  into  the  manganese 
solution,  there  should  be  added  just  enough  hydrochloric  acid  to  give  it  a  faint 
permanent  greenish-yellow  colour  after  gentle  agitation. 

*  J.  C.  S.  1879,  365,  also  J.  S.  C.  1. 10,  337. 


252  MANGANESE. 

The  object  of  this  addition  of  acid  is  to  prevent  a  precipitate  forming  when 
the  hypochlorite  is  added,  due  to  the  alkalinity  of  this  solution.  When  hydro- 
chloric acid  is  added  in  this  way  to  the  solution  of  calcium  hypochlorite,  the 
manganese  solution  remains  clear  on  the  addition  of  the  calcium  hypochlorite, 
and  any  possible  local  precipitation  of  manganese  in  a  lower  state  of  oxidation 
than  MnOo  is  obviated. 

Finally,  add  to  the  manganese  solution  about  3  gm.  of  calcium  carbonate 
diffused  in  about  15  c.c.  of  boiling  water.  After  the  first  evolution  of  carbonic 
acid  has  ceased,  during  which  time  the  cover  is  kept  on  the  beaker,  the 
precipitate  is  stirred  to  make  it  collect  together,  and  2  c.c.  of  alcohol  or  methylated 
spirit  are  added  and  it  is  again  stirred. 

The  precipitate  is  then  thrown  upon  a  large  filter  and  washed,  at  first  with 
cold  water  until  the  greater  part  of  the  chlorine  is  removed,  and  afterwards,  to 
make  the  washing  more  rapid,  with  warm  water  at  about  155°  F.  (65°  C.).  It  is 
washed  until,  after  draining,  a  drop  shaken  down  straight  from  the  precipitate 
by  gently  jolting  the  funnel,  shows  no  indication  of  chlorine  when  tested  with 
a  strip  of  iodized  starch-paper.  As  a  matter  of  practice  two  or  three  washings 
are  given  after  there  has  ceased  to  be  any  indication  of  chlorine. 

By  carrying  out  the  process  in  the  manner  here  described,  the  temperature  of 
the  liquid,  immediately  after  the  precipitation  is  complete,  is  about  170°  F.  (77°  C.), 
and  it  is  found  that  the  best  and  most  constant  results  are  obtained  when  the 
temperature  after  precipitation  is  near  this  point. 

70  c.c.  of  an  acidified  solution  of  ferrous  sulphate,  containing  about  0*7  gm.  of 
iron,  and  made  by  dissolving  crystallized  ferrous  sulphate  in  a  mixture  of  one 
part  of  monohydrated  sulphuric  acid  and  three  parts  of  water,  are  then  accurately 
measured  off  by  a  pipette  and  run  into  the  beaker  in  which  the  precipitation  was 
made.  The  filter  paper,  together  with  the  precipitate,  is  then  removed  from 
the  funnel  and  placed  in  the  solution  of  ferrous  sulphate  in  the  beaker.  The 
precipitate  readily  dissolves  even  in  the  cold  (sometimes  it  may  be  necessary  to 
add  a  little  more  acid  to  dissolve  the  ferric  hydrate  completely),  the  manganese 
dioxide  converting  its  equivalent  of  ferrous  sulphate  into  ferric  sulphate.  A 
sufficient  quantity  of  cold  water  is  now  added,  and  the  ferrous  sulphate  still 
remaining  is  titrated  with  a  standard  solution  of  potassium  dichromate. 

The  exact  amount  of  ferrous  sulphate  in  70  c.c.  of  the  ferrous  sulphate  solution 
is  determined  by  measuring  off  into  a  clean  beaker  another  portion  of  70  c.c. 
and  titrating  with  standard  dichromate  solution.  The  difference  between  the 
amounts  of  that  solution  required  gives  the  quantity  of  ferrous  sulphate  oxidized 
by  the  manganese  dioxide,  and  from  this  the  percentage  of  manganese  in  the 
sample  can  be  calculated. 

The  ferrous  sulphate  solution  should  be  standardized  from  day  to  clay,  as  it 
undergoes  slow  oxidation  on  exposure  to  air. 

A  solution  of  bromine  in  water  may  of  course  be  used  instead  of  the  hypo- 
chlorite solution,  in  which  case  no  acid  is  added  to  the  bromine  solution.  When 
using  bromine,  a  solution  containing  about  0-7  gm.  of  bromine  (about  22  gm.  per 
litre)  should  be  used,  and  90  c.c.  of  this  solution  used  for  precipitating  about  0!25 
gm.  of  manganese. 

The  unpleasantness  of  working  with  bromine  may  be  mitigated,  to  some  extent, 
by  adding  to  the  bromine  solution,  before  pouring  it  into  the  liquid  containing 
the  manganese,  a  few  drops  of  a  solution  of  sodium  hydrate  until  nearly  all,  but 
not  quite  all,  the  bromine  is  taken  up.  If  an  excess  of  sodium  hydrate  were 
added  to  the  bromine  it  would  produce  a  precipitate  on  pouring  it  into  the  man- 
ganese solution,  and  this  is  to  be  avoided. 

It  is  preferable  to  have  both  zinc  and  iron  in  solution  with  the  manganese. 
When  working  with  either  of  these  alone  all  the  manganese  is  obtained  in  the 
form  of  dioxide,  but  with  iron  alone  there  is  a  greater  tendency  to  the  formation 
of  permanganate  than  when  zinc  is  also  present.  This  point  was  also  noticed  by 
Wright  and  Menke.*  When  zinc  alone  is  present,  it  is  found  that  the  precipita- 
tion of  the  dioxide  does  not  take  place  so  rapidly  as  when  iron  is  also  present.  When 
both  iron  and  zinc  are  used,  there  is  very  seldom  any  permanganate  formed,  if 

6  J.  C.  S.  Trans.  1883,  43. 


MANGANESE.  253 

care  is  taken  not  to  use  an  unnecessarily  large  excess  of  chlorine  or  bromine,  but 
occasionally  there  is  a  small  quantity  formed,  especially  if  the  precipitate  is  left 
to  stand  some  considerable  time  before  filtering.  It  was  found  that  the  addition 
of  a  very  small  quantity  of  alcohol  immediately  after  the  precipitation  of  the 
manganese  is  complete  entirely  prevents  the  formation  of  permanganate  even 
when  a  large  excess  of  chlorine  has  been  used,  and  for  this  reason  it  is  well  to  add  it. 

When  filtering  paper  has  been  wetted  with  the  solution  containing  free 
chlorine  or  bromine  and  afterwards  washed  clean,  it  has  no  reducing  action  either 
upon  potassium  dichromate  or  upon  ferric  sulphate.  The  addition  of  the  filter 
together  with  the  precipitate  to  the  solution  of  ferrous  sulphate,  therefore,  does  not 
influence  the  result. 

It  must  be  pointed  out  that  the  presence  of  lead,  copper,  nickel,  cobalt,  and 
chromium  in  the  substances  under  examination  interferes  with  the  accuracy  of 
this  method  of  titrating  manganese. 

It  was  found  that  so  large  a  proportion  as  1  per  cent,  of  lead,  copper,  and  nickel 
does  not  greatly  interfere  with  the  test,  but  the  interference  of  cobalt,  and 
especially  of  chromium,  is  serious.  All  these  substances,  except  chromium, 
form,  under  the  conditions  of  the  test,  higher  oxides  insoluble  in  water,  which 
are  precipitated  with  the  manganese  dioxide,  and  which  oxidize  ferrous  sulphate 
to  ferric  sulphate  ;  whilst  chromium  forms  some  insoluble  chromate  which  goes 
down  with  the  manganese  dioxide. 

Fortunately  these  metals  rarely,  if  ever,  occur  in  the  ores  of  manganese  or  in 
spiegeleisen  and  ferromanganese  in  sufficient  quantity  to  affect  the  practical 
accuracy  of  this  test. 

This  volumetric  method^  cannot,  however,  be  applied  to  the  determination  of 
manganese  in  alloys  of  these  metals,  such  as  ferrochrome  or  in  ores  containing 
these  metals,  without  previously  separating  them  from  the  solution  containing 
the  manganese. 

1  c.c.  N/10  dichromate  =  0-002747  gm.  Mn. 

The  above  is  undoubtedly  one  of  the  best  volumetric  methods 
known  for  the  determination  of  manganese  in  various  compounds 
and  ores  ;  but  Saniter*  in  criticising  the  method  gives  it  the 
credit  for  yielding  slightly  low  results,  and  advocates  the 
standardizing  of  the  dichromate,  not  upon  iron,  but  upon  a  man- 
ganese oxide  of  known  composition. 

Atkinsonf  gives  the  following  short  description  of  the  method 
as  practically  in  daily  use  in  a  large  steel  works. 

Weigh  out  0-5  gm.  or  0-6  gm.  of  an  ore  containing  about  20  per  cent, 
manganese,  dissolve  in  7  or  8  c.c.  of  strong  HC1,  and  when  dissolved,  wash  the 
whole,  without  filtering,  into  a  large  narrow-sided  beaker.  When  cold  it  is 
neutralized  with  precipitated  calcium  carbonate,  until  the  liquid  assumes 
a  reddish  hue.  40  or  50  c.c.  of  saturated  bromine  water  are  added,  and  the 
mixture  allowed  to  stand  in  the  cold  for  half-an-hour.  At  the  expiration  of  that 
time  the  beaker  is  nearly  filled  up  with  boiling  water,  and  precipitated  calcium 
carbonate  added  until  there  is  110  further  effervescence,  and  part  of  the  carbonate 
is  evidently  unacted  upon.  A  small  quantity  of  alcohol  is  then  added,  the  whole 
well  stirred,  and  the  precipitate  allowed  to  settle.  The  clear  liquid  is  filtered  off 
and  fresh  boiling  water  added  to  the  residue  in  the  beaker,  a  little  alcohol  being 
used  to  reduce  any  permanganate  which  is  formed.  The  filtration  and  washing 
are  repeated  until  the  filtrate  when  cooled  no  longer  turns  iodized  starch-paper 
blue.  During  the  washing  about  1*9  to  2'5  gm.  of  pure  granular  ferrous- 
ammonium  sulphate  are  weighed  out,  washed  into  the  beaker  in  which  the 
precipitation  took  place,  and  about  30  to  50  c.c.  of  dilute  sulphuric  acid  added. 
The  filter  containing  the  precipitated  Mn02,  is  then  placed  in  the  beaker,  and 

*  J.  S.  C.  1. 13,  112.  f  J.  S.  C.  I.  5,  365. 


254  MANGANESE. 

the  latter  is  quickly  dissolved  by  the  oxidation  of  a  portion  of  the  ferrous  salt 
into  ferric  sulphate.  The  remaining  ferrous  iron  is  then  titrated  with  dichromate 
in  the  .usual  way.  The  difference  between  the  number  of  c.c.  of  dichromate  used 
and  the  number  which  the  original  weight  of  the  ferrous-ammonium  sulphate 
would  have. required  if  directly  titrated  is  a  measure  of  the  quantity  of  3fn02 
present.  For  ;rapidity  and  simplicity  this  volumetric  process  leaves  nothing  to 
be  desired  ;  duplicate  experiments  agree  within  very  narrow  limits  ;  and  if  the 
assumption  be  accepted  that  the  presence  of  ferric  chloride  effects  the  complete 
oxidation  of  the  manganese  to  the  state  of  peroxide,  no  other  process  can  compete 
with  it. 

Pattinson  prefers  to  use  bleach  solution  to  bromine,  because  the 
formation  of  permanganate  is  more  easily  seen.  In  any  case  not 
more  than  a  trace  of  permanganate  should  be  formed,  and  if  the 
first  experiment  shows  this  to  be  the  case,  another  trial  must  be 
commenced  with  less  oxidizing  material. 

J.  W.  Westmoreland  describes  a  modified  method  which  is 
designed  to  overcome  some  objections  raised  against  the  above 
processes. 

With  ferro-manganese  and  ores  containing  about  50  to  60  %  of  Mn,  about 
0'4  gm.  is  taken  ;  ores  with  40  %,  0'5  gm.  ;  manganiferous  iron  ores,  with  say 
about  20  %  each  of  Fe  and  Mn,  0'75  gm.  ;  spiegeleisen  and  silicospiegels,  with 
about  25  %  Mn,  the  same. 

The  material,  having  been  brought  into  solution  by  any  of  the  methods 
described,  is  concentrated  to  a  small  bulk  in  a  large  conical  beaker.  A  solution 
of  ferric  chloride,  containing  about  the  same  amount  of  iron  as  there  is 
approximately  of  Mn,  is  added,  together  with  "a  solution  of  zinc  chloride, 
containing  about  0'5  gm.  of  Zn.  The  excess  of  acid  is  then  neutralized  with 
caustic  potash,  so  that  the  bulk  of  liquid  is  about  80  c.c.  To  this  is  added 
about  60  c.c.  of  saturated  bromine  water  (more  for  ferro-manganese,  less  for 
manganiferous  iron  ores)  and  zinc  oxide  emulsion*  is  gradually  dropped  in  with 
shaking,  until  the  Fe  and  Mn  are  precipitated,  (care  must  be  taken  to  avoid  a 
large  excess  of  zinc  oxide).  The  beaker  is  then  filled  up  with  boiling  taprwater, 
and  the  clear  liquid  poured  through  a  filter,  previously  adding  a  few  drops  of 
alcohol.  The  beaker  is  then  filled  with  boiling  water  five  times  in  succession, 
the  precipitate  being  stirred  up  with  the  hot  water  each  time  of  washing  and 
allowed  to  settle.  It  is  then  brought  on  the  filter,  and  again  freely  washed  with 
boiling  distilled  water.  The  filter  and  contents  are  then  transferred  to  the 
beaker,  an  excess  of  acid  solution  of  ferrous  sulphate  added,  and  when  the 
precipitate  is  dissolved  the  liquid  is  diluted  with  cold  distilled  water,  and  the 
excess  of  ferrous  iron  determined  at  once  with  permanganate.  The  value  of  the 
iron  solution  in  metallic  iron  is  found  by  titrating  the  same  volume  of  iron 
solution  as  has  actually  been  used  for  dissolving  the  Mn  precipitate,  and  the  Fe- 
oxidized  multiplied  by  0'49 18  =Mn. 

It  is  absolutely  necessary,  in  order  to  get  accurate  results,  to 
wash  the  precipitate  as  thoroughly  as  mentioned. 

2KMn04  +  3MnO = K20  +  5Mn02 
1  c.c.  N/10  permanganate  =-00 16 48  gm.  Mn. 

*  The  emulsion  of  zinc  oxide  may,  of  course,  be  r'eadily  made  by  rubbing  down  pure 
zinc  oxide  in  water  so  as  to  be  of  about  the  consistence  of  cream.  Emnierton 
(Trans.  Amer.  Jnst.  Min.Eng.10,  201),  suggests  the  following  method  of  preparing 
this  reagent.  Dissolve  ordinary  zinc  white  in  IIC1,  add  the  powder  until  there 
remains  some  undissolved,  then  add  a  little  bromine  water;  heat  the  mixture,  filter 
and  precipitate  the  zinc  oxide  from  the  nitrate  with  the  slightest  possible  excess  of 
ammonia.  Wash  thoroughly  by  decantation,  and  finally  wash  into  a  bottle  with 
approxiniately  enough  water  to  give  a  proper  consistence.  By  this  method  a  very 
flnely  divided  oxide  is  obtained,  owing  to  its  not  being  dried. 


MANGANESE.  255 

2.     Volhard's  Permanganate  Method. 

This  is  now  largely  used  by  Continental  and  American  chemists, 
especially  with  certain  modifications* ;  the  details  of  the  original 
process  being  as  follows  : — 

A  quantity  of  material  is  taken  so  as  to  contain  from  0'3  to  0*5  gm.  Mn, 
dissolved  in  hydrochloric  or  nitric  acid,  evaporated  in  porcelain  to  dryness,  first 
adding  a  little  ammonium  nitrate,  then  heated  over  the  flame  to  destroy  organic 
matter.  The  residue  is  digested  with  HC1,  a  little  strong  H2SO4  added,  and 
again  evaporated  to  dryness,  first  on  the  water-bath,  then  with  greater  heat  till 
vapours  of  SO3  are  evolved.  It  is  then  washed  into  a  litre  flask  and  neutralized 
with  sodium  hydrate  or  carbonate  ;  sufficient  pure  zinc  oxide,  made  into  a  cream, 
is  added  to  precipitate  all  the  iron.  The  flask  is  filled  to  the  mark,  shaken,  and 
200  c.c.  filtered  off  into  a  boiling  flask,  acidified  with  2  drops  of  nitric  acid,  (sp. 
gr.  1-2),  heated  to  boiling,  and  titrated  with  N/io  permanganate  while  still  hot. 
Owing  to  the  presence  of  the  trace  of  nitric  acid,  most  operators  now  deduct  0'2  c.c. 
of  permanganate  before  calculating  the  manganese. 

Sarnstro  m's  method  of  using  this  process  for  determining 
manganese  in  iron  ores,  as  described  by  Mixer  and  Du  Bois,  is  to 
precipitate  the  iron  in  hot  dilute  solution  by  sodium  carbonate,  care 
being  taken  to  add  no  more  than  is  just  sufficient  to  precipitate 
the  iron  ;  then  titrating  (without  filtering  off  the  ferric  oxide)  with 
permanganate.  Using  the  soda  in  this  way  does  not  give  perfect 
neutralization,  yet  it  gives  excellent  results,  as  shown  by  both 
Mixer  and  Du  Bois  and  G.  Auchy.  This  is  difficult  to  explain. 
as  mentioned  by  the  latter  chemist,  because  in  working  either 
Volhard's  method  or  Stone's  modification  of  it,  there  must  be 
a  much  larger  quantity  of  zinc  emulsion  used  than  is  necessary  to 
precipitate  the  iron,  in  order  to  avoid  too  high  results.  But  all 
experiments  show  that  Sarnstrom's  process  is  quite  free  from 
this  error.  Auchy  is  of  opinion  that  either  the  ferric  oxide,  by 
its  presence,  in  some  way  prevents  high  results  being  obtained 
when  solutions  are  incompletely  neutralized,  or  by  its  presence 
prevents  the  precipitation  of  manganese  dioxide,  unless  the  solu- 
tion be  thoroughly  neutralized  when  titrated  ;  the  permanganate 
simply  colouring  the  solution,  and  no  manganese  being  precipitated 
unless  more  sodium  carbonate  is  added. 

G.  E.  Stone's  modification  of  the  original  Volhard's  method 
gives  an  easier  and  quicker  result,  as  no  evaporation  with  sulphuric 
acid  is  needed,  and  the  precipitate  of  ferric  oxide  rapidly  subsides 
in  the  faintly  acid  nitric  acid  solution. 

METHOD  OF  PROCEDURE  :  The  necessary  precautions,  as  given  by  G.  Auchy, 
are  printed  in  italics.  3 '3  gm.  of  drillings  are  dissolved  in  50  c.c.  of  nitric  acid 
(sp.  gr.  1-20)  and  washed  into  a  500  c.c.  measuring  flask.  Two-thirds  of  the 
amount  of  sodium  carbonate  solution  necessary  for  complete  neutralization  are 
added,  and  the  liquid  cooled.  Zinc  oxide  emulsion  is  then  added  until  the  solution 
stiffens,  an  excess  being  avoided.  After  dilution  to  about  three-fourths  the  capacity 
of  the  flask  the  whole  is  allowed  to  stand  until  the  ferric  oxide  begins  to  settle, 

*  Modifications  of  V  o  1  h  a  r  d  '  s  original  method  have  been  discussed  by 
Mayer,  Z.  angew.  Chem.  1907,  1080,  and  Analyst  1908,  34  ; 
H  eike,  StaM  u.  Eisen,  1909,  1921,  and  J.  S.  C.  I.  1910,  114 ; 
Deis  s,  Chem.  Zeit.  1910,  237,  and  J.  S.  C.  I.  1910,  456. 
Fischer,  Z.  anal.  Chem.  1909,  751,  and  J  S.  C.I.  1910,47. 


256  MANGANESE. 

and  a  considerable  excess  of  zinc  oxide  emulsion  then  added  to  the  colourless  solution. 
After  being  made  up  to  the  mark  and  well  shaken,  the  precipitate  is  allowed  to 
settle,  and  250  c.c.  of  the  clear  solution  heated  in  a  flask  to  boiling  and  titrated 
with  permanganate  of  strength  0-0056.  After  making  the  necessary  deductions 
for  impurities  in  the  sodium  carbonate  and  zinc  oxide  (which  have  been  previously 
ascertained),  the  number  of  c.c.  of  permanganate  taken  is  divided  by  10,  and 
OO2  per  cent,  deducted  from  the  result. 

Fischer's  Modification  of  Vol hard's  Method.*  This  modification 
is  claimed  to  give  accurate  results  in  the  presence  of  chlorides  and 
when  titrating  with  a  solution  of  permanganate  standardized  by 
oxalic  acid,  without  the  use  of  a  correcting  factor.  A  solution  of 
manganous  salt,  containing  hydrochloric  or  sulphuric  acid,  is 
treated  as  follows  : — 

Sodium  hydroxide  is  added  until  a  slight  precipitate  appears;  this  is  re- 
dissolved  with  a  few  drops  of  sulphuric  acid,  1  gin.  of  freshly  ignited  zinc  oxide 
and  10  gm.  of  zinc  sulphate  are  added,  and  the  solution  is  titrated  with  per- 
manganate, with  frequent  boiling  and  shaking.  Then  1  c.c.  of  pure  glacial  acetic 
acid  is  added  and  the  liquid  is  boiled  ;  this  discharges  the  pink  colour,  apparently 
owing  to  the  liberation  of  absorbed  manganous  salt  from  the  precipitate,  which 
becomes  flocculent  and  settles,  and  the  liquid  is  again  titrated  until  the  pink 
colour  is  restored,  the  total  volume  of  permanganate  used  being  taken  as  that 
required  to  react  with  the  manganese.  If  sulphates  only  be  present,  about  10  gm. 
of  zinc  sulphate  are  added  for  every  10  c.c.  of  N/io  permangate  used.  Iron  when 
present,  as  in  the  analysis  of  ferromanganese,  is,  as  usual,  precipitated  with  zinc 
oxide,  and  manganese  is  determined  in  the  filtrate. 

Cahen  and  Littlef  have  investigated  this  modification  and  find 
that  it  gives  very  consistent  results. 


3.     Determination  by  conversion  into  Permanganic  Acid. 

This  method,  in  which  the  manganese  is  oxidized  to  perman- 
ganic acid  by  bismuth  peroxide  in  the  presence  of  nitric  acid,  was 
originally  devised  by  Schneider.  Further  experiments  were 
afterwards  carried  on  by  Reddrop  and  Ramagef,  the  result  of 
which  was  that  they  used  sodium  bismuthate  in  the  solid  state  for 
the  conversion  of  the  manganese. 

Their  paper  gives  details  of  the  experiments  011  ferro-manganese, 
spiegel,  silico-spiegel,  iron  and  steel,  too  voluminous  to  be  reproduced 
here  ;  but  the  final  method  for  wronght  iron,  steel,  and  pig  iron  is 
as  follows  :— 

Weigh  out  1*1  gm.  of  the  sample,  and  place  it  in  a  beaker  or  boiling-tube  ;  add 
30  c.c.  of  dilute  nitric  acid  (sp.  gr.  1'2)  and  boil.  If  a  part  remains  undissolved, 
decant  the  solution,  filter  off  carbonaceous  matter  if  necessary,  a.nd  add  more 
nitric  acid  up  to  25  c.c.  If  the  sample  dissolves  completely,  use  the  25  c.c.  of  acitl 
to  Wash  the  tube.  If  the  sample  contains  much  silicon,  care  must  be  taken, 
when  boiling,  not  to  concentrate  the  acid  unduly,  or  the  silicic  acid  will  separate? 
in  a  form  which  blocks  up  the  filter. 

Cool  the  solution  in  a  beaker  to  about  16°,  oxidize  with  2  gm.  of  sodium 
bismuthate,  stir  for  three  minutes,  and  filter  through  an  asbestos  filter  into  a 
clean  flask.  Add  N/io  hydrogen  peroxide  solution  from  a  burette  until  the 

*  Z.  a.  Chem.  1909,  48,  751-760.  f  Analyst,  1911.  {  J.  C.  S.  66,  268. 


MANGANESE.  257 

reddish  colour  disappears;  then  add  from  1*5  to  3'0  c.c.  in  excess,  and  titrate 
with  N/io  potassium  permanganate. 

Each  c.c.  of  N/IO  hydrogen  peroxide  reduced  by  the  sample  =0*1  per  cent,  of 
manganese. 

The  reddish  colour  mentioned  above  is  produced  by  a  secondary  reaction  during 
the  titration,  probably  between  the  permanganic  acid  and  the  reduced  manganous 
nitrate.  A  small  quantity  of  manganic  salt  appears  to  be  formed,  and  the  yellow 
colour  of  this  masks  the  colour  of  the  permagnanic  acid.  This  yellow  solution  is 
more  difficult  to  reduce  than  the  solution  of  permanganic  acid,  and  the  tinctorial 
power  of  the  compound  is  much  less  than  that  of  the  acid  ;  hence  the  necessity 
for  adding  an  excess  of  hydrogen  peroxide. 

Hydrochloric  acid  must  be  absent.  The  results  are  within  O'Ol  per  cent,  of 
the  manganese  present. 

A  modification  of  this  method  has  been  adopted  by  Ibbotson 
and  Brearley.* 

These  authors  consider  that  the  reduction  of  the  permanganate 
during  and  after  filtration  is  caused  by  the  presence  of  carbon.  It 
is,  therefore,  essential  to  complete  the  oxidation  of  the  carbon  in 
hot  solution,  and  this  should  certainly  be  done  with  high  carbon 
steels.  The  method  generally  adopted  by  them  is  as  follows  : — 

Dissolve  I'l  gm.  of  the  metal  in  35  c.c.  of  1'20  nitric  acid,  and  then  add  sodium 
bismuthate  a  little  at  a  time  to  the  somewhat  cooled  solution  until  a  permanganate 
colour  persists,  or  on  boiling  is  decomposed  to  manganic  oxide.  Clear  up  the 
permanganate  or  the  dioxide  precipitate  with  a  little  hydrogen  peroxide, 
sulphurous  acid,  or  ferrous  sulphate  free  from  manganese.  Cool  the  solution, 
and  add  about  10  c.c.  of  water  and  a  considerable  excess  of  bismuthate.  Filter, 
wash  with  dilute  (3  or  4  per  cent.)  nitric  acid,  and  titrate  with  N/io  permanganate. 

Unlike  hydrogen  peroxide,  the  excess  of  ferrous  sulphate  does  not  react  with 
ferric  nitrate,  and  is  therefore  preferable.  It  can  be  safely  used  in  cold  nitric  acid 
solutions  as  above,  if  the  titration  is  not  needlessly  delayed.  Molybdenum, 
titanium,  and  vanadium  do  not  interfere  in  this  modification  as  they  do  in  R  e  d  d  r  o  p 
and  Ram  age's  process.  If  chromium  be  present,  it  is  liable  to  be  slowly  but 
completely  oxidized  by  bismuthate  to  chromic  acid,  thus  giving  high  results 
for  manganese.  The  bismuthate  should  therefore  be  added,  and  the  solution 
shaken  and  filtered  quickly.  The  presence  of  tungsten  introduces  no  error ; 
but  in  steels  containing  much  tungsten,  and  particularly  in  chrome  tungsten 
steels,  the  precipitate  has  a  tendency  to  pass  through  the  filter.  As  the  joint 
presence  of  tungsten  and  hydrofluoric  acid  causes  the  results  to  be  high  and  very 
erratic,  the  hydrofluoric  acid  should  be  previously  driven  off  with  sulphuric  acid. 

For  the  application  of  this  process  to  steels,  ores,  etc.,  see  A.  A. 
Blair,  J.  Am.  Chem.  Soc.,  26,  793. 

A  simplification  of  Reddrap  and  Ramage's  process  has  been 
made  by  Duf  tyf  for  use  in  iron  works. 

This  colorimetric  process  is  as  follows : — The  nitric  acid  solution  is  transferred 
(after  the  carton  has  been  determined  by  the  Eggertz  method)  to  a  graduated 
stoppered  test-mixer,  the  carbon  tube  being  rinsed  out  into  the  mixer,  and  the 
solution  then  made  up  to  a  definite  volume  with  nitric  acid  (sp.  gr.  1'20,  is  used 
throughout).  A  standard  steel,  of  known  Mn  content,  is  treated  in  like  manner, 
being  diluted  to  the  same  volume  as  the  sample.  Equal  quantities  of  bismuthate 
are  then  added  through  a  dry  funnel,  and  the  contents  thoroughly  mixed,  the 
mixing  being  repeated  five  minutes  later.  After  being  allowed  to  settle  in  a 
dark  cupboard,  which  usually  takes  about  thirty  minutes,  measured  quantities 

*  C.  N.  84,  247.  t  C.  N.  84,  248. 


258  MANGANESE. 

of  the  clear  pink  solutions  are  transferred  by  means  of  a  pipette  to  stoppered 
carbon  tubes,  and  the  colours  compared  in  the  usual  manner. 

In  practice  it  has  been  found  most  convenient  to  weigh  out  O'l  gm.  of  steel, 
dissolving  in  2  or  3  c.c.  HN03,  according  to  carbon  content,  and  after  the  carbon 
has  been  determined  transferring  to  25  c.c.  test-mixers,  and  diluting  with  HN03 
to  20  c.c.  if  the  manganese  be  under  0*8  per  cent.,  or  to  25  c.c.  if  over  that 
percentage.  0'2  gm.  bismuthate  is  then  added  to  each  tube,  and  after  mixing  and 
allowing  to  settle  exactly  5  c.c.  are  transferred  to  the  comparing  tubes,  the 
standard  being  diluted  to  a  convenient  tint  for  comparison  (e.g.,  0*82  standard  to 
16*4  c.c.).  The  standard  used  for  the  carbon  determination  may  conveniently  be 
used  for  determining  the  manganese,  and  one  standard  solution  will  serve  for 
a  batch  of  determinations  as  in  the  carbon  colorimetric  test. 

The  above  modification  of  the  "  bismuthate  process  "  has  been 
used  for  some  time  with  satisfaction.  For  steel  works  with  Siemens 
or  Bessemer  plant,  where  a  large  number  of  analyses  have  to  be 
made  as  rapidly  as  possible,  the  advantages  of  this  modification 
over  the  original  volumetric  process  are  obvious  ;  a  considerable 
saving  in  time  not  only  being  effected,  but  90  per  cent,  less 
"  bismuthate  "is  required  for  each  determination. 

Metzger  and  McCrackan*  Modification  of  the  Bismuthate 
Method. — This  depends  on  the  fact  that,  in  hot  sulphuric  acid 
solution,  sodium  bismuthate  oxidizes  manganese  to  the  quadrivalent 
condition  (not  to  permanganate),  and  to  the  further  fact  that  in 
cold  sulphuric  acid  solution  the  oxidizing  power  of  sodium  bismuth- 
ate  is  so  restricted,  that  it  is  unnecessary  to  remove  the  excess  of 
bismuthate  from  the  cooled  solution  by  filtration  before  proceeding 
to  the  titration  of  the  quadrivalent  manganese. 

METHOD  OF  PROCEDURE  :  To  the  manganese  solution  10  to  15  c.c.  of  sulphuric 
acid  are  added,  together  with  sufficient  water  to  bring  the  volume  up  to  60  or  70 
c.c.  The  mixture  is  allowed  to  cool,  and  then  1  to  2  gm.  of  sodium  bismuthate 
are  introduced  into  the  flask  in  such  a  way  that  none  sticks  to  the  neck  or  sides. 
The  flask  is  placed  in  a  cold  water-bath,  which  is  then  heated  to  boiling,  and  kept 
boiling  for  twenty  minutes.  The  contents  of  the  flask  are  cooled  under  the  tap, 
a  measured  amount  of  standard  ferrous  sulphate  solution,  more  than  sufficient  to 
react  with  the  tetravalent  manganese,  is  added,  the  solution  diluted  to  200  c.c., 
and  the  excess  of  ferrous  sulphate  determined  by  titration  with  permanganate. 
Mn=2Fe.  The  extreme  error  of  the  method  is  0'3  c.c.  of  N/IQ  solution,  and 
manganese  is  never^  over-estimated. 

4.    Persulphate  Method  (K  n  o  r  r  e).t 

In  this  method  the  manganese  is  oxidized  by  means  of  ammonium 
persulphate  and  precipitated  as  hydrated  manganese  dioxide,  which 
is  filtered  off,  treated  with  hydrogen  peroxide  or  ferrous  sulphate, 
and  titrated  with  permanganate,  as  in  the  previous  method. 

METHOD  OF  PROCEDURE  :  The  solution  containing  manganese  (such  a  quantity 
as  will  contain  about  O'l  gm.  of  the  metal)  is  placed  in  a  capacious  Erlenmeyer 
flask,  and  ammonium  persulphate  added  (50—100  c.c.  of  a  solution  containing 
about  60  gm.  per  litre).  The  liquid  is  boiled  for  about  five  minutes,  and  the 
precipitated  manganese  dioxide  filtered  and  washed.  It  is  then  transferred 
(with  the  filter)  to  the  flask  in  which  it  was  precipitated,  dilute  sulphuric  acid  is 
added,  followed  by  a  known  quantity  of  a  titrated  dilute  solution  of  hydrogen 

*/.  Amer.  Chem.  Soc.,  1910,  32,  1250.  f  Z.  angew.  Chem.,  1901,  1149. 


MANGANESE.  259 

4 

peroxide.  After  the  precipitate  is  completely  dissolved,  the  excess  of  hydrogen 
peroxide  is  determined  by  standard  potassium  permanganate,  and  the  manganese 
is  calculated  from  the  amount  of  permanganate  equivalent  to  the  hydrogen 
peroxide  destroyed.  Instead  of  hydrogen  peroxide,  ferrous  sulphate  may  be 
used. 

The  author  standardizes  his  permanganate  solution  by  carrying  the  method 
through  on  a  known  quantity  of  manganese  ammonium  sulphate,  potassium 
permanganate,  or  other  manganese  compound  of  definite  composition.  When 
the  permanganate  is  standardized  by  iron  or  ferrous  salts,  the  method  gives 
results  about  \\  per  cent,  too  low,  though  gravimetric  determinations  indicate 
that  the  manganese  is  completely  precipitated  by  the  boiling  with  persulphate. 

Schmidt*  oxidizes  the  manganese  by  persulphate  in  the  presence  of  silver 
nitrate,  and  makes  the  determination  colorimetrically. 

5.     Technical  Examination  of  Manganese  Ores  used  for 
Bleaching  Purposes,  etc. 

The  ore,  when  powdered  and  dried  for  analysis,  rapidly  absorbs 
moisture  on  exposing  it  to  the  air,  and  consequently  has  to  be 
weighed  quickly  ;  it  is  better  to  keep  the  powdered  and  dried  sample 
in  a  small  light  stoppered  bottle,  the  weight  of  which,  with  its 
contents  and  stopper,  is  accurately  known  About  1  or  2  gm.  or 
any  other  quantity  within  a  trifle,  can  be  emptied  into  the  proper 
vessel  for  analysis,  and  the  exact  quantity  found  by  reweighing 
the  bottle. 

A  hardened  steel  or  agate  mortar  must  be  used  to  reduce  the 
mineral  to  the  finest  possible  powder,  so  as  to  ensure  its  complete 
and  rapid  decomposition  by  the  hydrochloric  acid. 

Considerable  discussion  has  taken  place  as  to  the  best  processes 
for  determining  the  available  oxygen  in  manganese  ores,  arising 
from  the  fact  that  many  of  the  ores  now  on  the  market  contain  iron 
in  the  ferrous  state  ;  and  if  such  ores  be  analysed  by  the  usual  iron 
method  with  hydrochloric  acid,  a  portion  of  the  chlorine  produced 
is  employed  in  oxidizing  the  iron  contained  in  the  original  ore. 
Such  ores,  if  examined  by  Fresenius'  and  Wills'  method,  show 
therefore  a  higher  percentage  than  by  the  iron  method,  since  no 
such  consumption  of  chlorine  occurs  in  the  former  process.  Manu- 
facturers have  therefore  refused  to  accept  certificates  of  analysis 
of  such  ores  when  based  on  Fresenius'  and  Wills  'method.  This 
renders  the  volumetric  processes  of  more  importance,  and  hence 
various  experiments  have  been  made  to  ascertain  their  possible 
sources  of  error. 

The  results  show  that  the  three  following  methods  give  very 
satisfactory  results. t 

6.     Direct  Analysis  by  Distillation  with  Hydrochloric  Acid. 

This  is  the  quickest  and  most  accurate  method  of  finding  the 
quantity  of  available  oxygen  present  in  any  of  the  ores  of  manganese 

*  J.  Amer.Chem.  Soc.,  32,  965,  and  Analyst,  1910,  454. 

t  See  Schererand  Rumpf,  C.  N.  20,  302;  also  Pattinson,  ibid.  21,  266; 
and  Paul,  21,  16. 

S   2 


260  MANGANESE. 

or  mixtures  of  them.  It  also  possesses  the  recommendation  that 
the  quantity  of  chlorine  which  they  liberate  is  directly  expressed 
in  the  analysis  itself  ;  and,  further,  gives  an  estimate  of  the  quantity 
of  hydrochloric  acid  required  for  the  decomposition  of  any  particular 
sample  of  ore,  which  is  a  matter  of  some  moment  to  the  manu- 
facturer of  bleaching  powder. 

The  apparatus  for  the  operation  may  be  those  shown  in  figs.  38 
or  39.  For  precautions  in  conducting  the  distillation  see  p.  135 
et  seq. 

METHOD  OF  PROCEDURE  :  In  order  that  the  percentage  of  dioxide  shall  be 
directly  expressed  by  the  number  of  c.c.  of  N/io  thiosulphate  solution  used, 
0-4347  gm.  of  the  properly  dried  and  powdered  sample  is  weighed  and  put  into 
the  little  flask  ;  solution  of  potassium  iodide  in  sufficient  quantity  to  absorb  all 
the  iodine  set  free  is  put  into  the  large  tube  (if  the  solution  containing  T%  eq.  or 
33-2  gm.  in  the  litre  be  used,  about  70  or  80  c.c.  will  in  ordinary  cases  be 
sufficient) ;  very  strong  hydrochloric  acid  is  then  poured  into  the  distilling  flask, 
and  the  operation  conducted  as  on  p.  137.  Each  equivalent  of  iodine  liberated 
represents  1  eq.  Cl,  also  1  eq.  Mn02. 

Instead  of  using  a  definite  weight,  it  is  well  to  do  as  before 
proposed,  namely,  to  pour  about  the  quantity  required  out  of  the 
weighed  sample-bottle  into  the  flask,  and  find  the  exact  weight 
afterwards. 

Barlow*  records  a  good  method  of  separating  Mn  from  the 
metals  of  its  own  group  as  well  as  from  alkalies  and  alkaline 
earths. 

For  the  quantitative  determination  of  Fe  and  Mn  in  the  same 
solution  as  chlorides  (other  metals  except  Cr  and  Al  may  be  present, 
but  are  best  absent),  solution  of  NH4C1  is  first  added,  then  strong 
NH4HO  in  excess,  boil,  then  add  hydrogen  peroxide  so  long  as  a 
precipitate  falls,  boil  for  a  few  minutes,  filter,  wash  with  hot  water, 
ignite,  and  weigh  the  mixed  oxides  together  as  Fe2O3+Mn3O4. 

The  oxides  are  then  distilled  with  HC1,  and  the  amount  of  iodine 
found  by  thiosulphate. 

The  weight  of  mixed  oxides,  minus  the  Mn3O4,  gives  the  weight 
of  Fe2O3. 

Pickering*  has  pointed  out  that  pure  manganese  oxides,  freshly 
prepared,  or  the  dry  oxides  in  very  fine  powder,  may  be  rapidly 
determined  without  distillation  by  merely  adding  them  to  a  large 
excess  of  potassium  iodide  solution  in  a  beaker,  running  in  2  or 
3  c.c.  of  hydrochloric  acid,  when  the  oxides  are  immediately 
attacked  and  decomposed  ;  the  liberated  iodine  is  then  at  once 
titrated  with  thiosulphate.  Impure  oxides,  containing  especially 
ferric  oxide,  cannot  however  be  determined  in  this  way,  since  the 
iron  would  have  £he  same  effect  as  manganic  oxide  ;  hence  distilla- 
tion must  be  resorted  to  in  the  case  of  all  such  ores,  and  it  is 
imperative  that  the  strongest  hydrochloric  acid  should  be  used. 

Pickering's  modified  process  is  well  adapted  to  the  examination 
of  the  Weldon  mud  for  its  available  amount  of  manganese  dioxide. 

*  C-  N.  53,  41.  f  J-  C.  S.  1880,  128. 


MANGAK-ESE.  26 1 

7.     Determination  by  Oxalic  Acid. 

The  very  finely  powdered  ore  is  mixed  with  a  known  volume  of 
normal  oxalic  acid  solution,  sulphuric  acid  added,  and  the  mixture 
heated  and  well  shaken,  to  bring  the  materials  into  intimate  contact 
and  liberate  the  CO2.  When  the  whole  of  the  ore  is  decomposed, 
which  may  be  known  by  the  absence  of  brown  or  black  sediment, 
the  contents  of  the  vessel  are  made  up  to  a  definite  volume  (say 
300  c.c.),  and  100  c.c.  of  the  dirty  milky  fluid  well  acidified,  diluted, 
and  titrated  for  the  excess  of  oxalic  acid  by  permanganate.  If,  in 
consequence  of  the  impurities  of  the  ore,  the  mixture  be  brown  or 
reddish  coloured,  this  would  of  course  interfere  with  the  indication 
of  the  permanganate,  and  consequently  the  mixture  in  this  case 
must  be  filtered;  the  300  c.c.  are,  therefore,  well  shaken  and  poured 
upon  a  large  filter.  When  about  100  c.c.  have  passed  through,  that 
quantity  can  be  taken  by  the  pipette  and  titrated  as  in  the  former 
case. 

If  the  solution  be  not  dilute  and  freely  acid,  it  will  be  found  that 
the  permanganate  produces  a  dirty  brown  colour  instead  of  its 
well-known  bright  rose-red  ;  if  the  first  few  drops  of  permanganate 
produce  the  proper  colour  immediately  they  are  added,  the  solution 
is  sufficiently  acid  and  dilute. 

If  4* 347  gm.  of  the  ore  be  weighed  for  analysis,  the  number  of  c.c. 
of  normal  oxalic  acid  will  give  the  percentage  of  dioxide  ;  but  as 
that  is  rather  a  large  quantity,  and  takes  some  time  to  dissolve 
and  decompose,  half  the  quantity  may  be  taken,  when  the  per- 
centage is  obtained  by  doubling  the  volume  of  oxalic  acid  used. 

This  process  possesses  an  advantage  over  the  following,  inas- 
much as  there  is  no  fear  of  false  results  occurring  from  the  presence 
of  air.  The  analysis  may  be  broken  off  at  any  stage,  and  resumed 
at  the  operator's  convenience. 

8.     Determination  by  Iron. 

Iron  wire  of  99*8  %  purity  can  readily  be  obtained  ;  but  if 
a  perfectly  dry  and  unoxidized  double  iron  salt  be  at  hand,  its  use 
saves  time.  1  mol.  of  this  saltf  =  392*17),  representing  43*47  of  MnO2, 
consequently,  1  gm.  of  the  latter  requires  9*022  gm.  of  the  double 
salt ;  or  in  order  that  the  percentage  shall  be  obtained  without  calcula- 
tion 1*108  gm.  of  ore  may  be  weighed  and  digested  in  the  presence 
of  free  sulphuric  acid,  with  10  gm.  of  double  iron  salt,  the  whole  of 
which  would  be  required  supposing  the  sample  were  pure  dioxide. 
The  undecomposed  iron  salt  remaining  at  the  end  of  the  reaction 
is  determined  by  permanganate  or  dichromate  ;  the  quantity  so 
found  is  deducted  from  the  original  10  gm.,  and  if  the  remainder 
be  multiplied  by  10  the  percentage  of  dioxide  is  arrived  at. 

Instead  of  this  plan,  which  necessitates  exact  weighing,  any 
convenient  quantity  may  be  taken  from  the  tared  bottle,  as  before 
described,  and  digested  with  an  excess  of  double  salt,  the  weight  of 
which  is  known.  After  the  undecomposed  quantity  is  found  by 


262  MANGANESE. 

permanganate  or  dichromate,  the  remainder  is  multiplied  by  the 
factor  Olll  (or  |),  which  gives  the  proportion  of  dioxide  present, 
whence  the  percentage  may  be  calculated. 

The  decomposition  of  the  ore  may  be  made  in  any  of  the  apparatus 
used  for  titrating  ferrous  iron.  The  ore  is  first  put  into  the  de- 
composing flask,  then  the  iron  salt  and  water,  so  as  to  dissolve  the 
salt  to  some  extent  before  the  sulphuric  acid  is  added.  Sulphuric 
acid  should  be  used  in  considerable  excess,  and  the  flask  heated  till 
all  the  ore  is  decomposed  ;  the  solution  is  then  cooled,  diluted,  and 
the  whole  or  part  titrated  with  permanganate  or  dichromate. 

In  the  case  of  using  N/10  dichromate  for  the  titration,  the  follow- 
ing plan  is  convenient: — 100  c.c.  of  N/10  dichromate  =  3*  922  gm.  of 
double  iron  salt  (supposing  it  to  be  perfectly  pure),  therefore  if 
0-4347  gm.  of  the  sample  of  ore  be  boiled  with  3'922  gm.  of  the  double 
salt  and  excess  of  acid,  the  number  of  c.c.  of  dichromate  required 
deducted  from  100  will  give  the  number  corresponding  to  the 
percentage. 

MERCURY. 

Hg  =  200. 
1  c.c.  N/10  solution -0-0200  gm.  Hg. 

=0-0208  gm.  Hg20. 
=0-0271  gm.  HgCl2 
Double  iron  salt  x  0-5104  =  Hg. 

xO-6914=HgCl2. 

1.    Precipitation  as  Mercurous  Chloride. 

THE  solution  to  be  titrated  must  not  be  warmed,  and  must 
contain  the  metal  in  the  form  of  protosalt  only.  N/10  sodium 
chloride  is  added  in  slight  excess,  filtered,  and  the  precipitate 
washed  with  the  least  possible  quantity  of  water  to  ensure  the 
removal  of  all  the  sodium  chloride  ;  to  the  filtrate  a  few  drops  of 
chromate  indicator  are  added,  then  pure  sodium  carbonate  till 
the  liquid  is  of  a  clear  yellow  colour,  N/10  silver  is  then  delivered 
in  till  the  red  colour  appears.  The  quantity  of  sodium  chloride  so 
found  is  deducted  from  that  originally  used,  and  the  difference 
calculated  in  the  usual  way. 

2.    By  Ferrous  Oxide  and  Permanganate  (M  o  h  r). 

This  process  is  based  on  the  fact  that  when  mercuric  chloride 
(corrosive  sublimate)  is  brought  in  contact  with  an  alkaline  solution 
of  ferrous  oxide  in  excess,  the  latter  is  converted  into  ferric  oxide, 
while  the  mercuric  is  reduced  to  mercurous  chloride  (calomel). 
The  excess  of  ferrous  oxide  is  then  found  by  permanganate  or 
dichromate — 

2HgCl2+2FeCl2=Hg2Cl2+Fe2Cl6. 

It  is  therefore  advisable  in  all  cases  to  convert  the  mercury  to  be 


MERCURY.  263 

determined  into  the  form  of  sublimate,  by  evaporating  it  to  dryness 
with  nitro-hydrochloric  acid  ;  this  must  take  place,  however,  below 
boiling  heat,  as  vapours  of  chloride  escape  with  steam  at  100°  C. 
(Fresenius). 

Nitric  acid  or  free  chlorine  must  be  altogether  absent  during  the 
decomposition  with  the  iron  protosalt,  otherwise  the  residual 
titration  will  be  inexact,  and  the  quantity  of  the  iron  salt  must  be 
more  than  sufficient  to  absorb  half  the  chlorine  in  the  sublimate. 

EXAMPLE. — 1  gm.  of  pure  HgCl2  was  dissolved  in  warm  water,  and  3  gm.  of 
double  iron  salt  added,  then  solution  of  caustic  soda  till  strongly  alkaline.  The 
mixture  became  muddy  and  dark  in  colour,  and  was  well  shaken  for  a  few 
minutes,  then  sodium  chloride  and  sulphuric  acid  added,  continuing  the  shaking 
till  the  colour  disappeared  and  the  precipitate  of  ferric  oxide  dissolved,  leaving 
the  calomel  white ;  it  was  then  diluted  to  300  c.c.,  filtered  through  a  dry  filter, 
and  100  c.c.  titrated  with  N/io  permanganate,  of  which  13 '2  were  required — 
13'2x3=39'6,  which  deducted  from  76'5  c.c.  (the  quantity  required  for  3  gm. 
double  iron  salt),  left  36'9  c.c.  =1*447  gm.  of  undecomposed  iron  salt,  which 
multiplied  by  the  factor  0'6914,  gave  1O005  gm.  of  sublimate,  instead  of  1  gm., 
or  the  36 '9  c.c.  may  be  multiplied  by  the  N/io  factor  for  mercuric  chloride,  which 
will  give  1  gm.  exactly. 


3.     By  Iodine  and  Thiosulphate  (H  e  m  p  e  1). 

If  the  mercury  exist  as  a  protosalt  it  is  precipitated  by  sodium 
chloride,  the  precipitate  well  washed  and  together  with  its  filter 
pushed  through  the  funnel  into  a  stoppered  flask,  a  sufficient 
quantity  of  potassium  iodide  added,  together  with  N/10  iodine 
solution  (to  1  gm.  of  calomel  about  2*5  gm.  of  iodide,  and  100  c.c. 
of  N/10  iodine),  the  flask  closed,  and  shaken  till  the  precipitate  has 
dissolved — 

Hg2Cl2 + 6KI + 12 = 2HgK2I4 + 2KC1. 

The  brown  solution  is  then  titrated  with  N/10  thiosulphate  till 
colourless,  diluted  to  a  definite  volume,  and  a  measured  portion 
titrated  with  N/10  iodine  and  starch  for  the  excess  of  thiosulphate. 
1  c.c.  N/10  iodine =0-02  gm.  Hg. 

Where  the  mercurial  solution  contains  nitric  acid,  or  the  metal  exists  as 
peroxide,  it  may  be  converted  into  protochloride  by  the  reducing  action  of 
ferrous  sulphate,  as  in  M  o  h  r '  s  method.  The  solution  must  contain  hydro- 
chloric acid  or  common  salt  in  sufficient  quantity  to  transform  all  the  mercury 
into  calomel.  Ferrous  sulphate  in  solution  in  quantity  equal  to  at  least  three  times 
the  weight  of  mercury  present  is  to  be  added,,  then  caustic  soda  in  excess,  the 
muddy  liquid  well  shaken  for  a  few  minutes,  then  dilute  sulphuric  acid  added  in 
excess,  and  the  mixture  stirred  till  the  dark-coloured  precipitate  has  become 
perfectly  white.  The  calomel  so  obtained  is  collected  on  a  filter,  well  washed,  and 
titrated  with  N/io  iodine  and  thiosulphate  as  above. 

C.  Reichardt*  has  recommended  the  following  method  of  determining  mercury 
A  weighed  quantity  of  the  mercury  compound  is  dissolved  and  converted  into 
mercury  arsenate  by  boiling  with  standard  arsenious  acid  and  caustic  soda  in 
excess.  Metallic  mercury  is  precipitated  as  a  black  powder.  This  is  filtered  and 
washed,  and  the  amount  of  unoxidized  arsenious  acid  in  the  filtrate  is  found  by 
titration  with  standard  iodine  in  the  usual  way. 

*  Z.  a.  C.  1898,  749. 


264  MERCURY. 

4.     As  Mercuric  Iodide  (P  e  r  s  o  n  n  e).* 

This  process  is  founded  on  the  fact  that  if  a  solution  of  mercuric 
chloride  be  added  to  one  of  potassium  iodide,  in  the  proportion  of 
1  equivalent  of  the  former  to  4  of  the  latter,  red  mercuric  iodide  is 
formed,  which  dissolves  to  a  colourless  solution  until  the  balance  is 
overstepped,  when  the  brilliant  red  colour  of  the  iodide  appears 
as  a  precipitate,  which,  even  in  the  smallest  quantity,  communicates 
its  tint  to  the  liquid.  The  mercuric  solution  must  always  be  added 
to  the  iodide  ;  a  reversal  of  the  process,  though  giving  eventually 
the  same  quantitative  reaction,  is  nevertheless  much  less  speedy 
and  trustworthy.  The  mercurial  compounds  to  be  determined  by 
this  process  must  invariably  be  brought  into  the  form  of  neutral 
mercuric  chloride. 

The  standard  solutions  required  are  decinormal,  made  as 
follows  : — 

Solution  of  potassium  iodide. — 33*2  gm.  of  pure  salt  to  1  litre. 
1  c.c.  =0-01  gm.  Hg.  or  0-01355  gm.  HgCl2. 

Solution  of  mercuric  chloride. — 13-546  gm.  of  the  salt,  with  about 
30  gm.  of  pure  sodium  chloride  (to  assist  solution),  are  dissolved  to 
1  litre.  1  c.c.=0-01  gm.  Hg. 

METHOD  OF  PROCEDURE  :  The  conversion  of  various  forms  of  mercury  into 
mercuric  chloride  is,  according  to  Personne,  best  effected  by  heating  with 
caustic  soda  or  potash,  and  passing  chlorine  gas  into  the  mixture,  which  is  after- 
wards boiled  to  expel  excess  of  chlorine  (the  mercuric  chloride  is  not  volatile  at 
boiling  temperature  when  associated  with  alkali  chloride).  The  solution  is 
then  cooled  and  diluted  to  a  given  volume,  placed  in  a  burette,  and  delivered  into 
a  measured  volume  of  the  decinormal  iodide  until  the  characteristic  colour  appears. 
It  is  preferable  to  dilute  the  mercuric  solution  considerably,  and  make  up  to1 
a  given  measure,  say  300  or  500  c.c.  ;  and  as  a  preliminary  trial  take  20  c.c.  or  so 
of  iodide  solution,  and  titrate  it  with  the  mercuric  solution  approximately  with 
a  graduated  pipette  ;  the  exact  strength  may  then  be  found  by  using  a  burette  of 
sufficient  size. 

lodimetric  Method  (Rupp)t.  This  can  be  used  for  mercuric 
nitrate,  chloride,  or  sulphate,  as  well  as  for  mercuric  cyanide. 

METHOD  OF  PROCEDURE  :  The  solution  of  the  mercury  salt,  containing  about 
0'2  gram  of  mercury  in  25  to  50  c.c.,  is  treated  with  excess  of  potassium  iodide 
(1  gram)  so  that  the  mercuric  iodide  that  forms  is  redissolved.  The  liquid  is 
next  rendered  alkaline  with  sodium  hydroxide,  treated  with  2  or  3  c.c.  of  40  per 
cent,  formaldehyde  solution,  diluted  with  10  c.c.  of  water,  and  vigorously  and 
continuously  shaken  for  one  to  two  minutes.  It  is  then  acidified  with  acetic 
acid,  25  c.c.  of  N/io  iodine  solution  added,  and  the  excess  of  free  iodine  titrated 
with  N/io  sodium  thiosulphate  solution.  The  formaldehyde  precipitates  the 
mercury,  which  combines  with  the  iodine  to  form  mercuric  iodide,  and  the  excess 
of  iodine  is  titrated  as  described.  If  the  mercury  be  in  the  form  of  mercurous 
salts,  it  must  be  brought  into  the  mercuric  state  before  precipitation,  by  treat- 
ment with  Br.  water,  excess  of  Br.  being  removed  by  gentle  heating.  In  the  case 
of  mercuric  cyanide,  sulphuric  acid  should  be  used  instead  of  acetic  acid  for  the 
acidification,  so  as  to  decompose  any  cyanogen  iodide  that  may  have  been  formed. 

*  Compt.  Rend.  56,  (53.  f  Ber.  1906,  39,  3702. 


MERCURY*  265 

5.     By  Potassium  Cyanide  (H  a  n  n  a  y). 

This  process  is  exceedingly  valuable  for  the  determination  of 
almost  all  the  salts  of  mercury  when  they  occur,  or  can  be  separated, 
in  a  tolerably  pure  state.  Organic  compounds  are  of  no  consequence 
unless  they  affect  the  colour  of  the  solution. 

The  method  depends  on  the  fact  that  free  ammonia  produces 
a  precipitate  or  (when  the  quantity  of  mercury  is  very  small)  an 
opalescence  in  mercurial  solutions,  which  is  removed  by  a  definite 
amount  of  potassium  cyanide. 

The  delicacy  of  the  reaction  is  interfered  with  by  excessive 
quantities  of  ammoniacal  salts,  or  by  caustic  soda  or  potash  ;  but 
this  difficulty  is  lessened  by  the  modification  suggested  by  Tuson 
and  Neison.* 

Chapman  Jo  nest  nas  further  modified  the  process  so  as  to 
make  it  easier  to  detect  the  end-point,  and  says  of  the  method  as 
worked  by  Tuson  and  Neison  :  "  Their  general  method  consists 
in  dissolving  the  mercury  compound  in  acid,  as  may  be  convenient, 
adding  a  little  ammonium  chloride,  and  then  potassium  carbonate, 
until  an  opalescent  precipitate  appears.  The  cyanide  solution  is 
then  added.  They  give  experiments  showing  the  trustworthiness 
of  the  process  as  applied  to  the  nitrate,  sulphate,  acetate,  oxalate, 
sebate,  and  citrate  of  mercury  ;  and  state  that  the  presence  of 
nitrates,  sulphates,  chlorides,  acetates,  oxalates,  citrates,  and 
buty rates  of  potassium,  sodium,  calcium,  and  magnesium,  and 
organic  matter  as  far  as  tested,  does  not  interfere  with  the  accuracy 
of  the  method. 

From  my  experience,  I  cannot  affirm  that  these  methods  of 
working  are  satisfactory.  There  is  considerable  uncertainty  as  to 
the  end  of  the  reaction,  because  less  potassium  cyanide  will  effect 
a  clearance  if  longer  time  is  allowed. 

These  difficulties  and  uncertainties  can,  I  find,  be  entirely 
eliminated,  and  the  process  reduced  to  a  series  of  operations  which 
are  comparatively  simple  and  rapid,  by  performing  the  titration  in 
an  entirely  different  manner  from  either  variation  suggested  by  the 
authors  referred  to.  I  employ  a  solution  of  mercuric  chloride 
containing  0*01  gm.  of  metal  per  c.c.,  and  a  solution  of  crystallized 
potassium  cyanide  made  by  dissolving  7  gm.  to  the  litre,  the  exact 
value  of  which  is  found  by  titrating  it  against  the  mercury  solution. 
Strong  ammonia  diluted  to  ten  times  its  bulk,  and  some  diluted  to 
fifty  or  a  hundred  times  its  bulk,  are  convenient. 

METHOD  OF  PROCEDURE  :  If  the  mercury  solution  is  not  fit  for  titration,  the 
metal  is  precipitated  as  sulphide,  which,  after  washing,  is  washed  off  the  filter  and 
allowed  to  subside  ;  the  clear  water  is  then  decanted  off,  and  aqua  regia  added  to 
the  moist  residue.  The  precipitate,  with  the  paper  it  is  on,  might  doubtless 
be  treated  direct  with  aqua  regia,  as  Tuson  and  Neison  found  that  organic 
matter,  so  far  as  they  tried  it,  does  not  influence  the  result.  To  avoid  the 
possibility  of  volatilizing  the  mercury  salt,  the  aqua  regia  is  allowed  to  act  in 
the  cold.  In  a  few  hours,  sometimes  in  far  less  time,  the  residue  is  of  a  pale 

*  J.  C.  S.  1877,  679.  t  J-  C.  S.  61,  364. 


266  MERCURY. 

yellow  colour,  and  the  solution  may  be  diluted  and  filtered.  The  solution,  or  an 
aliquot  part  of  it,  is  then  coloured  distinctly  with  litmus,  treated  with  successive 
small  quantities  of  powdered  potassium  carbonate  until  alkaline,  warming  but 
slightly,  and  then  rendered  just  acid  with  dilute  hydrochloric  acid,  with 
subsequent  boiling  to  remove  the  C02.  The  mercury  is  not  precipitated  at  all, 
unless  the  C02  is  boiled  out  before  acidification.  After  cooling,  the  dilutest 
ammonia  mentioned  above  is  added,  a  drop  at 'a  time,  until  the  litmus  in  the 
solution  shows  that  the  excess  of  acid  is  very  slight,  or  in  just  insufficient 
quantity  to  produce  a  permanent  precipitate.  A  quantity  of  cyanide  solution, 
which  is  known  to  be  in  excess  of  that  required,  is  added,  and,  as  a  guide  for  the 
first  titration,  the  ammonia  may  be  added  until  a  slight  precipitate  is  produced, 
and  cyanide  until  the  solution  is  cleared.  Two  or  three  drops  (not  more)  of  the 
1  in  10  ammonia  are  introduced,  and  then  more  of  the  mercury  solution  is  added 
until  the  permanent  turbidity  produced  matches  that  obtained  by  adding  O'l  c.c. 
of  the  mercury  solution  to  about  the  same  bulk  of  water  as  the  test,  and 
containing  approximately  the  same  amounts  of  litmus  and  free  ammonia.  Each 
drop  of  the  mercury  solution  added  produces  its  maximum  turbidity  in  a  few 
seconds,  and  it  can  be  seen  at  a  glance,  if  the  flasks  are  properly  placed,  whether 
this  turbidity  is  equal  to  or  less  than  the  standard.  In  a  few  seconds  more  it  is 
quite  obvious  whether  the  turbidity  is  permanent  or  is  growing  less.  Too  much 
free  ammonia  causes  the  precipitate  to  clot  together,  and  so  vitiates  the  result. 
The  presence  of  the  litmus  tends,  in  my  experience,  to  lessen  the  error  due  to 
the  variation  in  the  state  of  aggregation  of  the  precipitate  when  too  much  ammonia 
has  been  added.  The  turbidities  so  obtained  will  remain  apparently  unchanged 
for  many  hours.  The  0*1  c.c.  excess  of  mercury  solution  is  of  course  allowed  for 
in  the  calculation." 

Acidimetric  Cyanide  Method  (Rupp)*. — The  solution  of  the  mercuric  salt  is 
made  neutral  by  the  addition  of  alkali  chloride,  followed  by  phenolphthalein  and 
sufficient  alkali  to  just  redden  the  solution,  and  then  an  excess  of  N/4  or  N/2 
potassium  cyanide,  sufficient  to  produce  an  intense  red  colour,  is  added,  and  the 
excess  over  that  necessary  to  form  mercuric  cyanide  is  titrated  with  N/i  or  N/6 
hydrochloric  or  sulphuric  acid,  using  methyl  orange  as  indicator.  For  mercuric 
chloride,  direct  titration  with  potassium  cyanide  and  phenolphthalein  is  allowable, 
but  in  solutions  containing  much  alkali  chloride,  as  obtained  with  mercuric  nitrate 
and  sulphate,  the  indirect  method  given  above  is  to  be  preferred.  In  order  to 
apply  the  method  to  mercuric  cyanide,  potassium  iodide  is  added,  so  as  to  form 
the  compound,  K2HgI4,  and  liberate  potassium  cyanide,  which  is  then  titrated 
as  above.  Mercuric  oxide  may  be  dissolved  in  potassium  iodide  solution  by 
shaking  and  the  potassium  hydroxide  formed  directly  titrated  with  acid,  using 
methyl  orange  as  indicator. 

6.    Thiocyanate  Method.f 

This  may  be  carried  out  as  for  silver  salts,  see  p.  145.  It  is 
inapplicable  when  chlorides,  mercurous  salts,  and  nitrous  acid  are 
present.  It  is  especially  applicable  in  the  presence  of  nitric  acid 
and  heavy  metals.  Rupp  and  N611J  have  adapted  it  to  the 
valuation  of  organic  mercury  compounds  thus  : — 

METHOD  OF  PKOCEDUKE:  In  order  to  oxidize  the  organic  matter,  0'3  gm.  of 
the  substance  is  heated  in  a  150  c.c.  flask  with  4  gm.  of  potassium  sulphate  and 
5  c.c.  of  concentrated  sulphuric  acid  to  gentle  boiling  until  quite  clear.  A  reflux 
tube ,  40-50  cm.  long  is  provided  to  prevent  loss  of  mercury  sulphate  by 
volatilization,  and  this  is  then  rinsed  with  5-10  c.c.  of  concentrated  sulphuric 
acid  and  removed;  0'1-0'2  gm.  of  potassium  permanganate  is  now  added  to  ensure 
the  mercury  being  in  the  mercuric  condition,  and  the  heating  continued  until  the 
pink  colour  vanishes.  After  cooling,  the  liquid  is  diluted  to  about  100  c.c.,  again 
allowed  to  cool,  2  c.c.  of  10  per  cent,  iron  alum  solution  added  as  indicator,  and 
then  titrated  with  N/io  thiocyanate,  the  flask  being  frequently  rotated. 

1  c.c.  N/io  thiocyanate  =0-010015  gm.  Hg. 

*  Chem.  Zeil.  1908,  32,  1077.  t Ber.,  35,  2015.  }  Arch.  Pharm.,  1905,  243,  1-5. 


KICKEL.  267 

NICKEL. 

THE  best  method  for  the  determination  of  this  metal  volu- 
metrically  is  that  of  T.  Moore,*  whose  description  of  the  method 
is  as  follows  : — • 

"If  to  an  ammoniacal  solution  of  nickel  containing  Agl  in 
suspension  (silver  iodide  being  almost  insoluble  in  weak  ammonia) 
there  is  added  potassium  cyanide,  the  solution  will  remain  turbid  so 
long  as  all  the  nickel  is  not  converted  into  the  double  cyanide  of 
nickel  and  potassium,  the  slightest  excess  of  cyanide  being  indicated 
by  the  clearing  up  of  the  liquid,  and  furthermore,  this  excess  may 
be  exactly  determined  by  adding  a  solution  of  silver  until  the 
turbidity  is  reproduced.  It  is  a  fortunate  circumstance  that  the 
complicated  side-reactions  existing  in  Parkes's  copper  assay  do 
not  appear  to  take  place  with  nickel  solutions,  at  least  not  when  the 
temperature  is  kept  below-  20°  C.  This  is  fully  borne  out  by  the 
fact  that  the  cyanide  may  be  standardized  on  either  silver  or  nickel 
solutions  with  equal  exactness.  In  practice  it  has  been  found  best 
to  proceed  in  the  following  manner  : — 

Standard  solution  of  silver  nitrate,  containing  about  3  gm,  of 
silver  per  litre.  The  strength  of  this  solution  must  be  accurately 
known. 

Potassium  iodide,  10  per  cent,  solution. 

Potassium  cyanide,  22  to  25  gm.  per  litre.  This  solution  must 
be  tested  every  few  days,  owing  to  its  liability  to  change. 

Standardizing  the  Cyanide  Solution. — This  may  be  accomplished  in  two  ways ; 
(a)  on  a  solution  of  nickel  of  known  metallic  content,  or  (b)  on  the  silver 
solution. 

(a)  First,  accurately  establish  the  relation  of  the  cyanide  to  the  silver  solution, 
by  running  into  a  beaker  3  or  4  c.c.  of  the  former ;  dilute  this  with  abou't  150  c.c. 
of  water,  render  slightly  alkaline  with  ammonia,  and  then  add  a  few  drops  of 
the  potassium  iodide.  Now  carefully  run  in  the  silver  solution  until  a  faint 
permanent  opalescence  is  produced,  which  is  finally  caused  to  disappear  by  the 
further  addition  of  a  mere  trace  of  cyanide.  The  respective  volumes  of  the 
silver  and  cyanide  solutions  are  then  read  off,  and  the  equivalent  in  cyanide  of 
1  c.c.  silver  solution  calculated.  A  solution  containing  a  known  quantity  of 
nickel  is  now  required.  This  must  have  sufficient  free  acid  present  to  prevent 
the  formation  of  any  precipitate  on  the  subsequent  addition  of  ammonia  to 
alkaline  reaction ;  if  this  is  not  so,  a  little  ammonium  chloride  may  be  added. 
A  carefully  measured  quantity  of  the  solution  is  then  taken,  containing  about 
O'l  gm.  of  nickel,  and  rendered  distinctly  alkaline  with  ammonia,  a  few  drops  of 
iodide  added,  and  the  liquid  diluted  to  150  or  200  c.c.  A  few  drops  of  the 
silver  solution  are  now  run  in,  and  the  solution  stirred  to  produce  a  uniform 
turbidity.  The  solution  is  now  ready  to  be  titrated  with  the  cyanide,  which  is 
added  slowly  and  with  constant  stirring  until  the  precipitate  wholly  disappears  ; 
a  few  extra  drops  are  added,  after  which  the  beaker  is  placed  under  the  silver 
nitrate  burette,  and  this  solution  gently  dropped  in  until  a  faint  permanent  turbidity 
is  again  visible  ;  this  is  now  finally  caused  to  dissolve  by  the  mere  fraction  of  a  drop 
of  the  cyanide.  A  correction  must  now  be  applied  for  the  excess  of  the  cyanide 
added,  by  noting  the  amount  of  silver  employed,  and  working  out  its  value  in 
cyanide  from  the  data  already  found  ;  this  excess  must  then  be  deducted,  the 
corrected  number  of  c.c.  being  then  noted  as  equivalent  to  the  amount  of  nickel 
employed. 

»  C.  N.  72,  92. 


268  KICKEL. 

(6)  Having  determined  the  relative  value  of  the  cyanide  to  the  silver  solution, 
and  knowing  accurately  the  metallic  content  of  the  latter,  then  Ag  x  0-272  gives 
the  nickel  equivalent.  This  method  is  quite  as  accurate  as  the  direct  titration. 

A  modification  of  the  above  process,  wherein  one  burette  only  is 
necessary,  has  been  found  very  convenient,  and  has  given  most 
excellent  results.  It  is  as  follows  : — 

When  a  solution  of  potassium  cyanide,  containing  a  small  quantity  of  silver 
cyanide  dissolved  in  it,  is  added  to  an  ammoniacal  solution  of  nickel  containing 
potassium  iodide,  it  is  seen  that  silver  iodide  is  precipitated,  and  the  turbidity 
thus  caused  in  the  solution  continues  to  increase  up  to  the  point  where  the 
formation  of  the  nickel-potassium  cyanide  is  complete  ;  any  further  addition 
after  this  stage  is  reached  will  produce  a  clearing  up  of  the  liquid,  until,  at  last, 
the  addition  of  a  single  drop  causes  the  precipitate  to  vanish.  This  final 
disappearance  is  most  distinct,  and  leaves  no  room  for  doubt.  Such  a  solution 
may  be  prepared  by  dissolving  20  to  25  gm.  of  potassium  cyanide  in  a  litre  of 
water,  adding  to  this  about  0-25  gm.  silver  nitrate,  previously  dissolved  in  a  little 
water.  For  large  quantities  of  nickel  the  quantity  of  silver  may  advantageously 
be  diminished,  and  vice  versd.  The  value  of  the  cyanide  is  best  ascertained,  in 
the  manner  already  described,  on  a  nickel  solution. 

Small  quantities  of  cobalt  do  not  seriously  affect  the  results,  but 
it  must  be  remembered  that  it  will  be  reckoned  with  the  nickel ; 
its  presence  is  at  once  detected  by  the  darkening  of  the  solution. 
Manganese  or  copper,  when  present,  render  the  process  valueless, 
so  also  does  zinc  ;  the  latter,  however,  in  alkali  pyrophosphate 
solution  exercises  no  influence.  In  the  presence  of  alumina, 
magnesia,  or  ferric  oxide,  citric  acid,  tartaric  acid,  or  pyrophosphate. 
of  soda  may  be  employed  to  keep  them  in  solution.  The  action 
of  iron  is  somewhat  deceptive,  as  the  solution,  once  cleared  up, 
often  becomes  turbid  again  on  standing  for  a  minute  :  should  this 
occur,  a  further  addition  of  cyanide  must  be  made  until  the  liquid 
is  rendered  perfectly  clear.  The  temperature  of  the  solution  should 
not  much  exceed  20°  C.  :  above  this  the  results  v become  irregular. 
The  amount  of  free  ammonia  has  also  a  disturbing  influence  ; 
a  large  excess  hinders  or  entirely  prevents  the  reaction  ;  the  liquid 
should,  therefore,  be  only  slightly,  but  very  distinctly,  alkaline. 
A  word  of  caution  must  be  given  regarding  the  potassium  cyanide, 
as  many  of  the  reputed  pure  samples  are  very  far  from  being  so. 
The  most  harmful  impurity  is,  however,  sulphur,  as  it  gives  rise 
to  a  darkening  of  the  solution,  owing  to  the  formation  of  the  less 
readily  soluble  silver  sulphide  ;  to  get  rid  of  the  sulphur  impurity 
it  is  necessary  to  thoroughly  agitate  the  cyanide  liquor  with  oxide 
of  lead,  or,  what  is  far  preferable,  oxide  of  bismuth. 

As  regards  the  exactness  of  the  methods,  it  may  be  said  that, 
after  a  prolonged  experience,  extending  over  many  thousands  of 
determinations,  they  have  been  found  to  be  more  accurate  and 
reliable  than  either  the  electrolytic  or  gravimetric  methods,  and 
when  time  is  a  consideration  the  superiority  is  still  more  pronounced. 
The  employment  of  organic  acids  or  sodium  pyrophosphate  in  the 
case  when  iron,  zinc,  etc.,  are  present,  allows  the  operator  to 
dispense  with  the  tedious  separation  which  their  presence  otherwise 


NICKEL.  269 

entails  ;  and  this  is  a  matter  of  considerable  importance  in  the 
assay  of  nickel  mattes  or  German  silver." 

Another  modification  of  this  method  has  been  adopted  by  the 
author  for  nickel  ores  existing  in  New  Caledonia  which  contain  iron, 
manganese,  etc. 

METHOD  OF  PROCEDURE:  Two  solutions  are  prepared:  (a)  11  gm.  of  98  per 
cent,  potassium  cyanide,  0-5  gm.  of  silver  nitrate,  and  1  litre  of  distilled  water  ; 
and  (b)  50  gm.  of  citric  acid,  38  gm.  (approximately)  of  sodium  carbonate,  7*5  gm. 
of  potassium  iodide,  and  500  c.c.  of  distilled  water  ;  35  gm.  of  the  sodium  carbonate 
are  first  added,  and  then  the  remainder,  decigram  by  decigram,  until  neutrality 
is  attained,  before  adding  the  potassium  iodide.  It  is  important  that  solution 
h  be  either  absolutely  neutral  or  only  very  slightly  alkaline.  2-5  gm.  of  the  ore 
(after  drying  at  100°  C)  are  placed  in  a  250  c.c.  flask,  dissolved  in  20  c.c.  of  HC1, 
and  the  solution  made  up  to  250  c.c.  with  water,  and  then  well  agitated.  The 
insoluble  silica,  etc.,  is  then  filtered  off,  and  to  50  c.c.  of  the  filtrate  10  c.c>  of 
solution  b  are  added,  then  dilute  ammonia  in  slight  excess  till  the  Characteristic 
blue  colouration  is  produced,  and  the  solution  is  cooled.  The  liquid  is  then  titrated 
with  solution  a,  added  gradually  and  with  stirring.  A  white,  cloudy  precipitate 
forms  at  first,  but  disappears  on  the  addition  of  the  last  drop  of  solution  a.  A 
standard  solution  of  pure  nickel  is  prepared  and  titrated  in  the  same  manner  as 
the  ore. 

The  process  takes  about  thirty  minutes,  and  the  results,  although  usually 
a  little  too  high,  are  very  concordant.  The  method  is  not  applicable  to  ores 
containing  large  quantities  of  iron,  manganese,  or  cobalt,  25  per  cent,  being  the 
limit  for  iron  and  manganese,  and  1  per  cent,  for  cobalt. 

Jamieson*  has  recently  published  the  following  modification 
of  the  above  process  for  the  determination  of  nickel  in  steel  : — 

Dissolve  0'5  gm.  of  borings  in  10  c.c.  dilute  nitric  acid  (1  :  1)  in  a  150  c.c.  flask, 
according  to  the  directions  given  for  the  ferrocyanide  method.  If  the  metal 
contains  more  than  0*5  %  of  manganese,  remove  it  according  to  the  same  directions. 
Add  to  the  nitric  acid  solution  2-3  gm.  citric  acid  and  2  gm.  anhydrous  sodium 
pyrophosphate,  then  add  ammonia  slowly  with  stirring  until  the "  precipitate  at 
first  formed  just  dissolves  and  the  solution  acquires  a  very  faint  odour  of  ammonia. 
If  too  much  ammonia  has  been  used,  it  must  be  nearly  neutralized  by  the  careful 
addition  of  nitric  acid.  Dilute  to  about  150  c.c.  and  cool  to  a  temperature  below 
20°,  add  a  few  drops  of  a  10  %  solution  of  KI  and  enough  N/iq  silver  nitrate 
solution — the  volume  of  which  must  be  noted — to  produce  a  distinct  turbidity. 
Then  run  in  KCy  solution  very  slowly  with  stirring  until  the  turbidity  just  dis- 
appears, and  the  solution  lightens  to  a  golden-yellow  colour.  The  solution  should 
remain  bright  for  five  minutes,  otherwise  the  titration  is  incomplete.  Sometimes 
it  happens  that  the  turbidity  disappears  when  only  one-third  to  one-half  of  the 
required  amount  of  cyanide  has  been  used,  but  in  this  case  on  the  addition  of  a  drop 
or  two  of  silver  nitrate,  or  on  waiting  a  moment,  the  turbidity  reappears.  The 
end-point  is  best  observed  when  the  beaker  is  placed  on  a  white  paper  having  an 
elliptical  hole  in  it  under  which  is  placed  a  black  glazed  paper  for  contrast.  In 
making  the  calculation  the  proper  deduction  for  the  silver  nitrate  used  should  be 
made. 

The  potassium  cyanide  solution  should  be  standardized  by  means  of  the  N/iO 
silver  nitrate.  To  do  this,  pipette  20  c.c.  into  a  beaker,  dilute  to  about  150  c.c. 
with  cold  water,  add  ammonia  until  the  odour  is  distinct  but  slight,  add  a  few 
drops  of  10  %  KI  solution,  run  in  silver  nitrate  until  a  distinct  turbidity  is  pro- 
duced, and  then  finish  by  slowly  adding  the  cyanide  solution  until  the  turbidity 
just  disappears.  The  theoretical  amount  of  nickel  per  c.c.  of  N/iO  solution  is 
0-002934  gm.,  but  instead  of  using  this  value  it  is  perhaps  preferable  to  standardize 
the  solution  with  a  steel  of  known  content  of  nickel. 

•C.N.  1910,  102,  51. 


270  NICKEL. 

Nickel-plating  Solutions. — These  contain,  as  a  rule,  only  nickel  sulphate  and 
ammonia,  and  the  nickel  can  be  determined  with  a  simple  solution  of  potassium 
cyanide  previously  standardized  on  pure  nickel  ammonium  sulphate.  The  nickel 
solution  to  be  tested  should  be  fairly  concentrated  and  rendered  feebly  alkaline 
with  ammonia.  If  there  is  iron  present  some  ammonium  tartrate  should  be  added 
to  prevent  the  precipitation  of  it  by  the  ammonia.  The  cyanide  is  used  in  small 
quantities  with  constant  shaking  until  a  drop  produces  a  clear  yellowish  solution. 
Copper,  zinc,  and  cobalt  must  not  be  present. 

Determination  of  Nickel  and  Cobalt  by  Potassium  Ferrocyanide. — Standard 
K4FeCy6  -20  gm.  of  the  cryst.  salt  per  litre  (1  c.c.  =  about  0'003  gm.  Ni  or  Co). 

This  method  is  referred  to  by  Canto ni  and  Rosenstein*  and  is  described 
in  detail  by  J  a  m  i  e  s  o  nf  ;  it  is  applicable  for  the  determination  of  nickel  and 
cobalt,  but  cannot  be  used  in  the  presence  of  copper,  zinc  and  manganese.  The 
following  procedure  is  recommended  by  Jamie  son  for  standardizing  the  ferro- 
cyanide  solution,  a  similar  course  being  pursued  with  the  nickel  or  cobalt  solution 
to  be  tested  : — 

A  solution  of  nickel  or  cobalt  is  made  from  a  pure  salt  and  of  known  strength. 
Three  equal  portions,  each  containing  about  0-1  gm.  of  the  metal,  are  measured 
into  beakers.  10  c.c.  of  a  10  %  solution  of  ferric  chloride  and  2 — 3  gm.  of  citric 
acid  are  added  to  each,  and  then  ammonia  with  stirring  until  the  liquid  has  a 
faint  smell  of  the  reagent — a  large  excess  being  carefully  avoided.  The  solutions 
are  then  diluted  to  about  100  c.c.  with  hot  water  and  brought  to  a  temperature 
of  63 — 75°  C.  The  ferrocyanide  is  then  run  in  slowly  from  a  burette  with  constant 
stirring.  A  drop  of  the  solution  is  from  time  to  time  transferred  to  a  paraffined 
white  plate  and  acidified  with  a  drop  of  dilute  acetic  acid.  The  titration  is  finished 
when  a  greenish  colour  appears  after  five  minutes'  standing.  The  first  portion 
is  used  to  get  an  approximate  result ;  the  exact  end-point  being  determined  in 
the  other  two. 

Determination  of  Nickel  in  Steel. — Dissolve  1  gm.  of  the  borings  in  10 — 15  c.c. 
dilute  nitric  acid  (1  :  1)  in  a  150  c.c.  flask  covered  with  an  inverted  crucible  cover. 
When  the  first  violent  action  is  over,  remove  cover,  and  boil  gently  with  constant 
motion  over  a  bunsen  flame  till  the  steel  is  dissolved,  adding,  if  necessary,  a  few 
drops  of  strong  HC1  or  a  crystal  of  KC103  to  complete  solution.  Now  add  10  c.c. 
of  cone.  HNOS,  heat  to  boiling  again,  and  add  0'5  gm.  KC103.  Boil  off  the  chlorine, 
then  add  another  0'5  gm.  of  chlorate,  and  boil  for  two  minutes.  Allow  the  flask 
to  cool  a  little  and  filter  off  the  Mn02  on  a  G  o  o  c  h  crucible,  and  wash  with  as  small 
a  quantity  of  cold  water  as  possible.  Then  proceed  according  to  the  method 
described  above.  As  the  presence  of  a  large  amount  of  iron  somewhat  retards 
the  appearance  of  the  end-reaction,  the  ferrocyanide  solution  used  should  be 
standardized  in  the  presence  of  about  the  same  amount  of  iron  as  is  contained 
in  1  gm.  of  steel.  The  results  are  accurate. 

Determination  of  Cobalt  and  Nickel  (Rupp  and  Pfennig^. 

Cobalt. — The  following  method  is  based  on  the  formation  of  a  double  cyanide, 
which  is  decomposed  by  excess  of  cobalt  ions  with  formation  of  insoluble  cobalt 
cyanide.  1  atom  of  cobalt  was  found  to  correspond  to  5  molecules  of  KCN. 

The  cobalt  solution,  which  must  be  neutral  and  contain  between  0'02  and 
0'75  %  of  cobalt,  is  run  from  a  burette  into  a  measured  volume  (5-25  c.c.)  of 
N/2  potassium  cyanide  (undiluted)  until  a  permanent  brownish  turbidity  is  pro- 
duced. 

1  c.c.  w/a  KCN  =0-0059  gm.  cobalt. 

N/2  KCN  prepared  from  pure  cyanide  can  be  standardized  by  N/4  or  N/2  HC1 
or  H2S04  using  methyl  orange  as  indicator. 

*The  Analyst,  1908,  33,  107.  ^C.N.  1910,  102,  51. 

IChem.  Zeit.  1910,  34,  322,  and  J.  8:  C.  I.  1910,  29,  518. 


NITRATES.  271 

Nickel. — Exactly  the  same  procedure  is  followed  with  regard  to  nickel.  The 
neutral  solution,  diluted  to  contain  O'4-l'O  %  nickel,  is  run  into  10  c.c.  of  N/g 
cyanide  until  a  permanent  turbidity  is  seen.  Before  commencing  the  titration 
5-20  drops  of  10  %  ammonia  should  be  added  to  the  cyanide  solution,  as  an 
increased  sharpness  of  the  end -reaction  is  thus  secured  :  the  addition  of  more 
than  5  c.c.  however,  leads  to  very  erroneous  results. 

1  c.c.  N/2  KCN  =0-007335  gm.  Ni. 

The  method  may  also  be  carried  out  in  the  reverse  way — for  nickel,  but  not  for 
cobalt — as  follows  :  To  the  (neutral)  nickel  solution  add  10  drops  of  a  1  % 
phenolphthalein  solution,  then  run  in  N/g  cyanide  from  a  burette  until  the 
precipitate  formed  is  redissolved  and  a  red  tint  obtained,  showing  the  formation 
of  the  double  cyanide.  The  addition  of  one  drop  more  causes  the  formation  of 
the  usual  red  colour. 

Another  modification,  also  applicable  to  nickel  only,  is  as  follows  :  To  the 
neutral  nickel  solution  is  added  a  measured  quantity  of  cyanide,  more  than  sufficient 
to  form  the  double  cyanide,  and  the  excess  determined  by  titration  with  N/2 
HC1  or  H2SOj,  using  methyl  orange  as  indicator. 


NITROGEN    AS    NITRATES    AND    NITRITES. 

Nitric  Anhydride. 

N205- 108*02. 

Nitrous  Anhydride. 

N0  =  76-02. 


Normal  acid 
Ditto 

X 
X 

0-0540  =N2O5 
0-1011  =KN03 

Metallic  iron 

X 

0-3761  =HNO3 

Ditto 

X 

0-6035  =KN03 

Ditto 

X 

0-3224  =N2O5 

THE  accurate  determination  of  nitric  acid  in  combination  presents 
great  difficulties,  and  can  only  be  made  by  indirect  means  ;  the 
methods  here  given  are  sufficient  for  most  purposes.  Very  few  of 
them  can  be  said  to  be  simple,  but  it  is  to  be  feared  that  no  simple 
process  can  ever  be  obtained  for  the  determination  of  nitric  acid 
in  many  of  its  combinations. 

1.     Determination  by  conversion  into  Ammonia  (S  c  h  u  1  z  e  and 
Vernon  Harcourt). 

This  method  is  based  on  the  fact  that  when  a  nitrate  is  heated 
with  a  strong  alkaline  solution,  and  zinc  added,  ammonia  is  evolved  ; 
when  zinc  alone  is  used,  however,  the  quantity  of  ammonia  liberated 
is  not  a  constant  measure  of  the  nitric  acid  present.  Vernon 
Harcourt  and  Si  e  wart*  appear  to  have  arrived  independently 
at  the  result  that  by  using  a  mixture  of  zinc  and  iron  the  reaction 
became  quantitative.  - 

A  convenient  form  of  apparatus  is  shown  in  fig.  47. 

*  J.  C.  S.  1862,  381 ;  An.  Chem.  u.  Phar.  125,  293. 


272  NITRATES. 

METHOD  OF  PROCEDURE  :  The  distilling  flask  holds  about  200  c.c.  and  is  closely 
connected  by  a  bent  tube  with  another  smaller  flask,  in  such  a  manner  that  both 
may  be  placed  obliquely  upon  a  sand-bath,  the  bulb  of  the  smaller  flask  coming 
just  under  the  neck  of  the  larger.  The  oblique  direction  prevents  the  spirting 
of  the  boiling  liquids  from  entering  the  exit  tubes,  but  as  a  further  precaution 
these  latter  are  in  both  flasks  turned  into  the  form  of  a  hook  ;  from  the  second  flask, 
which  must  be  somewhat  wide  in  the  mouth,  a  long  tube  passes  through  a  L  i  e  b  i  g '  s 
condenser  (which  may  be  made  of  wide  glass  tube)  into  an  ordinary  tubulated 
receiver,  containing  normal  sulphuric  acid  coloured  with  an  indicator.  The  end 
of  the  distilling  tube  reaches  to  about  the  middle  of  the  receiver,  through  the 
tubulure  of  which  Harcourt  passes  a  bulb  apparatus  of  peculiar  form,  containing 
also  coloured  normal  acid  ;  instead  of  this  latter,  however,  a  chloride  of  calcium 


Fig.  47. 


tube,  filled  with  broken  glass,  and  moistened  with  acid,  wtll  answer  the  purpose. 
The  distilling  tube  should  be  cut  at  about  two  inches  from  the  cork  of  the  second 
flask,  and  connected  by  means  of  a  well-fitting  vulcanized  tube  ;  by  this  means 
water  may  be  passed  through  the  tube  when  the  distillation  is  over  so  as  to  remove 
any  traces  of  ammonia  which  may  have  been  retained  on  its  sides.  All  the  corks 
of  the  apparatus  should  be  soaked  in  hot  paraffin,  so  as  to  fill  up  the  pores. 

All  being  ready,  about  50  gm.  of  finely  granulated  zinc  (best  made  by  pouring 
molten  zinc  into  a  warm  iron  mortar  while  the  pestle  is  rapidly  being  rubbed 
round)  are  put  into  the  larger  flask  with  about  half  the  quantity  of  clean  iron 
filings  which  have  been  ignited  in  a  covered  crucible  (fresh  iron  and  zinc  should 
be  used  for  each  analysis)  •  the  weighed  nitrate  is  then  introduced,  either  in 
solution,  or  with  water  in  sufficient  quantity  to  dissolve  it,  strong  solution  of 
caustic  potash  added,  and  the  flask  immediately  connected  with  the  apparatus, 
and  placed  on  a  small  sand-bath,  which  can  be  heated  by  a  gas-burner,  a  little 
water  being  previously  put  into  the  second  flask.  Convenient  proportions  of 
material  are  |  gm.  nitre,  and  about  25  c.c.  each  of  water  and  solution  of  potash 
of  spec.  grav.  1'3.  The  mixture  should  be  allowed  to  remain  at  ordinary 
temperature  for  about  an  hour  (Eder). 

Heat  is  now  applied  to  that  part  of  the  sand-bath  immediately  beneath  the 
larger  flask,  and  the  mixture  is  gradually  raised  to  the  boiling  point.  When 
distillation  has  actually  commenced,  the  water  in  the  second  flask  is  made  to  boil 
gently  ;  by  this  arrangement  the  fluid  is  twice  distilled,  and  any  traces  of  fixed 
alkali  which  may  have  escaped  the  first  are  sure  to  be  retained  in  the  second  flask. 
The  distillation  with  the  quantities  above  named  will  occupy  about  an  hour  and 
a  half,  and  is  completed  when  hydrogen  is  pretty  freely  liberated  as  the  potash 
becomes  concentrated.  The  lamp  is  then  removed,  and  the  whole  allowed  to 
cool,  the  distilling  tube  rinsed  into  the  receiver,  also  the  tube  containing  broken 
glass ;  the  contents  of  the  receiver  are  then  titrated  with  N/io  caustic  potash  or 
soda  as  usual. 

Eder  recommends  that  an  ordinary  retort,  with  its  neck  set  upwards,  should 


NITRATES.  273 

be  used  instead  of  the  flask  for  holding  the  nitrate,  and  that  an  aspirator  should 
be  attached  to  the  exit  tube,  so  that  a  current  of  air  may  be  drawn  through  during 
and  after  the  distillation. 

Chlorides  and  sulphates  do  not  interfere  with  the  process. 
This    method    is    simplified    in    some    agricultural    experiment 
stations  for  the  analysis  of  sodium  and  potassium  .nitrates. 

METHOD  OF  PROCEDURE  :  0'5  gm.  of  the  nitrate"  is  dissolved  in  about  50  c.c. 
of  water  in  a  convenient  flask  fitted  with  a  bulb  distilling  tube  such  as  is  shown 
in  either  fig.  29  or  30.  To  the  liquid  is  added  about  5  gm.  each  of  zinc  dust  and 
iron  filings,  then  80  c.c.  of  sodium  hydrate  solution  (sp.  gr.  1'3).  The  mixture 
is  allowed  to  stand  at  ordinary  temperature  for  an  hour,  when  the  distillation  is 
commenced  by  heating  up  carefully  and  distilling  until  at  least  100  c.c.  are  received 
into  standard  acid  through  a  condenser,  as  in  the  Kjeldahl  process. 

Schmitt  has  suggested  a  further  modification  of  this  method, 
technically  useful  for  mixed  manures. 

METHOD  OF  PROCEDURE  :  About  1  gm.  of  the  substance  in  which  the  nitrate 
is  to  be  determined  is  dissolved  in  water,  5  c.c.  of  glacial  acetic  acid  and  3  gm. 
of  a  mixture  of  equal  weights  of  finely  powdered  iron  and  zinc  added,  and  the  flask 
gently  heated  for  10  or  15  minutes.  When  cooled,  25  c.c.  of  sulphuric  acid  are 
cautiously  added  and  a  little  solid  paraffin  to  prevent  frothing  The  flask  is  then 

fntly  heated    to  drive    off   the  acetic   acid,   and  the  residue  boiled  as   in  the 
j  e  1  d  a  h  1  method  until  colourless.    Caustic  soda  in  excess  is  then  added  and  the 
distillation  commenced  in  the  usual  way,  receiving  the  distillate  into  standard 
acid.     (See  p.  87). 


2.     By  Oxidation  of  Ferrous  Salts  (P  e  1  o  u  z  e). 
(Not  available  in  the  presence  of  Organic  Matter.) 

The  principle  upon  which  this  well-known  process  is  based  is  as 
followrs  : — 

(a)  When  a  nitrate  is  brought  into  contact  with  a  solution  of 
ferrous  oxide,  mixed  with  free  hydrochloric  acid,  and  heated,  part 
of  the  oxygen  contained  in  the  nitric  acid  passes  over  to  the  iron, 
forming  a  persalt,  while  the  base  combines  with  hydrochloric  acid, 
and  nitric  oxide  (NO)  is  set  free.  3  eq.  iron  (  =  167*55)  are  oxidized 
by  1  eq.  nitric  acid(  =  63*02).  If,  therefore,  a  weighed  quantity  of 
the  nitrate  be  mixed  with  an  acid  solution  of  ferrous  chloride  or 
sulphate  of  known  strength  in  excess,  and  the  solution  boiled  to 
expel  the  liberated  nitric  oxide,  then  the  amount  of  unoxidized 
iron  remaining  in  the  mixture  being  found  by  a  suitable  method  of 
titration,  the  quantity  of  iron  converted  from  the  ferrous  into  the 
ferric  state  will  be  the  measure  of  the  original  nitric  acid  in  the 
proportion  of  167*55  to  63*02  ;  or  by  dividing  63*02  by  167*55  the 
factor  0*3761  is  obtained,  so  that  if  the  amount  of  iron  changed 
as  described  be  multiplied  by  this  factor,  the  product  will  be  the 
amount  of  nitric  acid  present. 

This  method,  though  theoretically  perfect,  is  in  practice  liable  to 
serious  errors,  owing  to  the  readiness  with  which  a  solution  of 
a  ferrous  salt  absorbs  oxygen  from  the  atmosphere.  On  this 


274  NITRATES. 

account  accurate  results  are  only  obtained  by  conducting  hydrogen 
or  carbon  dioxide  through  the  apparatus  while  the  boiling  is  being 
carried  on.  This  modification  has  been  adopted  by  Fresenius 
with  very  satisfactory  results. 

The  boiling  vessel  may  consist  of  a  small  tubulated  retort,  supported  in  such 
a  manner  that  its  neck  inclines  upward  ;  a  cork  is  fitted  into  the  tubulure,  and 
through  it  is  passed  a  small  tube  connected  with  a  vessel  for  generating  either 
carbonic  acid  or  hydrogen.  If  a  weighed  quantity  of  pure  metallic  iron  is  used 
for  preparing  the  solution,  the  washed  carbonic  acid  or  hydrogen  should  be 
passed  through  the  apparatus  while  it  is  being  dissolved  ;  the  solution  so  obtained, 
or  one  of  double  sulphate  of  iron  and  ammonia  of  known  strength,  being 
already  in  the  retort,  the  nitrate  is  carefully  introduced,  and  the  mixture  heated 
gently  by  a  small  lamp,  or  by  the  water  bath,  for  ten  minutes  or  so,  then  boiled 
until  the  dark-red  colour  of  the  liquid  disappears  and  gives  place  to  the  brownish- 
yellow  of  ferric  compounds.  The  retort  is  then  allowed  to  cool,  the  current  of 
carbonic  acid  or  hydrogen  still  being  kept  up,  then  the  liquid  diluted  freely,  and 
titrated  with  N/io  permanganate. 

thving  to  the  irregularities  attending  the  use  of  permanganate 
with  hydrochloric  acid,  it  is  preferable,  in  case  this  acid  has  been 
used,  to  dilute  the  solution  less,  and  titrate  with  dichromate.  Two 
grams  of  pure  iron,  or  its  equivalent  in  double  iron  salt,  0*5  gm.  of 
saltpetre,  and  about  60  c.c.  of  strong  hydrochloric  acid,  are  con- 
venient proportions  for  the  analysis. 

Eder*  has  modified  Fresenius's  improvements  as  follows: — 

1'5  gm.  of  very  thin  iron  wire  is  dissolved  in  30  to  40  c.c.  of  pure  fuming 
hydrochloric  acid,  placed  in  a  retort  of  about  200  c.c.  capacity  ;  the  neck  of  the 
retort  points  upwards,  at  a  moderately  acute  angle,  and  is  connected  with 
a  U-tube,  which  contains  water.  Solution  of  the  iron  is  hastened  by  applying 
a  small  flame  to  the  retort.  Throughout  the  entire  process  a  stream  of  C02  is 
passed  through  the  apparatus.  When  the  iron  is  all  dissolved  the  solution  is 
allowed  to  cool,  the  stream  of  C02  being  maintained  ;  the  weighed  quantity  of 
nitrate  contained  in  a  small  glass  tube  (equal  to  about  0'2  gm.  HN03)  is  then 
quickly  passed  into  the  retort  through  the  neck  ;  the  heating  is  continued  under 
the  same  conditions  as  before,  until  the  liquid  assumes  the  colour  of  ferric  chloride. 
The  whole  is  allowed  to  cool  in  a  stream  of  C02 ;  water  is  added  in  quantity,  and 
the  unoxidized  iron  is  determined  .by  titration  with  permanganate.  The  results 
are  exceedingly  good. 

If  the  CO2  be  generated  in  a  flask,  with  a  tube  passing  downwards 
for  the  reception  of  the  acid,  air  always  finds  its  way  into  the  retort, 
and  the  results  are  unsatisfactory.  Eder  recommends  the  use  of 
Kipp's  CO2  apparatus.  By  carrying  out  the  operation  exactly  as 
is  now  to  be  described,  he  has  obtained  very  good  results  with 
ferrous  sulphate  in  place  of  chloride. 

The  same  apparatus  is  employed  ;  the  tube  through  which  C02  enters  the  retort 
passes  to  the  bottom  of  the  liquid  therein,  and  the  lower  extremity  of  this  tube 
is  drawn  out  to  a  fine  point.  The  bubbles  of  C02  are  thus  reduced  in  size,  and 
the  whole  of  the  nitric  acid  is  removed  from  the  liquid  by  the  passage  of  these 
bubbles.  The  iron  wire  is  dissolved  in  excess  of  dilute  sulphuric  acid  (strength 
1  :  3  or  1  :  4).  When  the  liquid  in  the  retort  has  become  cold,  a  small  tube 
containing  the  nitrate  is  quickly  passed,  by  means  of  a  piece  of  platinum  wire 
attached  to  it,  through  the  tubulus  of  the  retort,  and  the  cork  is  replaced  before 

*  Z.  a.  C.  16,  267. 


NITRATES.  275 

the  tube  has  touched  the  liquid  ;  C02  is  again  passed  through  the  apparatus  for 
some  time,  after  which,  by  slightly  loosening  the  cork,  the  tube  containing  the 
nitrate  is  allowed  to  fall  into  the  liquid.  The  whole  is  allowed  to  remain  at  the 
ordinary  temperature  for  about  an  hour — this  is  essential — after  which  time  the 
contents  of  the  retort  are  heated  to  boiling,  C02  being  passed  continuously  into 
the  retort,  and  the  boiling  continued  till  the  liquid  assumes  the  light  yellow  colour 
of  ferric  sulphate.  After  cooling,  water  is  added  (this  may  be  omitted  with 
dichromate),  and  the  unoxidized  iron  is  determined  by  permanganate. 

Eder  also  describes  a  slight  modification  of  this  process,  allowing 
of  the  use  of  a  flask  in  place  of  the  retort,  and  of  ammonio-ferrous 
sulphate  in  place  of  iron  wire.  Although  the  titration  with  per- 
manganate is  more  trustworthy  when  sulphuric  acid  is  employed 
than  when  hydrochloric  acid  is  used,  he  nevertheless  thinks  that 
the  use  of  ferrous  chloride  is  generally  to  be  recommended  in 
preference  to  that  of  ferrous  sulphate.  When  the  chloride  is  em- 
ployed, no  special  concentration  of  acid  is  necessary  ;  the  nitric 
oxide  is  more  readily  expelled  from  the  liquid,  and  the  process  is 
finished  in  a  shorter  time.  Some  magnesium  sulphate  should, 
however,  be  used  to  prevent  the  disturbing  effect  of  HC1  when 
permanganate  is  used  for  titration. 

The  final  point  in  the  titration  with  permanganate,  when  the 
sulphate  is  employed,  is  rendered  more  easy  of  determination  by 
adding  a  little  potassium  sulphate  to  the  liquid. 

(b)  Direct  titration  of  the  resulting  Ferric  salt  by  Stannous 
Chloride. — Fresenius  has  adopted  the  use  of  stannous  chloride  for 
titrating  the  ferric  salt  with  very  good  results. 

The  following  plan  of  procedure  is  recommended  by  the  same 
authority. 

A  solution  of  ferrous  sulphate  is  prepared  by  dissolving  100  gm.  of  the  crystals 
in  500  c.c.  of  hydrochloric  acid  of  spec.  grav.  I'lO ;  when  used  for  the  analysis, 
the  small  proportion  of  ferric  oxide  invariably  present  in  it  is  found  by  titrating 
with  stannous  chloride.  The  nitrate  being  weighed  or  measured,  is  brought 
together  with  50  c.c.  (more  or  less,  according  to  the  quantity  of  nitrate)  of  the 
iron  solution  into  a  long-necked  flask,  through  the  cork  of  which  two  glass  tubes 
are  passed,  one  connected  with  a  C02  apparatus,  and  reaching  to  the  middle 
of  the  flask,  the  other  simply  an  outlet  for  the  passage  of  the  gas.  When  the 
gas  has  driven  out  all  the  air,  the  flask  is  at  first  gently  heated,  and  eventually 
boiled,  to  dispel  all  the  nitric  oxide.  The  C02  tube  is  then  rinsed  into  the  flask, 
and  the  liquid,  while  still  boiling  hot,  titrated  for  ferric  chloride,  as  on  p.  127. 

The  liquid  must,  however,  be  allowed  to  cool  before  titrating  with 
iodine  for  the  excess  of  stannous  chloride.  While  cooling,  the 
stream  of  CO2  should  still  be  continued.  The  quantity  of  iron 
changed  into  peroxide,  multiplied  by  the  factor  0'3761,  will  give 
the  amount  of  nitric  acid  (HNO3). 

EXAMPLE  :  (1)  A  solution  of  stannous  chloride  was  used  for  titrating  10  c.c. 
of  solution  of  pure  ferric  chloride  containing  0'2151  gm.  Fe  ;  25'65  c.c.  of  tin 
solution  were  required,  therefore  that  quantity  was  equal  to  0*0809  gm.  of  HNOa, 
or  0-06932  gm.  of  N205. 

(2)  50  c.c.  of  acid  ferrous  sulphate  were  titrated  with  tin  solution  for  ferric 
oxide,  and  0'25  c.c.  was  required. 

(3)  1  c.c.  tin  solution  =3'3  c.c.  iodine  solution. 

T   2 


276  NITRATES. 

(4)     0*2177  gm.  of  pure  nitre  was  boiled,  as  described,  with  50  c.c.  of  the  acid 
ferrous  sulphate,  and  required  45-05  c.c.  tin  solution,  and  4*7  c.c.  iodine — 
4*7  c.c.  iodine  solution  =1-42  c.c.  SnCl2 

The  peroxide  in  the  protosulphate  solution  =0-25  c.c. 

1-67 

45 -05 -1-67  =43 -38 
.-.   25-65  :  43-38=0-06932  :       x 
x    =0-1172  N205 

instead  of  0'1163,  or  53*69  per  cent,  instead  of  53*42.  A  mean  of  this  and  three 
other  determinations,  using  varying  proportions  of  tin  and  iron  solutions,  gave 
exactly  53*42  per  cent.  In  the  case  of  pure  materials,  therefore,  the  process  is 
entirely  satisfactory. 

The  above  process  is  slightly  modified  by  Eder.  About  10  gm. 
of  ammonium-ferrous  sulphate  are  dissolved  in  a  flask  in  about 
50  c.c.  of  hydrochloric  acid  (sp.  gr.  1-07)  in  a  stream  of  CO2.  The 
tube  through  which  the  CO2  enters  is  drawn  to  a  point ;  an  exit 
tube,  somewhat  trumpet-shaped,  to  admit  of  any  liquid  that  may 
spirt  finding  its  way  back  into  the  flask,  passes  downwards  into 
water.  After  solution  of  the  double  salt,  the  nitrate  is  dropped  in 
with  the  precautions  already  detailed,  and  the  liquid  is  boiled  until 
the  nitric  oxide  is  all  expelled.  The  hot  liquid  is  diluted  with  twice 
its  own  volume  of  water,  excess  of  stannous  chloride  solution  is 
run  in,  the  whole  is  allowed  to  cool  in  a  stream  of  CO2,  and  the 
excess  of  tin  is  determined  by  means  of  standard  iodine. 

(c)  Holland's  Modification  of  the  P  e  1  o  u  z  e 
Process. — The  arrangement  of  apparatus 
shown  in  fig.  48  obviates  the  use  of  an 
atmosphere  of  H  or  CO2.  A  is  a  long-necked 
assay  flask  drawn  off  at  B,  so  as  to  form  a 
shoulder,  over  which  is  passed  a  piece  of  stout 
pure  india-rubber  tube,  D,  about  6  centimetres 
long,  the  other  end  terminating  in  a  glass 
tube,  F,  drawn  off  so  as  to  leave  only  a  small 
orifice.  On  the  elastic  connector  D  is  placed 
a  screw  clamp.  At  c,  a  distance  of  3  centi- 
F.  48  metres  from  the  shoulder,  is  cemented  with 

a    blow-pipe    a   piece    of   glass    tube    about 

2  centimetres  long,  surmounted  by  one  of  stout  elastic  tube,  rather 
more  than  twice  that  length.  The  elastic  tubes  must  be  securely 
attached  to  the  glass  by  binding  with  wire.  After  binding,  it  is  as 
well  to  turn  the  end  of  the  conductor  back,  and  smear  the  inner 
surface  with  fused  caoutchouc,  and  then  replace  it  to  render  the 
joint  air-tight. 

METHOD  OF  PROCEDURE  :  A  small  funnel  is  inserted  into  the  elastic  tube  at 
c,  the  clamp  at  D  being  for  the  time  open  ;  after  the  introduction  of  the  solution, 
followed  by  a  little  water  which  washes  all  into  the  flask,  the  funnel  is  removed, 
and  the  flask  supported,  by  means  of  the  wooden  clamp,  in  the  inclined  position 
it  occupies  in  the  figure.  The  contents  are  now  made  to  boil  so  as  to  expel  all 
air  and  reduce  the  volume  of  the  fluid  to  about  4  or  5  c.c.  When  this  point  is 


>  NITRATES.  277 

reached  a  piece  of  glass  rod  is  inserted  into  the  elastic  tube  at  c,  which  causes  the 
water  vapour  to  escape  through  F. 

Into  the  small  beaker  is  put  about  50  c.c.  of  a  previously  boiled  solution  of 
ferrous  sulphate  in  hydrochloric  acid  (the  amount  of  iron  already  existing  in 
the  ferric  state  must  be  known). 

The  boiling  is  still  continued  for  a  moment  to  ensure  perfect  expulsion  of  air 
from  F,  the  lamp  is  then  removed,  and  the  caoutchouc  connector  slightly  com- 
pressed with  the  first  finger  and  thumb  of  the  left  hand.  As  the  flask  cools  the 
solution  of  iron  is  drawn  into  it :  when  the  whole  has  nearly  receded  the  elastic 
tube  is  tightly  compressed  with  the  fingers,  whilst  the  sides  of  the  beaker  are 
washed  with  a  jet  of  boiled  water,  which  is  also  allowed  to  pass  into  the  flask. 
The  washing  may  be  repeated,  taking  care  not  to  dilute  more  than  is  necessary  or 
to  admit  air.  Whilst  F  is  still  full  of  water,  the  elastic  connector  previously 
compressed  with  the  fingers  is  now  securely  closed  with  the  clamp,  the  screw  of 
which  is  worked  with  the  right  hand.  Provided  the  clamp  is  a  good  one,  F  will 
remain  full  of  water  during  the  subsequent  digestion. 

After  heating  in  a  water  bath  at  100°  for  half  an  hour,  the  flask  is  removed 
from  the  water  bath  and  cautiously  heated  with  a  small  flame,  the  fingers  at  the 
same  time  resting  on  the  elastic  connector  at  the  point  nearest  the  shoulder ;  as 
soon  as  the  tube  is  felt  to  expand,  owing  to  the  pressure  from  within,  the  lamp  is 
removed  and  the  screw  clamp  released,  the  fingers  maintaining  a  secure  hold  of 
the  tube,  the  gas-flame  is  again  replaced,  and  when  the  pressure  on  the  tube  is 
again  felt,  this  latter  is  released  altogether,  thus  admitting  of  the  escape  of  the 
nitric  oxide,  through  F,  wrhich  should  be  below  the  surface  of  water  in  the  beaker 
whilst  these  manipulations  are  performed.  The  contents  of  the  flask  are  now 
boiled  until  the  nitric  oxide  is  entirely  expelled,  and  the  solution  of  iron  shows 
only  the  brown  colour  of  the  perchloride.  At  the  completion  of  the  operation, 
the  beaker  is  first  removed,  and  then  the  lamp. 

It  now  only  remains  to  transfer  the  ferric  solution  to  a  suitable  vessel  and 
determine  the  perchloride  with  stannous  chloride  as  in  b. 

A  mean  of  six  experiments  for  the  percentage  determination  of 
N2O5  in  pure  nitre  gave  53*53  per  cent,  instead  of  53*42.  The 
process  is  easy  of  execution,  and  gives  satisfactory  technical  results. 
The  point  chiefly  requiring  attention  is  that  the  apparatus  should 
be  air-tight,  which  is  secured  by  the  use  of  good  elastic  tubes  and 
clamp. 


3.     Schlosing's    Method    (available   in   the   presence   of 
Organic  Matter). 

The  solution  of  nitrate  is  boiled  in  a  flask  till  all  air  is  expelled, 
then  an  acid  solution  of  ferrous  chloride  drawn  in,  the  mixture 
boiled,  and  the  nitric  oxide  gas  collected  over  mercury  in  a  balloon 
filled  with  mercury  and  milk  of  lime  ;  the  gas  is  then  brought, 
without  loss,  in  contact  with  oxygen  and  water,  so  as  to  convert  it 
again  into  nitric  acid,  then  titrated  with  N/10  alkali  as  usual. 

This  method  was  devised  by  Sc hi 6 sing  for  the  determination 
of  nitric  acid  in  tobacco,  and  is  especially  suitable  for  that  and 
similar  purposes,  where  the  presence  of  organic  matter  would 
interfere  with  the  direct  titration  of  the  iron  solution.  Where  the 
quantity  of  nitric  acid  is  not  below  0*15  gm.  the  process  is  fairly 
accurate,  but  needs  a  special  and  rather  complicated  arrangement 
of  apparatus,  the  description  of  which  may  be  found  in  Fresenius's 
Quant.  Anal,  sixth  edition. 


278 


NITRATES. 


An  arrangement  of  apparatus,  dispensing  with  the  use  of  mercury, 
has  been  devised  by  Wildt  and  Scheibe,*  which  simplifies  the 
method  and  gives  accurate  results  with  not  less  than  O25  gm. 
N2O5.  With  smaller  quantities  the  results  are  too  low.  Fig.  49 
shows  the  apparatus  used. 


A  is  a  conical  flask  of  250  c.c.  capacity,  containing  the  solution 
to  be  analysed.  B  is  a  round-bottomed  flask  of  250 — 300  c.c. 
capacity,  half  filled  with  caustic  soda,  to  absorb  any  HC1  which 
might  be  carried  over  from  A.  C  is  a  conical  flask  of  750  c.c. 
capacity,  containing  a  little  water  to  absorb  the  nitric  acid.  D  is 
a  tube  containing  water  to  collect  any  nitric  acid  not  absorbed  by 
the  water  in  C.  The  tube  d  is  bent,  as  shown  in  the  diagram,  and 
drawn  out  to  a  point,  to  diminish  the  size  of  the  bubbles.  The  tube 
e  is  wide,  and  cut  obliquely  to  prevent  water  collecting  and  passing 
into  C. 

METHOD  OF  PROCEDURE  :  The  clip  b  is  closed  and  c  opened,  and  the  tube  e 
disconnected  from  /.  The  solutions  in  A  and  B  are  then  boiled  for  20  minutes 
to  remove  all  oxygen.  The  tubes  e  and/  are  again  connected,  the  clip  c  is  closed, 
the  flame  under  B  increased  to  prevent  the  liquid  in  C  from  being  drawn  back, 
and  the  clip  b  is  opened.  As  soon  as  steam  issues  from  the  tube  a,  it  is  dipped 
into  a  conical  glass  containing  50  c.c.  of  ferrous  chloride  prepared  according  to 
Schlosing's  directions,and  the  flame  under  A  is  removed,  when  the  ferrous 
chloride  enters  the  flask.  The  clip  b  is  regulated  with  the  finger  and  thumb, 
so  as  to  prevent  the  entry  of  air  into  the  flask.  The  conical  vessel  is  rinsed  two 
or  three  times  with  water,  and  this  is  allowed  to  enter  the  flask,  and  the  clip  b  is 
then  closed,  and  the  vessel  A  heated.  The  liquid  in  A  turns  brown  in  a  short 
time,  and  nitric  oxide  is  evolved.  The  clip  c  is  opened  slightly  from  time  to  time 

*  Z.  a.  C.  23, 151. 


TITRATES.  279 

Until  the  pressure  is  high  enough,  when  it  is  opened  entirely.  The  flames  must 
be  so  regulated  that  a  slow  current  of  gas  bubbles  through  the  water  in  C.  The 
hydrochloric  acid  is  removed  by  the  caustic  soda  in  B,  and  the  nitric  oxide  on 
coming  in  contact  with  the  air  in  C  is  oxidized,  and  the  nitric  acid  absorbed  by  the 
water.  In  case  the  current  of  gas  is  too  rapid,  the  escaping  nitric  acid  is  absorbed 
in  D.  After  an  hour  the  tubes  e  and  /  are  disconnected,  while  the  solutions  in 
A  and  B  are  still  boiling,  and  the  nitric  acid  is  titrated  with  dilute  caustic  soda 
about  (£  normal).  The  vessel  C  must  be  well  cooled  during  the  whole  experiment, 
which  occupies  about  an  hour  and  a  half. 

Good  results  were  obtained  with  nitrates  of  potash  and  soda,  both 
alone  and  mixed  with  ammonium  sulphate,  superphosphate,  and 
amido  compounds.  With  superphosphate  the  solution  should  be 
made  slightly  alkaline,  to  prevent  the  liberation  of  nitric  acid. 

Warington*  has  made  a  series  of  experiments  on  the  original 
Sch losing  process,  for  the  purpose  of  testing  its  accuracy  when 
small  quantities  of  nitric  acid  have  to  be  determined  in  the  presence 
of  organic  substances,  such  for  instance  as  in  soils,  the  sap  of  beet- 
root, etc.  ;  but  instead  of  re-converting  the  nitric  oxide  into  nitric 
acid  as  in  the  original  method,  he  collected  the  gas  either  over 
caustic  soda,  as  recommended  byReichardt,or  over  mercury,  and 
ascertained  its  amount  by  measurement  in  Frankland's  gas 
apparatus.  The  results  obtained  by  Warington  plainly  showed 
that  even  in  the  most  favourable  circumstances  the  method  as 
usually  worked  in  Germany,  either  by  the  alkalimetric  titration  or 
by  measurement  of  the  gas,  invariably  gave  results  much  too  low, 
especially  if  the  quantity  of  nitrate  operated  on  was  small,  say  5 
or  6  centigrams  of  nitre  ;  moreover,  when  sugar  or  similar  organic 
substance  was  present  the  resulting  gas  was  very  impure,  and  the 
distillates  were  highly  coloured  from  the  presence  of  some  volatile 
products.  The  nitric  oxide  also  suffered  considerable  diminution 
of  volume  when  left  for  any  length  of  time  in  contact  with  the 
distillate,  especially  when  over  caustic  soda.  This  being  the  case, 
the  following  modification  originally  recommended  by  Schlosing 
was  adopted  in  which  C02  was  employed,  both  to  assist  in  expelling 
the  air  from  the  apparatus,  and  to  chase  out  the  nitric  oxide  pro- 
duced. 

The  form  of  apparatus  adopted  by  Warington  is  shown  in  fig.  50.  The 
vessel  in  which  the  reaction  takes  place  is  a  small  tubulated  receiver,  the 
tubulure  of  which  has  been  bent  near  its  extremity  to  make  a  convenient  junction 
with  the  delivery  tube,  which  dips  into  a  trough  of  mercury  on  the  left.  The 
long  supply  tube  attached  to  the  receiver  is  of  small  bore,  and  is  easily  filled  by 
a  ^  c.c.  of  liquid.  The  short  tube  to  the  right  is  also  of  small  bore,  and  is  connected 
by  a  caoutchouc  tube  and  clamp  with  an  apparatus  for  the  continuous  production 
of  carbon  dioxide. 

In  using  this  apparatus  the  supply  tube  is  first  filled  with  strong  HC1,  and  C02 
is  passed  through  the  apparatus  till  a  portion  of  the  gas  collected  in  a  jar  over 
mercury  is  found  to  be  entirely  absorbed  by  caustic  potash.  The  current  of  gas 
is  then  stopped  by  closing  the  clamp  to  the  right.  A  chloride  of  calcium  bath  at 
140°  is  next  brought  under  the  receiver,  which  is  immersed  one-half  or  more  in 
the  hot  fluid  ;  the  temperature  of  the  bath  is  maintained  throughout  the  operation 
by  a  gas  burner  placed  beneath  it.  By  allowing  a  few  drops  of  HC1  to  enter  the 
hot  receiver,  the  CO2  it  contains  is  almost  entirely  expelled.  A  jar  filled  with 

*J.  C.  S.  1880,  468. 


280 


NITRATES. 


mercury  is  then  placed  over  the  end  of  the  delivery  tube,  and  all  is  ready  for  the 
commencement  of  a  determination. 

The  nitrate,  which  should  be  in  the  form  of  a  dry  residue  in  a  small  beaker  or 
basin,  is  dissolved  in  about  2  c.c.*  of  strong  ferrous  chloride  solution,  1  c.c.  of 
strong  HC1  is  added,  and  the  whole  is  then  introduced  into  the  receiver  through 
the  supply  tube,  being  followed  by  successive  rinsings  with  HC1,  each  rinsing  not 
exceeding  a  \  c.c.,  as  the  object  is  to  introduce  as  small  a  bulk  of  liquid  as  possible. 


Fig.  50. 

The  contents  of  the  receiver  are  in  a  few  minutes  boiled  to  dryness  ;  a  little  C02 
is  admitted  before  dryness  is  reached,  and  again  afterwards  to  drive  over  all 
remains  of  nitric  oxide.  If  the  gas  is  not  to  be  analysed  till  next  day,  it  is  advisable 
to  use  more  C02,  so  as  to  leave  the  nitric  oxide  diluted  with  several  times  its  volume 
of  that  gas.  As  soon  as  one  operation  is  concluded  the  apparatus  is  ready  for 
another  charge. 

This  mode  of  working  presents  the  following  advantages  : — 

(1)  The  volume  of  liquid  introduced  into  the  apparatus  is  much  diminished, 
and  with  this  of  course  the  amount  of  dissolved  air  contributed  from  this  source. 

(2)  By  evaporation  to  dryness  a  complete  reaction  of  the  nitrate  and  ferrous 
chloride,  and  a  perfect  expulsion  of  the  nitric  oxide  formed,  is  as  far  as  possible 
attained. 

(3)  The  nitric  oxide  in  the  collecting  jar  is  left  in  contact  with  a  much  smaller 
volume  of  acid  distillate,  and  its  liability  to  absorption  is  greatly  diminished  by 
its  dilution  with  CO2. 

The  results  obtained  with  this  apparatus  by  Warington  on  small  quantities 
of  nitre  alone,  and  when  mixed  with  varying  quantities  of  ammonium  salts  and 
organic  substances  including  sugar,  showed  a  marked  improvement  upon  the 
method  as  usually  carried  out. 

»  Supposing  the  ferrous  chloride  to  contain  2  gm.  of  iron  per  10  c.c.,  then  1  c.c. 
the  solution  will  be  nearly  equivalent  to  0-1207  gm.  of  nitre,  or  0'0167   gm.   of 
nitrogen.    A  considerable  excess  of  iron  should,  however,  always  be  used. 


NITRATES.  281 

A  further  improvement  has  been  made  in  this  method  by 
Warington,*  and  described  by  him  as  follows  : — 

The  apparatus  now  employed  is  quite  similar  to  that  shown  in  fig.  50,  with  the 
only  difference  that  the  bulb  retort  in  which  the  reaction  takes  place  is  now  only 
If  inch  in  diameter,  thus  more  exactly  resembling  the  form  employed  by 
Schlosing.  A  bulb  of  this  size  is  sufficient  for  the  analysis  of  soil  extracts  ; 
for  determinations  of  nitrates  in  vegetable  extracts  a  larger  bulb  is  required. 

The  chief  improvement  consists  in  the  use  of  C02  as  free  as  possible  from  oxygen. 
The  generator  is  formed  of  two  vessels.  The  lower  one  consists  of  a  bottle  with 
a  tubulure  in  the  side  near  the  bottom  ;  this  bottle  is  supported  in  an  inverted 
position,  and  contains  the  marble  from  which  the  gas  is  generated.  The  upper 
vessel  consists  of  a  similar  bottle  standing  upright ;  this  contains  the  HC1  required 
to  act  on  the  marble.  The  two  vessels  are  connected  by  a  glass  tube  passing 
from  the  side  tubulure  of  the  upper  vessel  to  the  inverted  mouth  of  the  lower 
vessel ;  the  acid  from  the  upper  vessel  thus  enters  below  the  marble.  C02  is 
generated  and  removed  at  pleasure  by  opening  a  stop-cock  attached  to  the  side 
tubulure  of  the  lower  vessel,  thus  allowing  HC1  to  descend  and  come  in  contact 
with  the  marble.  The  fragments  of  marble  used  have  been  previously  boiled  in 
water.  The  boiling  is  conducted  in  a  strong  flask.  After  boiling  has  proceeded 
some  time,  a  caoutchouc  stopper  is  fixed  in  the  neck  of  the  flask,  and  the  flame 
removed  ;  boiling  will  then  continue  for  some  time  in  a  partial  vacuum.  The  lower 
reservoir  is  nearly  filled  with  the  boiled  marble  thus  prepared.  The  HC1  has  been 
also  well  boiled,  and  before  it  is  introduced  into  the  upper  reservoir  it  has  dissolved 
in  it  a  moderate  quantity  of  cuprous  chloride.  As  soon  as  the  acid  has  been 
placed  in  the  upper  reservoir  it  is  covered  by  a  layer  of  oil.  The  apparatus  being 
thus  charged  is  at  once  set  in  active  work  by  opening  the  stop-cock  of  the  marble 
reservoir  ;  the  acid  descends,  enters  the  marble  reservoir,  and  the  C02  produced 
drives  out  the  air  which  is  necessarily  present  at  starting.  As  the  acid  reservoir 
is  kept  on  a  higher  level  than  the  marble  reservoir,  the  latter  is  always  under  internal 
pressure,  and  leakage  of  air  from  without  cannot  occur. 

The  presence  of  the  cuprous  chloride  in  the  hydrochloric  acid  not  only  ensures 
the  removal  of  dissolved  oxygen,  but  affords  an  indication  to  the  eye  of  the 
maintenance  of  this  condition.  So  long  as  the  acid  remains  of  an  olive  tint, 
oxygen  will  be  absent ;  but  should  the  acid  become  of  a  clear  blue-green,  it  is 
no  longer  certainly  free  from  oxygen,  and  more  cuprous  chloride  must  be  added. 

A  further  slight  improvement  adopted  consists  in  the  use  of  freshly-boiled 
reagents,  which  are  employed  in  as  small  a  quantity  as  possible.  When  boiling 
the  hydrochloric  acid  it  is  well  to  add  a  few  drops  of  ferrous  chloride,  in  order 
more  certainly  to  remove  any  dissolved  oxygen. 

The  mode  of  operation  is  as  follows  : — The  apparatus  is  fitted  together,  the  long 
funnel  tube  attached  to  the  bulb  retort  being  filled  with  water.  Connection  is 
made  with  the  glass  stop-cock  of  the  C02  generator  by  means  of  a  short  stout 
caoutchouc  tube,  provided  with  a  pinch-cock.  The  pinch-cock  being  opened,  the 
stop-cock  is  turned  till  a  moderate  stream  of  bubbles  rises  in  the  mercury  trough  ; 
the  stop-cock  is  left  in  this  position,  and  the  admission  of  gas  is  afterwards 
controlled  by  the  pinch-cock,  pressure  on  which  allows  a  few  bubbles  to  pass 
at  a  time.  The  heated  chloride  of  calcium  bath  is  next  raised,  so  that  the  bulb 
retort  is  almost  submerged  ;  the  temperature,  shown  by  a  thermometer  which 
forms  part  of  the  apparatus,  should  be  130-140°.  By  boiling  small  quantities 
of  water  or  hydrochloric  acid  in  the  bulb  retort  in  a  stream  of  C02  the  air  present 
is  expelled  ;  the  supply  of  gas  must  be  stopped  before  the  boiling  has  ceased,  so 
as  to  leave  little  in  the  retort.  Previous  to  very  delicate  experiments  it  is  advisable 
to  introduce  through  the  funnel  tube  a  small  quantity  of  nitre,  ferrous  chloride, 
and  hydrochloric  acid,  rinsing  the  tube  with  the  latter  reagent ;  any  trace  of 
oxygen  remaining  in  the  apparatus  is  then  consumed  by  the  nitric  oxide  formed, 
and  after  boiling  to  dryness,  and  driving  out  the  nitric  oxide  with  C02,  the  apparatus 
is  in  a  perfect  condition  for  a  quantitative  experiment. 

Soil  extracts  may  be  used  without  other  preparation  than  concentration. 
Vegetable  juices,  which  coagulate  when  heated,  require  to  be  boiled  and  filtered, 

*  J.  C.  S.  1882,  3d5. 


282  NITRATES. 

or  else  evaporated  to  a  thin  syrup,  treated  with  alcohol  and  filtered.  A  clear 
solution  being  thus  obtained,  it  is  concentrated  over  a  water  bath  to  the  smallest 
volume,  in  a  beaker  of  smallest  size.  As  soon  as  cool,  it  is  mixed  with  1  c.c.  of  a 
cold  saturated  solution  of  ferrous  chloride  and  1  c.c.  HC1,  both  reagents  having 
been  boiled  and  cooled  immediately  before  use.  In  mixing  with  the  reagents 
care  must  be  taken  that  bubbles  of  air  are  not  entangled  ;  this  is  especially  apt  to 
occur  with  viscid  extracts.  The  quantity  of  ferrous  chloride  mentioned  is  amply 
sufficient  for  most  soil  extracts,  but  it  is  well  perhaps  to  use  2  c.c.  in  the  first 
experiment  of  a  series  ;  the  presence  of  a  considerable  excess  of  ferrous  chloride 
in  the  retort  is  thus  ensured.  With  bulky  vegetable  extracts  more  ferrous  chloride 
should  be  employed  ;  to  the  syrup  from  20  gm.  of  mangel  sap  should  be  added  5  c.c. 
of  ferrous  chloride,  and  2  c.c.  of  hydrochloric  acid. 

The  mixture  of  the  extract  with  ferrous  chloride  and  HC1  is  introduced 
through  the  funnel  tube  and  rinsed  in  with  three  or  four  successive  |  c.c.  of  HC1. 
The  contents  of  the  retort  are  then  boiled  to  dryness,  a  little  C02  being  from 
time  to  time  admitted,  and  a  more  considerable  quantity  used  at  the  end  to  expel 
any  remaining  nitric  oxide.  The  most  convenient  temperature  is  140°,  but  in 
the  case  of  vegetable  extracts  it  is  well  to  commence  at  130°,  as  there  is  some  risk 
of  the  contents  of  the  retort  frothing  over.  The  gas  is  collected  in  a  small  jar 
over  mercury.  As  soon  as  one  operation  is  completed,  the  jar  is  replaced  by  another 
full  of  mercury,  and  the  apparatus  is  ready  to  receive  a  fresh  extract.  A  series 
of  five  determinations,  with  all  the  accompanying  gas  analyses,  may  be  readily 
performed  in  one  day.  The  bulb  retort  becomes  encrusted  with  charcoal,  when 
extracts  rich  in  organic  matter  are  the  subject  of  analysis  ;  it  is  best  cleaned  first 
with  water,  and  then  by  heating  oil  of  vitriol  in  it. 

Mercury,  contrary  to  the  statement  in  most  text-books,  is  gradually  attacked 
by  hydrochloric  acid  in  the  presence  of  air  ;  the  mercury  in  the  trough  is  thus 
apt  to  become  covered  with  a  grey  chloride,  and  it  is  quite  necessary  to  keep  the 
store  of  mercury  in  contact  with  sulphuric  acid  to  preserve  its  mobile  condition. 

The  gas  analysis  is  of  a  simple  character  ;  the  gas  is  measured  after  absorption 
of  the  CO2  by  potash,  and  again  after  absorption  of  the  nitric  oxide,  the  difference 
giving  the  amount  of  this  gas.  For  the  absorption  of  nitric  oxide,  a  saturated 
solution  of  ferrous  chloride  was  for  some  time  employed.  This  method  is  not, 
however,  perfectly  satisfactory  when  the  highest  accuracy  is  required,  the  nitric 
oxide  being  generally  rather  under-estimated,  unless  the  process  of  absorption  is 
repeated  with  a  fresh  portion  of  ferrous  chloride.  The  error  is  greater  in  pro- 
portion to  the  quantity  of  unabsorbed  gas  present.  Thus,  with  a  mixture  of 
nitrogen  and  nitric  oxide  containing  little  of  the  former  absorption  of  the  nitric 
oxide  by  successive  treatment  with  oxygen  and  pyrogallol  over  potash  showed 
97 '8  per  cent,  of  nitric  oxide  ;  while  the  same  gas,  analysed  by  a  single  absorption 
with  ferrous  chloride  (after  potash),  showed  97 '5  per  cent,  of  nitric  oxide.  With 
a  mixture  containing  more  nitrogen,  the  oxygen  method  showed  65 '9  per  cent,  of 
nitric  oxide  ;  while  one  absorption  with  ferrous  chloride  gave  64'2  per  cent.,  and 
a  second  absorption,  in  which  the  ferrous  chloride  was  plainly  discoloured,  66 -2 
per  cent.  The  use  of  ferrous  chloride  as  an  absorbent  for  nitric  oxide  has  now 
been  given  up,  and  the  oxygen  method  substituted.  All  the  measurements  of 
the  gas  are  now  made  without  shifting  the  laboratory  vessel ;  the  conditions  are 
thus  favourable  to  extreme  accuracy. 

The  chief  source  of  error  attending  the  oxygen  process  lies  in  the 
small  quantity  of  carbonic  oxide  produced  during  the  absorption 
with  pyrogallol ;  this  error  becomes  negligible  if  the  oxygen  is  only 
used  in  small  excess.  The  difficulty  of  using  the  oxygen  in  nicely 
regulated  quantity  may  be  removed  by  the  use  of  Bischof's  gas 
delivery-tube.  This  may  be  made  of  a  test-tube,  having  a  small 
perforation  half  an  inch  from  the  mouth.  The  tube  is  partly  filled 
with  oxygen  over  mercury,  and  its  mouth  is  then  closed  by  a  finely- 
perforated  stopper,  made  from  a  piece  of  wide  tube,  and  fitted 
tightly  into  the  test-tube  by  means  of  a  covering  of  caoutchouc. 


NITRATES.  283 

When  this  tube  is  inclined,  the  side  perforation  being  downwards, 
the  oxygen  is  discharged  in  small  bubbles  from  the  perforated  stopper 
while  mercury  enters  through  the  side  opening.  Using  this  tube, 
the  supply  of  oxygen  is  perfectly  under  control,  and  can  be  stopped 
as  soon  as  a  fresh  bubble  ceases  to  produce  a  red  tinge  in  the  labora- 
tory vessel.  The  trials  made  with  this  apparatus  have  been  very 
satisfactory.  If  nitrites  are  to  be  determined  by  this  method,  it  is 
necessary  first  to  convert  them  into  nitrates  with  excess  of  hydrogen 
peroxide,  which  is  entirely  destroyed  by  the  subsequent  evaporation 
to  dryness. 

4.     By  the  K  j  e  1  d  a  h  1  Process. 

By  the  Gunning-Jodlbauer  modified  method  described  on  p.  88 
it  is  now  quite  possible  to  determine  the  nitrogen  in  commercial 
nitrates  with  great  accuracy  and  very  little  personal  attention. 

5.     Ulsch's  Method. 

This  is  a  simple  and  ready  plan  of  determining  alkali  nitrates, 
or  the  amount  of  them  existing  in  manures  when  there  is  no  salt 
of  ammonia  or  other  form  of  nitrogen  present.  It  depends  on  the 
fact  that  when  nitrate  of  soda  or  potash  is  boiled  with  dilute 
sulphuric  acid,  together  with  iron  reduced  by  hydrogen,  the  nitrogen 
becomes  converted  into  ammonium  sulphate.  The  ammonia  is 
then  distilled  off  by  boiling  with  caustic  soda  as  in  Kjeldahl's 
method. 

METHOD  OF  PROCEDURE  :  0*5  gm.  of  an  alkali  nitrate,  dissolved  in  25  c.c.  of 
water,  or  a  volume  of  manure  solution  containing  about  that  quantity,  which 
should  not  measure  more  than  25  or  30  c.c.,  is  put  into  a  small  (150  c.c.)  flask. 
5  gm.  of  reduced  iron  and  20  c.c.  of  dilute  sulphuric  acid  (1*3)  are  then  added, 
and  the  flask  placed  in  an  inclined  position  and  the  reaction  allowed  to  continue 
in  the  cold  until  effervescence  ceases.  The  mixture  is  then  boiled  for  six  or  seven 
minutes,  and  allowed  to  cool.  The  liquid  is  then  transferred  to  a  Kjeldahl 
distilling  flask,  an  excess  of  caustic  soda  with  a  few  pieces  of  zinc  added,  and  the 
ammonia  collected  in  standard  acid  and  titrated  as  usual  in  the  Kjeldahl  process. 
The  calculation  into  nitrogen  or  alkali  nitrate  presents  no  difficulty. 

Some  operators  have  obtained  high  results  by  this  method,  owing  to  the  reduced 
iron  containing  some  form  of  nitrogen  or  ammonia.  A  blank  experiment  should 
therefore  be  made  with  the  iron  used  to  find  whether  such  impurity  exists. 
Brandt  found  that  cyanogen  was  the  offending  agent,  and  this  was  removed  by 
ignition  of  the  iron  again  in  hydrogen. 

The  official  method*  to  be  used  in  the  analysis  of  fertilizers  under 
the  Fertilizers  and  Feeding  Stuffs  Act,  1906,  is  as  follows  :— 

• 

NITROGEN  IN  NITRATES  IN  THE  ABSENCE  OF  AMMONIUM 
SALTS  AND  OF  ORGANIC  NITROGEN. 

1  gram  of  the  sample  shall  be  placed  in  a  half -litre  Erlenmeyer  flask  with  50  c.c. 
of  water,  10  grams  of  reduced  iron  and  20  c.c.  of  sulphuric  acid  of  1-35  sp.  gr.f 
shall  be  added.  The  flask  shall  be  closed  with  a  rubber  stopper  provided  with 

*The  Fertilizers  and  Feeding  Stuffs  (Methods  of  Analysis)  Regulations,  1908. 
No.  964. 

t  Made  by  mixing  1  vol.  of  T84  acid  with  2  vols.  of  water. 


284  NITRATES. 

a  thistle  tube,  the  head  of  which  shall  be  half  filled  with  glass  beads.  The  liquid 
shall  be  boiled  for  five  minutes,  and  the  flask  shall  then  be  removed  from  the 
flame,  any  liquid  that  may  have  accumulated  among  the  beads  being  rinsed 
back  with  water  into  the  flask.  The  solution  shall  be  boiled  for  three  minutes 
more,  and  the  beads  again  washed  with  a  little  water.  The  quantity  of  ammonia 
shall  then  be  determined  by  distillation  into  standard  acid  after  liberation  with 
alkali.  (Caustic  Soda  is  generally  used.) 

6.     Technical  method  for  the  P  e  1  o  u  z  e  process  with  Alkali 
Nitrates  and  Nitrated  Manures. 

Wagner  has  arranged  a  simple  form  of  the  Schlosing  method, 
which  gives  fairly  good  results  and  permits  of  rapid  working. 

The  following  is  the  slightly  modified  arrangement,  as  adopted 
at  the  Halle  Agricultural  Experiment  Station,  for  the  determination 
of  nitrogen  as  nitrates  in  fertilizers. 

A  250  c.c.  flask  is  fitted  with  a  two-hole  rubber  stopper.  One  hole  carries  an 
ordinary  gas  delivery-tube,  and  the  other  a  thistle  funnel,  having  a  stop-cock 
below  the  funnel.  The  end  of  this  tube  is  narrowed,  and  does  not  quite  reach 
the  liquid  in  the  flask. 

A  solution  of  200  gm.  of  iron  wire  in  hydrochloric  acid  is  made  and  diluted  to 
1  litre.  50  c.c.  of  this  solution,  and  the  same  quantity  of  10  %  HC1,  are  placed 
in  the  flask,  and  the  air  expelled  by  boiling.  10  c.c.  of  a  standard  solution  of  pure 
sodium  nitrate,  containing  25  gm.  per  litre,  are  then  placed  in  the  funnel,  and 
allowed  gradually  to  drop  into  the  boiling  solution  of  iron.  A  gas  tube  graduated 
to  100  c.c.  is  filled  with  40  %  solution  of  caustic  potash,  and  the  nitric  oxide 
collected  in  the  usual  way.  When  the  nitre  solution  is  nearly  all  dropped  in, 
the  funnel  is  filled  with  10  per  cent.  HC1,  and  run  down  drop  by  drop,  and  when 
no  more  nitric  oxide  is  evolved  the  tube  containing  the  gas  set  aside  in  a  large 
jar  containing  distilled  water.  10  c.c.  of  the  solution  to  be  tested  are  now  put 
into  the  funnel,  taking  care  that  not  more  than  100  c.c.  of  gas  will  result.  The 
gas  is  collected  as  before  in  a  fresh  tube  precisely  as  in  the  case  of  the  pure  nitrate. 
In  this  manner  ten  or  twelve  determinations  can  be  made  with  one  and  the  same 
ferrous  solution.  Finally,  a  fresh  test  is  made  with  standard  nitre  solution  ; 
the  readings  of  the  tubes  are  taken,  and  as  they  will  all  be  of  the  same  temperature 
and  pressure  no  correction  is  necessary,  all  being  allowed  to  cool  to  the  same 
point.  The  comparison  between  the  pure  nitrate  and  the  sample  will  afford  the 
calculation. 

Technical  use  of  the  P  e  1  o  u  z  e  Process  for  Mixed  Manures. — 
Vincent  Edwards*   adopts   the  following   method  for  manures 
containing  nitrates  together  with  ammonia  and  other  matters. 
The  solutions  required  are  : — 

Standard  potassium  dichromate,  14*742  gm.  per  litre.  1  c.c.= 
0-0085  gm.  NaNO3  or  0-0101  gm.  KNO3. 

Ferrous  sulphate.  100  gm.  of  crystallized  salt  with  100  c.c.  of 
concentrated  H2SO4  per  litre. 

The  exact  working  strength  of  these  two  solutions  in  practice  is 
found  by  boiling  50  c.c.  of  the  iron  solution  till  it  becomes  thick  in 
a  stout  well  annealed  glass  flask,  preferably  of  Jena  glass,  which  is 
fitted  with  a  B  u  n  s  e  n  valve,  made  by  cutting  the  rubber  tube  with  a 
sharp  razor,  the  glass  tube  to  which  it  is  fitted  passing  through  a  light 
fitting  rubber  stopper  ;  after  boiling  the  flask  is  set  aside  to  cool, 

«c.  N.  71,  307. 


NITRATES.  285 

then  100  c.c.  or  so  of  water  are  added,  and  the  titration  made  with 
dichromate  in  the  usual  way  with  fresh  solution  of  ferricyanide  as 
indicator. 

METHOD  OF  PROCEDURE  :  10-20  gm.  of  the  nitrated  manure,  according  to  its 
richness,  are  exhausted  with  water  and  the  liquid  made  up  to  200  c.c. 

20  c.c.  of  this  solution  are  placed  in  the  boiling  flask  together  with  50  c.c.  of 
the  iron  solution,  the  stopper  with  valve  is  then  inserted,  and  the  mixture  boiled 
until  it  becomes  thick,  and  semi-solid  drops  are  splashed  against  the  sides  of  the 
flask  ;  the  flask  is  then  enveloped  in  a  cloth,  and  removed  to  cool  ;  when  cool, 
100  c.c.  or  so  of  water  are  run  into  the  flask,  well  shaken,  then  titrated  with  the 
dichromate  as  in  the  case  of  the  blank  experiment. 

EXAMPLE  :  The  blank  titration  showed  that  50  c.c.  of  iron  solution  required 
54  c.c.  of  dichromate.  20  c.c.  of  the  manure  solution  (=1  gm.  manure)  were 
treated  as  above  described,  and  required  31  c.c.  of  dichromate,  therefore  54—31 
=23  c.c.  which  multiplied  by  0'0085  =0'1955  or  19'55  %  of  NaN03  in  the  manure. 
The  manure  was  known  to  be  a  mixture  of  20  %  of  nitrate  of  soda,  of  95 -5  % 
strength,  with  80  per  cent,  of  an  ammoniacal  guano. 

This  technical  process  is,  of  course,  chiefly  valuable  where  the 
nitrate  is  required  to  be  determined  apart  from  the  ammonia. 

7.    Devarda's  Method.* 

The  nitrate  is  reduced  to  ammonia  by  the  evolution  of  hydrogen 
generated  from  alkali  hydroxide  and  an  alloy  of  aluminium  with 
copper  and  zinc  (Al  45,  Cu  50,  Zn  5),  the  ammonia  being  distilled 
into  standard  acid  as  in  K  j  e  1  d  a  h  1 '  s  method.  The  alloy,  which  can 
be  readily  purchased,  is  brittle.  It  should  be  powdered  sufficiently 
finely  to  pass  through  a  No.  60  sieve,  and  five  times  the  weight  of 
the  nitrate  taken  for  the  determination  should  be  used.  Cahen,f 
who  has  recently  published  results  obtained  with  the  method, 
distils  the  ammonia  with  steam,  in  preference  to  distillation  by 
boiling. 

METHOD  OF  PROCEDURE:  About  0'5  gm.  of  the  nitrate  is  dissolved  in  110 
c.c.  of  water  in  a  Jena  glass  bulb  flask,  and  2-3  gm.  of  the  alloy,  5  c.c.  of  alcohol, 
and  50  c.c.  of  caustic  alkali  (sp.  gr.  1'3)  added.  The  flask  is  then  quickly  connected 
to  the  Kj  eld  a  hi  distilling  apparatus  and  allowed  to  stand  for  30  minutes,  when 
the  brisk  reaction  will  be  complete.  The  contents  of  the  flask  are  then  slowly 
raised  to  the  boiling  temperature,  and  steam  passed  for  30  minutes,  when  all  the 
ammonia  will  have  distilled  over. 


8.     Gasometric   Determination,   as   Nitric   Oxide,    of   Nitrogen 
in  Nitrates  and  Nitrites. 

1  c.c.  NO  at  N.T.P.  =0-6257  mgm.  N 

=  1-3402  „  NO 

=  1-6975  „  N203 

=  2-4121  „  N205 

=4-5176  „  KN03 

=  3-7986  „  NaN03 

=  2-8144  „  HN03 

*  Zeit.  /.  anal.  Chem.  1894,  33,  113.  f  Analyst,  1910,  307. 


286 


NITRATES    AND    NITRITES. 


The  two  following  methods  give  the  nitrogen  existing  as  both 
nitrate  and  nitrite  in  the  substances  analysed. 

(i.)  The  Crum  method.  This  method  was  described  by 
W.  Crum  as  far  back  as  1847.  Originally  devised  for  the  analysis 
of  nitrates  and  of  gun-cotton,  it  was,  in  an  improved  form,  used 
by  Frank  land  and  Armstrong  for  the  determination  of  nitrates  in 
water,  for  which  purpose  it  is  still  largely  used  (see  Water  Analysis 
section). 

(ii.)  Lunge's  nitrometer  method.  This  is  fully  described  in 
the  section  on  technical  gas  analysis.  It  is  used  for  the  analysis 
of  nitrous  vitriol  in  sulphuric  acid  manufacture  and  for  several 
other  purposes. 

NITRITES. 

1.     lodimetric  Method. 

Dunstan  and  Dymond*  have  devised  a 
method  for  the  determination  of  N203  in  organic 
and  inorganic  combination  which  is  both  simple 
in  operation  and  accurate  in  results.  The 
authors  point  out  that  although  the  inorganic 
nitrites  may  be  accurately  analysed  by  gaso- 
metric  methods,  or  by  permanganate,  it  is 
impossible  to  use  such  methods  for  the  organic 
compounds  or  their  alcoholic  solutions.  The 
reaction  upon  which  the  method  depends  is  not 
new,  being  based  on  the  following  equation — 

2HI + 2HNO2  =  2H2O  +  2NO + I2. 

The  liberated  iodine  is  titrated  with  N/10  thio- 
sulphate  in  the  usual  way.  The  chief  merit  in 
the  process  is  the  simple  form  of  apparatus  used, 
which  is  shown  in  fig.  51. 

A  stout  glass  flask,  having  a  capacity  of 
about  100  c.c.,  is  closed  by  a  tightly  fitting 
rubber  stopper,  through  which  passes  a  piece 
of  rather  wide  glass  tubing  (C),  one  end  of  which 
(that  within  the  flask)  is  cut  off  obliquely,  so  that 
liquid  may  flow  freely  through  it.  The  other 
end  of  the  tube  is  connected  by  means  of  a  piece 
of  thick  rubber  tubing  with  a  large  glass  tube, 
which  forms  a  lipped  funnel  (A).  A  steel  screw 
clamp  (B)  regulates  communication  between  the 
funnel  and  the  tube,  and  the  short  interval  of 
rubber  which  is  not  occupied  by  glass  tubing 
forms  a  hinge  upon  which  the  flask  may  be 
moved  into  a  position  at  right-angles  to  the 
Fig.  51.  funnel,  in  order  to  mix  by  agitation  the  liquids 

*  Pharm.  Journ.  [3]  19,  741. 


NITRITES.  287 

which  are  introduced  into  the  apparatus.  The  absence  of  any 
leak  in  the  apparatus  is  ascertained  by  boiling  about  50  c.c.  of  water 
in  the  flask  until  steam  has  continuously  issued  from  the  funnel  for 
some  few  minutes,  when  the  screw  clip  is  quickly  closed  and 
simultaneously  the  source  of  heat  is  removed.  A  little  water  is 
now  placed  in  the  funnel  and  the  flask  is  cooled  by  immersion 
in  water.  On  sharply  inverting  the  flask  the  "click"  of  the 
water  against  the  airless  flask  should  be  quite  distinct.  No  water 
should  be  drawn  from  the  funnel  or  from  any  of  the  joints  into 
the  flask,  and  no  diminution  in  the  intensity  of  the  "  click"  should 
be  observed  after  the  apparatus  has  been  standing,  neither  when 
the  flask  is  inverted  and  the  funnel  empty  should  any  bubbles 
of  air  pass  through  into  the  liquid.  Having  thus  proved  the 
absence  of  any  leak  in  the  apparatus,  it  is  ready  for  use.  The 
flask  is  now  free  from  all  but  mere  traces  of  oxygen.  A  conclusive 
proof  of  this  is  obtained  by  boiling  in  the  flask  a  solution  of 
potassium  iodide,  acidified  with  diluted  sulphuric  acid,  and  then, 
after  the  closed  flask  has  been  cooled,  the  funnel  removed  and  its 
place  taken  by  a  smaller  glass  tube  filled  with  air-free  water,  the 
apparatus  is  connected  with  a  reservoir  of  pure  nitric  oxide. 
When  the  clamp  is  unscrewed  nitric  oxide  is  drawn  into  the  flask, 
and  should  any  oxygen  be  present  nitrous  acid  will  be  produced, 
and  consequently  iodine  will  be  set  free.  This  experiment  has 
often  been  made  by  the  authors,  who  have  failed  to  observe  any 
but  an  insignificant  trace  of  liberated  iodine. 

METHOD  or  PROCEDURE  :  5  c.c.  of  a  10  per  cent,  solution  of  potassium  iodide,  ' 
5  c.c.  of  a  10  per  cent,  solution  of  sulphuric  acid,  and  40  c.c.  of  water  are 
introduced  into  the  flask,  which  is  securely  fitted  with  the  cork  carrying  the  funnel 
and  tube.  The  screw  clip  being  open,  and  a  free  passage  left  for  the  escape  of  steam, 
the  liquid  is  boiled.  After  a  few  minutes,  when  any  iodine  which  may  have  been 
liberated  has  been  expelled,  and  the  upper  part  of  the  flask  is  completely  filled 
with  steam,  which  is  also  freely  issuing  from  the  funnel,  the  clip  is  tightly  closed, 
and  at  the  same  moment  the  source  of  heat  is  removed.  A  little  water  is  now 
put  into  the  funnel,  and  also  on  the  rim  of  the  flask,  as  a  safeguard  against  a  possible 
minute  leakage,  and  the  vessel  is  cooled,  by  immersion  in  water.  A  solution 
containing  a  known  weight  of  the  nitrite  (equivalent  to  about  0*1  gm.  of  nitrous  . 
acid)  is  placed  in  the  funnel,  and  slowly  drawn  into  the  flask  by  cautiously 
unscrewing  the  clip.  The  liquid  which  adheres  to  the  funnel  is  washed  into  the 
flask  with  recently  boiled  and  air-free  water,  care  being  taken  that  during  this 
operation  no  air  is  admitted  into  the  flask.  When  experiments  are  being  made 
with  organic  nitrites  which  are  insoluble  in  water,  they  are  dissolved  in  alcohol, 
and  alcohol  is  also  used  to  wash  the  funnel.  When  the  nitrite  is  very  volatile, 
a  little  cold  alcohol  should  be  put  in  the  funnel,  and  the  point  of  the  pipette 
containing  the  nitrite  should  be  held  at  the  bottom  of  the  funnel  beneath  the  alcohol, 
and  the  liquid  quickly  drawn  from  the  pipette  into  the  flask.  The  nitrite  having 
been  introduced,  the  flask  is  well  shaken  and  the  liberated  iodine  is  titrated  with 
a  standard  solution  of  thiosulphate,  small  quantities  of  which  are  delivered  from 
a  burette  into  the  funnel  and  gradually  drawn  into  the  flask  ;  the  screw  clip  renders 
it  quite  easy  to  admit  minute  quantities  of  the  solution.  As  soon  as  the  iodine  is 
decolorized  any  standard  solution  remaining  in  the  funnel  is  returned  to  the  burette. 
Or  the  funnel  may,  before  the  titration  is  commenced,  be  replaced  by  the  burette 
itself,  and  the  standard  solution  delivered  direct  into  the  flask.  Starch  may  be  used 
as  an  indicator,  but  it  is  usually  quite  easy  to  observe  the  complete  disappearance 
of  the  yellow  colour  of  the  dissolved  iodine.  From  the  volume  of  the  standard 


288  NITRITES. 

solution  used,  the  amount  of  nitrous  acid  is  calculated  from  the  equation  befoio 
given. 

It  is  obvious  that  the  apparatus  might  be  improved  in  several 
respects,  as  for  example,  by  constructing  it  entirely  of  glass,  with 
a  ground  stopper  and  tap,  as  well  as  by  the  use  of  a  graduated 
funnel  to  deliver  the  standard  solution,  and  also  in  other  ways. 

The  authors  quote  numerous  experiments,  comparing  the  method 
with  careful  determinations  of  sodium  and  ethyl  nitrites  gaso- 
metrically,  showing  excellent  results. 

As  a  further  test  of  the  accuracy  of  the  process,  experiments  were 
made  with  various  organic  nitrites  of  known  purity.  In  each 
instance  a  solution  of  the  nitrite  was  made  by  weight,  and  a  weighed 
quantity  was  used  for  the  determination.  To  prevent  any  loss  of 
these  volatile  nitrites  the  experiments  wrere  conducted  in  the 
following  manner  : — A  well-stoppered  bottle  half  filled  with  the 
alcohol  corresponding  to  the  nitrite*  to  be  determined  was  weighed. 
Enough  of  the  nitrite  was  now  introduced  by  means  of  a  pipette  to 
constitute  approximately  a  2  per  cent  solution,  and  the  liquid 
again  weighed.  The  exact  strength  of  the  solution  having  been 
thus  determined,  the  contents  of  the  bottle  were  well  mixed,  and  the 
neck  and  stopper  of  the  bottle  dried.  The  bottle  was  now  re- weighed, 
and  about  2  c.c.  of  the  solution  removed  by  a  pipette,  care  being 
taken  not  to  wet  the  neck  of  the  bottle.  The  liquid  having  been 
introduced  into  the  flask  without  exposure  to  air,  in  the  manner 
which  has  been  previously  described,  the  bottle  containing  the 
solution  was  again  weighed.  The  results  obtained  with  ethyl 
nitrite  were  : — 

Taken.  Found. 

0-088  gm.  0-089  gm. 

0-176     „  0-179     „ 

0-113     „  0-115     „ 


2.     Analysis  of  Alkali  Nitrites  by  Permanganate. 

Kinnicutt  and  Nef  have  devised  the  following  method,  and  it 
gives  good  results  if  carefully  managed. 

The  sample  of  nitrite  is  dissolved  in  cold  water  in  the  proportion  of  about  1  to 
300  :  to  this  liquid  N/iO  permanganate  is  added,  drop  by  drop,  till  it  has  a  per- 
manent red  colour  ;  then  2  or  3  drops  of  dilute  H2S04,  and  immediately  afterwards 
a  known  excess  of  the  permanganate.  The  liquid,  which  should  now  be  of  a  dark 
red  colour,  is  strongly  acidified  with  pure  H2S04,  heated  to  boiling,  and  the  excess 
of  permanganate  determined  by  means  of  freshly  prepared  N/iO  oxalic  acid. 
1  c.c.  permanganate  =0-00345  gm.  NaN02,  or  0-00425  gm.  KN02. 

Of  course,  there  must  be  no  reducing  substance  other  than  the 
nitrite  present  in  the  material  examined,  and,  to  ensure  accuracy, 

*  The  corresponding  alcohol  was  employed  to  prevent  loss  consequent  on  the 
occurrence  of  a  reverse  chemical  change,  which  takes  place  when  a  lower  homologous 
alcohol  is  mixed  with  the  nitrite  corresponding  to  a  higher  homologous  alcohol :  tor 
example,  a  solution  of  amyl  nitrite  in  ethyl  alcohol  soon  becomes  a  solution  of  ethyl 
nitrite  in  amyl  alcohol,  from  which  the  ethyl  nitrite  rapidly  volatilizes 


NITRITES.  289 

a  blank  experiment  should  be  made  with  the  like  proportions  of 
H2SO4  and  oxalic  acid. 

3.     Gasometric  Method. 

P.  Frankland*  adopts  this  method  for  the  determination  of 
nitrous  acid  in  small  quantity,  but  too  large  for  colori metric 
determination,  and  where  also  ammonia,  organic  matters,  and 
nitrates  may  co-exist.  It  is  based  on  the  fact  that  when  nitrous 
acid,  together  with  excess  of  urea,  is  mixed  with  sulphuric  acid  in 
the  cold,  the  reaction  is 

CO(NH2)2  +  2HNO2=2N2+CO2+3H2O. 

The  decomposition  is  made  in  the  Crum-Frankland  shaking 
tube,  described  and  figured  in  Part  VI.,  and  the  evolved  nitrogen 
gas  measured  in  the  usual  gas  apparatus.  The  ordinary  nitrometer 
may  also  be  used  for  large  quantities  of  NO  by  the  same  method. 

In  the  case  of  an  ordinary  alkali  nitrite,  the  dry  substance,  or  its 
solution  evaporated  to  dryness,  is  mixed  with  excess  of  crystallized 
urea,  and  dissolved  in  about  2  c.c.  of  boiling  water  in  a  beaker,  then 
transferred,  with  the  rinsings,  to  the  cup  of  the  apparatus,  and 
passed  into  the  tube.  A  few  c.c.  of  dilute  sulphuric  acid  (1  :  5) 
are  then  passed  in.  A  vigorous  evolution  of  gas  takes  place,  and 
continues  for  some  five  minutes  ;  the  gas  is  a  mixture  of  nitrogen 
and  carbonic  anhydride.  The  decomposition  is  complete  in  fifteen 
minutes.  A  solution  of  pure  sodium  hydrate  (1  :  3)  is  now  added 
through  the  cup,  and  the  mixture  violently  shaken,  until  the  CO2 
is  absorbed.  The  gas  and  liquid  are  then  transferred,  by  means 
of  another  mercury  trough,  to  the  laboratory  vessel,  and  the  gas, 
which  is  double  the  volume  of  the  N  existing  as  N2O3,  measured  in 
a  gas  apparatus,  and  its  weight  calculated  in  the  usual  way. 

EXAMPLE  :  A  solution  of  sodium  nitrite  was  made  and  standardized  with 
permanganate,  the  result  being  that  10  c.c.  =0-01346  gm.  N.  10  c.c.  of  the  same 
solution  were  evaporated  to  dryness  in  a  small  beaker,  about  0-2  gm.  of  urea 
added,  the  whole  dissolved  in  2  c.c.  of  hot  water,  which,  with  the  rinsings,  were 
transferred  through  the  cup  into  the  tube,  treated  with  sulphuric  acid  and  caustic 
soda,  then  transferred  to  the  gas  apparatus  with  the  following  results : — Volume 
of  N,  13-8  c.c. ;  mercurial  pressure,  127 '5  mm. ;  temperature,  17 '7°  C.  The  weight 
of  N  thus  found,  after  the  necessary  corrections,  was  OO1346  gm. 

The  Crum-Frankland  mercury  method,  described  in  the  section 
on  Water  Analysis,  and  in  which  the  same  shaking  tube  is  used, 
does  not  distinguish  between  nitric  and  nitrous  nitrogen ;  but 
P.  Frankland  required  a  method  for  the  determination  of  nitrous 
acid  in  a  mixture  of  nitrates,  peptones,  sugar,  and  various  salts 
occurring  in  a  solution  used  for  cultivation  of  micro-organisms.  The 
experiments  carried  out  by  him  showed  that  when  such  a  mixture 
was  evaporated  to  dryness  the  loss  of  HNO2  was  considerable, 
and  the  results  came  out  much  too  low.  Further  experiment, 
however,  showed  that  the  addition  of  a  slight  excess  of  caustic 

*  J.  CJS.  63,  364. 


290  NITRITES. 

potash  during  evaporation  prevented  the  loss  of  any  HNO2  ;  and 
on  the  other  hand  the  addition  of  a  slight  excess  of  ammonium 
chloride  entirely  destroyed  it.  Therefore  by  a  combination  of  the 
mercury  and  the  urea  methods,  the  determination  of  nitric  and 
nitrous  acids  may  be  satisfactorily  accomplished,  the  destruction 
of  the  HN02  on  the  one  hand  being  effected  by  excess  of  NH4C1, 
whilst  on  the  other  hand  all  loss  of  HNO2  may  be  avoided  by 
evaporation  with  caustic  alkali.  The  mode  of  procedure  has  the 
advantage  over  all  differential  methods  in  that  each  acid  is 
determined  individually  and  independently  of  the  other. 

4.     Mixtures  of  Nitrites  with  Alkali  Sulphites  and  Thiosulphates. 

Lunge  and  Smith*  have  shown  that  the  only  satisfactory  method 
of  completely  oxidizing  sulphites  and  thiosulphates  by  permanganate 
is  to  add  to  the  solution  a  large  excess  of  permanganate,  more  than 
sufficient  for  complete  oxidation,  and  accompanied  with  formation 
of  Mn02.  Excess  of  FeSO4  is  then  added,  and  again  permanganate 
till  pink.  When  such  a  mixture  contains  nitrites,  they  will  of 
course  be  oxidized  to  nitrates. 

To  find  the  amount  of  nitrites  present,  therefore,  the  following 
plan  is  adopted  :  — 

METHOD  OF  PROCEDURE  :  The  solution  of  the  substance  in  not  too  large  quantity 
is  oxidized  exactly  as  described,  a  known  volume  of  standard  ferrous  sulphate 
is  added,  together  with  a  large  excess  of  strong  H2S04.  The  mixture  is  boiled 
nearly  to  dryness  in  a  flask  with  slit  valve,  diluted,  and,  when  cool,  titrated  with 
permanganate.  The  difference  between  the  volume  then  required  and  that 
required  by  the  original  FeS04,  represents  the  nitric  acid  which  has  been  reduced 
and  escaped  as  NO. 

The  exceedingly  delicate  colorimetric  method  of  determining 
nitrites  originally  devised  by  Griess,  and  improved  by  others, 
will  be  described  in  the  section  on  Water  Analysis. 

DISSOLVED    OXYGEN. 


THE  volumetric  determination  of  the  dissolved  oxygen  in  water 
is  an  operation  of  some  importance  in  water  analysis.  It  is  well 
known  that  organic  and  bacterial  contamination  generally  exist 
side  by  side  ;  the  organic  matter  offering  suitable  pabulum  for  the 
growth  of  bacterial  life.  Water  thus  contaminated  is  de-oxygenated 
by  the  living  organisms  which  consume  oxygen  during  their  growth  ; 
hence  the  importance  of  the  determination  of  dissolved  oxygen  in 
water,  as  a  means  of  ascertaining  the  co-existence  of  the  two  kinds 
of  impurity. 

In  brewing  also  a  knowledge  of  the  state  of  aeration  of  the  wort 
is  sometimes  of  importance,  especially  at  the  fermentation  stage  of 
the  process. 

Several    methods    have    been   proposed   for    carrying    out    the 

*  J.  S.  C.  I.  2,  465. 


DISSOLVED    OXYGEN.  291 

determination.  Mohr's  method,  depending  on  the  exidation  of 
ferrous  compounds,  with  subsequent  titration  by  permanganate, 
has  not  come  greatly  into  use.  W inkier*  proposed  to  take 
advantage  of  the  oxidation  of  manganous  hydroxide  (obtained  by 
mixing  solutions  of  a  manganous  salt  and  caustic  alkali)  by  dissolved 
oxygen,  the  higher  oxide  formed  being  decomposed  by  sulphuric 
acid  and  potassium  iodide  with  liberation  of  iodine,  which  is 
determined  by  titration  with  sodium  thiosulphate.  This  method 
is  interfered  with  by  the  presence  of  nitrites,  which  also  liberate 
iodine  from  acidified  potassium  iodide  ;  great  organic  contamination 
also  interferes,  inasmuch  as  the  impurities  present  take  up  a  portion 
of  the  liberated  iodine. 

Schiitzenberger's  method,  |  f  ully  described  in  the  sixth  edition 
of  this  book,  has  received  great  attention  from  many  operators, 
some  of  whom  have  reported  favourably,  whilst  others  find  the 
process  unreliable.  The  reason  for  the  anomalies  apparent  in  the 
reports  of  the  various  experimenters  is  shown  in  the  results  of  an 
interesting  and  critical  investigation  of  the  process  carried  out  by 
Roscoe  and  Lunt,J  They  show  that  an  important  disturbing 
influence  had  been  overlooked,  and  explain  many  previously 
ill-understood  points  in  the  process. 

Schiitzenberger's  original  process  depends  on  the  reducing 
action  of  sodium  hyposulphite  Na2SO2,  prepared  by  the  action  of 
zinc  dust  on  a  saturated  solution  of  sodium  bisulphite  containing 
an  excess  of  sulphurous  acid.  The  determination  was  originally 
carried  out  in  a  large  Woullfe's  bottle,  of  about  two  litres  capacity, 
filled  with  pure  hydrogen.  About  20-30  c.c.  of  water  were  intro- 
duced and  slightly  coloured  blue  by  indigo-carmine  solution. 
The  blue  colour  was  then  cautiously  discharged  by  the  careful 
dropping  in  of  hyposulphite  solution.  To  the  yellow  reduced 
liquid  thus  produced,  the  water  to  be  examined  was  added  from 
a  pear-shaped  vessel  holding  about  250  c.c.  The  dissolved  oxygen 
restored  the  blue  colour  by  oxidation,  and  the  amount  of  hypo- 
sulphite required  again  to  decolorize  the  liquid  was  noted. 

Schiitzenberger  showed  that  when  a  small  amount  of  indigo 
was  employed  in  the  determination,  the  yellow  colour  produced  when 
the  titration  was  completed  quickly  returned  to  blue,  and  this  when 
decolorized  again  turned  blue,  and  so  on  for  some  time,  until  double 
the  amount  of  hyposulphite  first  added  had  been  used.  He  showed 
also  that  by  using  a  much  larger  amount  of  indigo  the  double 
portion  of  hyposulphite  was  required  at  once. 

By  titrating  an  ammoniacal  solution  of  copper  sulphate  with  the 
hyposulphite  used  he  arrived  at  a  value  (though  an  erroneous  one) 
for  the  hyposulphite  employed  in  his  experiments,  and  concluded 
that,  at  the  first  yellow  colour  produced  in  a  titration  where  a  small 
amount  of  indigo  was  used,  only  half  the  oxygen  actually  present 

*  Berichte,  1888,2851. 

t  See  "  Fermentation  "by  P.  Sohutzenberger  (International  Scientific  Series). 
U.C.S.  1889,552. 

U    2 


292  DISSOLVED    OXYGEN. 

had  been  obtained.  The  other  half  he  accounted  for  by  saying  that 
the  reaction  between  hyposulphite  and  dissolved  oxygen  is  such 
that  one-half  the  oxygen  becomes  latent  as  hydrogen  peroxide, 
which  slowly  gives  up  half  its  oxygen.  He  thus  accounted  for  the 
return  of  the  blue  colour,  as  well  as  his  observation  that  only  half 
the  oxygen  was  at  once  obtained.  To  explain  the  observation 
that  when  a  large  amount  of  indigo  was  employed  the,  whole  of  the 
dissolved  oxygen  was  found,  he  assumed  that  a  different  reaction 
takes  place,  one  between  dissolved  oxygen  and  reduced  indigo,  in 
which  the  peroxide  of  hydrogen  is  not  formed. 

Rams  ay  and  Williams,*  whilst  agreeing  withSchiitzenberger 
and  with  Dupre,t  that  the  process  gives  reliable  results,  throw 
a  doubt  on  the  chemical  explanation  given  of  the  above 
experiments. 

Instead  of  the  ratio  1  :  2,  they  find  3  :  5  to  be  the  ratio  between 
the  first  and  the  total  quantity  of  hyposulphite  required  when  a  small 
amount  of  indigo  is  employed,  but  give  it  only  as  the  mean  expression 
of  the  varying  ratios  they  obtain,  and  add  "  but  it  is  difficult  to 
devise  an  equation  which  will  in  a  rational  manner  account  for  this 
partition  of  oxygen  "  into  two  stages  of  the  process.  Roscoe 
and  Lunt's  investigation;]:  has  thrown  a  new  light  on  these 
experiments.  They  show  (1)  that  a  series  of  fifteen  determinations 
carried  out  with  every  care  in  improved  apparatus,  and  under 
apparently  identical  conditions,  gave  discordant  results,  varying 
between  4*55  and  6'50  c.c.  of  hyposulphite  for  the  same  volume  of 
water,  showing  a  difference  of  0-35  per  cent,  of  the  mean  value. 
(2)  The  rapidity  of  titration  has  a  great  influence  on  the  result. 
The  mean  of  a  series  of  ten  determinations  carried  out  drop  by 
drop  was  5-47,  whilst  ten  experiments  with  the  same  sample  of 
water  gave  a  mean  of  7*12  when  the  titration  was  performed 
quickly.  (3)  Not  only  is  a  low  result  obtained  by  a  slow  titration 
and  a  high  result  by  a  quick  one,  but  by  varying  the  time  of  titration 
still  more,  extreme  variations  in  the  result  are  obtained ;  any  value 
between  1  and  100  per  cent,  of  the  total  oxygen  present  being 
shown  to  be  possible.  (4)  The  ratio  between  the  first  reading  and 
the  total  quantity  of  hyposulphite  required  is  not  a  constant  one, 
and  is  shown  to  be  capable  of  an  infinite  range  of  variation. 

The  key  to  the  explanation  of  these  remarkable  results  is  given 
by  the  authors  as  follows  : — "  The  conclusion  "  from  their  experi- 
ments "  was,  that  when  aerated  water  is  introduced  into  an 
atmosphere  of  pure  hydrogen,  it  immediately  begins  to  lose  oxygen 
by  diffusion  into  the  hydrogen  until  an  equilibrium  is  established." 
By  the  recognition  of  this  disturbing  influence,  the  previous 
anomalies  are  easily  explainable  on  the  following  data. 

(1)  Discordant  results  are  obtained  from  the  same  water,  because 
the  several  titrations  are  not  performed  in  exactly  the  same  time, 
therefore,  varying  amounts  of  oxygen  diffuse,  and  leave  a  varying 
residue  for  titration. 

*  J.  C.  S.  1886,  751.  t  Analyst  10,  156.  J  J.  C.  S.  1889,  552. 


DISSOLVED    OXYGEN.  293 

(2)  The  high  results  of  a  quick  titration  are  accounted  for  by  the 
fact  that  a  large  amount  of  oxygen  is  titrated  and  fixed  before  it 
has  had  time  to  diffuse,  whilst  the  slow  titration  gives  a  low  result, 
because  a  large  amount  of  oxygen  has  already  diffused  from  the 
liquid  before  the  titration  is  completed.  No  greater  proof  of  the 


Fig.  62. 

rapidity  with  which  the  water  under  examination  lost  oxygen  by 
the  old  process  need  be  given  than  the  fact  that  Schiitzenberger's 
results  show  that  half  the  oxygen  had  left  the  liquid  by  diffusion 
before  the  determination  could  be  completed. 

(3)     The  return  of  the  blue  colour  is  due  to  the  re-absorption  of 
the  diffused  oxygen  by  the  sensitive  yellow  liquid,  oxidation  by 


294  DISSOLVED    OXYGEN. 

gaseous  oxygen  producing  the  blue  colour,  which  is  thus  not  due 
to  a  reaction  within  the  liquid. 

(4)  The  whole  of  the  oxygen  is  obtained  when  a  large  amount 
of  indigo  is  used,  because  when  reduced  it  is  capable  of  at  once 
fixing  the  whole  of  the  dissolved  oxygen  and  thus  prevents  diffusion. 
The  use  of  so  large  a  quantity  of  indigo,  necessary  to  effect  this 
result,  however,  so  disturbs  the  end-reaction  that  "it  is  difficult 
to  fix  the  point  at  which  the  last  trace  of  blue  has  been  discharged 
with  any  degree  of  accuracy  "  (Dupre  loc  cit.).  Hence  a  new 
method  must  be  resorted  to  in  which  diffusion  is  eliminated,  and 
K/oscoe  and  Lunt  have  devised  the  following  method  to  satisfy 
the  conditions  of  the  case.  The  apparatus  employed  by  them  is 
shown  in  fig.  52. 

It  consists  essentially  of  (1)  an  apparatus  for  the  continuous 
generation  and  purification  of  hydrogen,  by  the  action  of  dilute 
sulphuric  acid  on  zinc  ;  (2)  a  200  c.c.  wide-mouthed  bottle,  fitted 
with  three  burettes  with  glass  taps,  inlet  and  outlet  tubes  for 
a  current  of  hydrogen,  and  an  outlet  tube  for  the  titrated  liquid  ; 
(3)  Winchester  stock  bottles  of  hyposulphite,  indigo  (not  shown), 
and  water  (sample),  communicating  with  their  respective  burettes 
by  glass*  siphons.  The  hydrogen  generated  in  A  passes  through 
two  wash-bottles  containing  caustic  potash,  thence  through  two 
Emmerling's  tubes  filled  with  glass  beads,  moistened  with  an 
alkaline  solution  of  potassium  pyrogallate,  an  arrangement  being 
made  whereby  the  beads  may  be  re-moistened  with  fresh  pyrogallate 
from  the  bottles  beneath,  the  liquid  being  forced  up  by  hydrogen 
pressure.  Pure  hydrogen  is  supplied  continuously  (1)  to  the  stock 
bottle  of  hyposulphite,  (2)  to  the  hyposulphite  burette,  and  (3)  to 
the  titration  bottle. 

Preparation  of  the  Reagents. — The  reagents  required  are- 
Hyposulphite  solution. 
Indigo  solution. 
Standard  aerated  distilled  water. 

The  Hyposulphite  solution  is  prepared  by  dissolving  125  gm.  of 
sodium  bisulphite  in  250  c.c.  of  water,  and  passing  a  current  of 
SO2  through  the  solution  until  saturation  is  effected.  The  solution 
is  poured  into  a  stoppered  bottle  of  about  500  c.c.  capacity,  con- 
taining 50  gm.  of  zinc  dust,  the  bottle  is  almost  filled  up  with  water, 
and  the  mixture  well  shaken  for  five  minutes,  after  which  the  bottle 
is  placed  beneath  a  running  tap  to  cool.  The  mixture  is  again 
agitated  after  a  quarter  of  an  hour  and  left  to  deposit  the  excess  of 
zinc.  The  clear  liquid  is  poured  off  from  the  sediment  into 
a  Winchester  quart  bottle  half  full  of  water.  Milk  of  lime  is  added 
in  excess,  and  the  solution  made  up  to  fill  the  bottle  almost 

*  India-rubber  tiibing  must  not  be  used  for  the  conveyance  of  the  hyposulphite 
solution  (or  the  water  under  examination),  as  atmospheric  oxygen  rapidly  diffuses 
through  the  india-rubber  and  affects  the  strength  of  the  solution. 


DISSOLVED    OXYGEN.  295 

completely.     The  mixture  is  now  thoroughly  shaken  and  allowed 
to  stand  (best  overnight)  until  clear. 

The  solution  thus  obtained  is  much  too  strong  for  use.  200  c.c. 
of  this  may  be  poured  into  a  Winchester  quart  bottle  of  water 
(never  into  a  bottle  filled  with  air)  and  well  shaken  with  as  little 
air  as  possible.  The  approximate  strength  of  this  dilute  solution 
must  now  be  found  by  titrating  good  tap  water  in  the  apparatus 
already  described.  The  strength  should  be  such  that  100  c.c.  of 
water  requires  about  5  c.c.  of  hyposulphite,  and  the  solution  should 
be  made  up  approximately  to  this  value.  It  slowly  loses  strength 
on  keeping,  even  in  hydrogen,  and  its  value  should  be  determined 
daily  as  required  to  be  used. 

The  Indigo-carmine  solution  is  really  sodium  or  potassium 
sulphindigotate,  and  is  prepared  by  shaking  up  200  gm.  of  this 
substance  in  a  Winchester  quart  bottle  of  water,  and  filtering  the 
blue  solution,  which  must  be  diluted  to  such  a  strength  that  20  c.c. 
require  about  5  c.c.  of  the  above  hyposulphite  solution  for 
decolorization. 

Standard  Aerated  Distilled  Water — Two  Winchester  quart 
bottles  half  filled  with  freshly  distilled  water  are  vigorously 
agitated  for  five  minutes,  and  the  air  renewed  several  times  by  filling 
up  one  bottle  with  the  contents  of  the  other,  and  again  dividing 
into  two  portions,  which  are  repeatedly  shaken  with  fresh  air. 
Finally,  one  bottle  being  filled,  the  temperature  of  the  water  is 
taken,  and  also  the  barometric  pressure,  after  which  the  bottle  is 
allowed  to  stand  stoppered  for  half  an  hour,  to  get  rid  of  minute 
air-bubbles.  Table  No.  8,  due  to  Roscoe  and  Lunt,  gives 
the  volume  of  oxygen  contained  in  this  standard  aerated  water, 
and  the  results  show  that  Buns  en's  co-efficients,  previously  used, 
are  inaccurate. 

THE  DETERMINATION  :  The  burette  having  been  filled,  and  a  preliminary  trial 
made — 

(1)  20  c.c.  of  the  water  are  introduced  into  the  small  bottle  and  about  3  c.c.  of 
indigo  solution  added. 

(2)  A  moderate  current  of  hydrogen  is  passed  through  the  blue  liquid  by  a 
very  fine  jet  for  three  minutes  to  free  both  water  and  supernatant  gas  from  free 
oxygen. 

(3)  Hyposulphite  is  now  carefully  added,  during  the  flow  of  hydrogen,  until 
the  change  from  blue  to  yellow  occurs,  taking  care  not  to  overstep  this  point. 

(4)  A  further  measured  quantity  of  hyposulphite  is  now  added  (say  10  c.c.) 
sufficient  to  combine  with  all  the  dissolved  oxygen  in  the  volume  of  water 
(50-100  c.c.)  proposed  to  be  used  in  the  determination. 

(5)  The  important  point  is  that  the  water  be  now  quickly  run  in  from  a  burette 
by  a  capillary  tube  passing  beneath  the  surface  of  the  liquid  to  the  bottom  of  the 
vessel.     The  water  is  thus  introduced  into  a  liquid  which  will  at  once  fix  the  free 
oxygen  and  thus  prevent  its  diffusion  on  coming  in  contact  with  the  hydrogen, 
the  reduced  indigo  acting  as  an  indicator  for  the  complete  oxidation  of  the  hypo- 
sulphite.    The  liquid  is  kept  in  constant  motion  during  the  addition  of  the  water, 
which  is  shut  off  the  moment  a  permanent  blue  colour  appears. 

(6)  The  blue  is  decolorized  by  a  further  slight  addition  of  hyposulphite.     The 


296  DISSOLVED    OXYGEN. 

volume  of  water  used  and  the  total  hyposulphite,  minus  the  first  addition,  are 
noted  and  the  determination  repeated  for  confirmation. 

When  the  water  contains  very  little  oxygen  the  second  addition 
of  hyposulphite  may  be  omitted,  the  reduced  indigo  being  sufficient 
to  take  up  all  the  dissolved  oxygen.  In  this  case,  care  must  be 
taken  that  the  oxygen  added  should  require  not  more  than  half 
the  hyposulphite  first  added  to  decolorize  the  indigo. 

Standardizing  the  Hyposulphite. — In  order  to  complete  the 
determination  it  is  necessary  to  know  the  strength  of  the  hypo- 
sulphite solution  employed,  and  for  this  purpose  the  bottle  of 
standard  aerated  distilled  water  is  titrated.  This  method  has  the 
great  advantage  that  it  is  a  titration  carried  out  under  almost  the 
same  conditions  as  the  examination  of  the  sample.  The  result  of 
a  determination  is  easily  obtained  by  the  following  formula  : — 

d  x  hs  x  Od 

=x  c.c.  dissolved  Oxygen  per  litre  or  water 

s  xhd 

where  d  and  s  =  the  volumes  of  distilled  water  and  sample 
respectively  used,  hd  and  As  =  the  hyposulphite  required  for  the 
distilled  water  and  sample  respectively,  and  Od  the  volume  of 
dissolved  oxygen  contained  in  one  litre  of  the  standard  water. 

Standardizing  the  Indigo. — When  once  the  hyposulphite  has 
been  carefully  standardized  by  distilled  water,  the  rather  trouble- 
some aeration  may  be  avoided  by  finding  the  oxygen  value  of  the 
indigo  solution.  This  solution  remaining  constant  may  be  used 
for  the  subsequent  standardizing  of  the  hyposulphite. 

It  is  only  necessary  to  take  a  suitable  quantity  of  indigo  solution, 
diluted  with  water  if  necessary,  free  it  from  all  dissolved  oxygen 
by  a  current  of  pure  hydrogen  continued  for  five  minutes,  then 
carefully  decolorize  with  hyposulphite,  the  value  of  which  has  been 
found  by  using  aerated  distilled  water. 

The  authors  show  that  Schiitzenberger's  method  of  standard- 
ization, depending  on  the  decolorization  of  ammoniacal  copper 
sulphate,  gives  inaccurate  results. 

Free  acids  or  alkalies  greatly  disturb  the  process.  Bicarbonates 
have  no  effect.  Of  course  when  substances  other  than  oxygen, 
which  decompose  hyposulphite,  are  present,  the  accuracy  of  the 
method  is  proportionately  disturbed.  The  authors  have  applied 
the  process  to  waters  of  very  varied  character,  and  containing 
widely  different  amounts  of  oxygen,  and  show  that  the  method  is 
capable  of  giving  good  results,  compared  with  the  actual  volume 
of  oxygen  found  by  extracting  the  gases  by  boiling  in  vacuo. 

The  delicacy  of  the  reaction  is  such  that  one  part  of  oxygen  in 
two  million  parts  of  water  is  easily  detected. 

The  following  numbers  were  obtained  from  five  different  samples 
of  London  tap  water  collected  on  five  different  days. 


DISSOLVED    OXYGEN. 


297 


(1) 

(2) 

(3) 

(4) 

(5) 

Nitrogen    

c.c. 
13-22 

c.c. 
13-95 

c.c. 
13-36 

c.c. 
13-43 

c.c. 
13-49 

Oxygen      

5-15 

5-91 

5-38 

6-31 

5-80 

Carbon  dioxide     

7-98 

9-29 

6-70 

7-35 

8-11 

Total  Gas      

26-35 

29-15 

25-44 

27-09 

27-40 

Oxygen     by     the     new 
volumetric  method 
Gas  obtained        

5-52 
5-15 

6-13 
5-91 

5-64 
5-38 

6-41 
6-31 

6-24 
5-80 

Difference      

0-37 

0-22 

0-26 

0-10 

0-44 

Mean  difference 

0-28  c.c. 

oxygen  p 

er  litre  of 

water. 

The  oxygen  values  obtained  by  the  two  methods  show  close 
agreement,  considering  the  possible  experimental  error  in  so 
complex  a  comparison. 

M.  A.  Adams*  describes  and  figures  a  very  convenient  arrange- 
ment for  carrying  out  this  process,  which  is  well  adapted  for 
technical  work,  and  less  cumbrous  than  the  apparatus  here 
described. 


lodimetric  Method. 

A  simpler  method  than  the  foregoing  has  been  proposed  by 
Thresh,  t  which  by  comparison  with  Roscoe  and  Lunt's 
method  appears  to  give  satisfactory  results  when  aerated  distilled 
water  was  under  titration,  the  differences  occurring  only  in  the 
second  decimal  place.  The  author  was  led  to  investigate  the  method 
by  observing  the  larger  amount  of  iodine  which  a  very  minute 
quantity  of  a  nitrite  caused  to  be  liberated  when  potassium  iodide 
and  dilute  sulphuric  acid  were  added  to  water  containing  it.  The 
amount  of  iodine  liberated  varies  with  the  length  of  exposure  to 
air.  If  air  is  excluded  no  increase  of  free  iodine  occurs  after  the 
first  few  minutes,  and  if  the  water  is  previously  boiled  and  cooled 
in  an  air-free  space  still  less  iodine  is  liberated.  In  this  latter 
case  the  action  is  represented  by  the  equation  — 

2HI  +  2HNO2  =  I2  +  2H20  +  2NO. 

When  oxygen  has  access  to  the  solution,  the  nitric  oxide  acts  as 
a  carrier,  and  more  hydrogen  iodide  is  decomposed,  the  nitric 
oxide  apparently  remaining  unaffected,  and  capable  of  causing 
the  decomposition  of  an  unlimited  quantity  of  the  iodide. 

This  reaction  is  the  one  utilized  in  the  process  devised  by  Thresh 

«  J.  C.  S.  61,  310.  t  J>  C.  S.  57,  185. 


298  DISSOLVED    OXYGEN. 

for  determining  the  oxygen  dissolved  in  water.     As  16  parts  by 
weight  of  oxygen  will  liberate  253-84  parts  of  iodine,  thus — 


and  as  the  latter  element  admits  of  being  accurately  determined, 
theoretically  the  oxygen  should  be  capable  of  very  precise  determi- 
nation. Practically  such  is  the  case  ;  the  oxygen  dissolved  in 
drinking  waters  admits  of  being  determined  both  rapidly  and  with 
precision.  It  is  only  necessary  to  add  to  a  known  volume  of  the 
water  a  known  quantity  of  sodium  nitrite,  together  with  excess  of 
potassium  iodide  and  acid,  avoiding  access  of  air,  and  then  to 
determine  volumetricaUy  the  amount  of  iodine  liberated.  After 
deducting  the  proportion  due  to  the  nitrite  used,  the  remainder 
represents  the  oxygen  which  was  dissolved  in  the  water  and  in 
the  volumetric  solution  used. 

The  following  are  the  reagents  required  :— 

(1)  Solution  of  sodium  nitrite  and  potassium  iodide  : — 

Sodium  nitrite 0-5  gm. 

Potassium  iodide     20-0  gm. 

Distilled  water     100  c.c. 

(2)  Dilute  sulphuric  acid  : — 

Pure  sulphuric  acid     1  volume 

Distilled  water  3  volumes 

(3)  A  clear,  freshly  made  solution  of  starch. 

(4)  A  volumetric  solution  of  sodium  thiosulphate  : — 

Pure  crystals  of  thiosulphate,  7 '75  gm. 

Distilled  water  to  1  litre. 

1  c.c.  corresponds  to  0'25  milligram  of  oxygen. 

The  apparatus  required  is  very  simple,  and  can  readily  be  fitted 
up.  It  consists  of  a  wide-mouthed  white  glass  bottle  (A,  fig.  53) 
of  about  500  c.c.  capacity,  closed  with  a  caoutchouc  stopper  having 
four  perforations.  Through  one  passes  the  tube  B,  drawn  out  at 
its  lower  extremity  to  a  rather  fine  point,  and  connected  at  the 
upper  end,  by  means  of  a  few  inches  of  rubber  tubing,  with  the 
burette  C,  containing  the  thiosulphate.  Through  another  opening 
passes  the  nozzle  of  a  separatory  tube  D,  having  a  stopper  and 
stopcock.  The  capacity  of  this  tube  when  full  to  the  stopper 
must  be  accurately  determined.  Through  the  third  opening 
passes  a  tube  E,  which  can  be  attached  to  an  ordinary  gas  supply. 
Through  the  last  aperature  is  passed  another  tube,  for  the  gas 
exit,  and  to  this  is  attached  a  sufficient  length  of  rubber  tubing 
to  enable  the  cork  G  at  its  end  to  be  placed  in  the  neck  of  the  tube 
D  when  the  stopper  is  removed.  A  small  piece  of  glass  tube 
projects  through  the  cork,  to  allow  of  the  escaping  gas  being 
ignited. 

The  apparatus  is  used  in  the  following  manner  : — The  bottle  A 


DISSOLVED    OXYGEN. 


299 


being  clean  and  dry,  the  perforated  bung  is  inserted,  the  burette 
charged,  and  the  tube  B  fixed  in  its  place.  E  is  connected  with  the 
gas  supply.  The  tube  D  is  filled  to  the  level  of  the  stopper  with 
the  water  to  be  examined,  1  c.c.  of  the  solution  of  sodium  nitrite 
and  potassium  iodide  added  from  a  1  c.c.  pipette,  then  1  c.c.  of  the 
dilute  acid,  and  the  stopper  instantly  fixed  in  its  place,  displacing 
a  little  of  the  water,  and  including  no  air.  If  the  pipette  be  held 
in  a  vertical  position  with  its  tip  just  under  the  surface  of  the 
water,  both  the  saline  solution  and  the  acid,  being  much  denser 
than  the  water,  flow  in  a  sharply  defined  column  to  the  lower  part 
of  the  tube,  so  that  an  infinitesimally  small  "quantity  (if  any)  is 
lost  in  the  water  which  overflows  when  the  stopper  is  inserted. 
The  tube  is  next  turned  upside  down  for  a  few  seconds  for  uniform 
admixture  to  take  place,  and  then  the  nozzle  is  pushed  through 
the  bung  of  the  bottle,  and  the  whole  allowed  to  remain  at  rest 
for  15  minutes,  to  enable  the  reaction  to  become  complete.  A 


Fig.  53. 

rapid  current  of  coal  gas  is  now  passed  through  the  bottle  A,  until 
all  the  air  is  displaced  and  the  gas  burns  at  G  with  a  full  luminous 
flame  ;  the  flame  is  now  extinguished,  the  stopper  of  D  removed, 
and  the  cork  G  rapidly  inserted.  On  turning  the  stopcock,  the 
water  flows  into  the  bottle  A.  The  stopcock  is  turned  off,  the  cork 
G  removed,  and  the  supply  of  gas  so  regulated  that  a  small  flame 
only  is  produced  when  this  gas  is  ignited  at  G.  Thiosulphate  is 


300  DISSOLVED    OXYGEN. 

now  run  in  until  the  colour  of  the  iodine  is  nearly  discharged. 
A  little  starch  solution  is  then  poured  into  D,  and  about  1  c.c. 
allowed  to  flow  into  the  bottle  by  turning  the  stopcock.  The 
titration  with  thiosulphate  is  then  completed.  After  the  dis- 
charge of  the  blue  colour,  the  latter  returns  faintly  in  the  course 
of  a  few  seconds,  due  to  the  oxygen  dissolved  in  the  volumetric 
solution  ;  after  standing  about  two  minutes,  from  0'05  to  0*1  c.c. 
of  thiosulphate  must  be  added  to  effect  the  final  discharge.  The 
amount  of  volumetric  solution  used  must  now  be  noted.  This 
will  represent  a,  the  oxygen  dissolved  in  the  water  examined, +  6, 
the  nitrite  in  the  1  c.c.  of  solution  used,  and  the  oxygen  in  the  acid 
and  starch  solution  -f-c,  a  portion  of  the  dissolved  oxygen  in  the 
volumetric  solution.  To  find  the  value  of  a,  it  is  obvious  that  b 
and  c  must  be  ascertained.  This  can  be  effected  in  many  ways, 
and  once  known  does  not  require  re-determination  unless  the 
conditions  are  changed. 

To  find  the  value  of  b. — Probably  the  best  plan  is  to  complete 
a  determination  as  above  described,  and  then,  by  means  of  the 
stoppered  tube,  introduce  into  the  bottle  in  succession  5  c.c.  of 
nitrite  solution,  dilute  acid,  and  starch  solution.  After  standing 
a  few  minutes,  titrate.  One-fifth  of  the  thiosulphate  used  will  be 
the  value  required. 

To  Find  the  Value  of  c. — This  correction  is  a  comparatively  small 
one,  and  admits  of  determination  with  sufficient  accuracy  if  it  is 
assumed  that  the  thiosulphate  solution  normally  contains  as  much 
dissolved  oxygen  as  distilled  water  saturated  at  the  same  tempera- 
ture. Complete  a  determination  as  above  described,  then  remove 
the  stoppered  tube,  and  insert  a  tube  similar  to  that  attached  to 
the  burette,  and  drop  in  from  it  10  or  20  c.c.  of  saturated  distilled 
water  exactly  as  the  thiosulphate  is  dropped  in.  Allow  to  stand 
a  few  minutes  and  titrate.  One-tenth  or  one-twentieth  of  the 
volumetric  solution  used,  according  to  the  number  of  c.c.  of  water 
added,  will  represent  the  correction  for  each  c.c.  of  volumetric 
solution  used.  Call  this  value  d. 

Let  e  be  the  number  of  c.c.  of  thiosulphate  used  in  an  actual 
determination  of  the  amount  of  oxygen  in  a  sample  of  water ; 

/=the  capacity  in  c.c.  of  the  tube  employed  — 2  c.c.,  the  volume 

of  reagents  added  : 
0=the  amount  of  oxygen  in  milligrams  dissolved  in  1  litre  of 

the  water  ; 

then  g  =  l®*>(e--1,-ed). 

With  a  tube  made  to  hold  exactly  250  c.c.,  the  most  convenient 

1000 
quantity  to  use,  -— -  becomes  unity,  and 

g=e—b—ed. 

In  the  author's  experiments  two  nitrite  solutions  were  used ;  in  the 
first  6  =  2-1  c.c.,  in  the  second  3-1  c.c.  A  number  of  determinations 


DISSOLVED    OXYGEN.  301 

of  d  were  made,  at  temperatures  varying  from  40°  to  60°  F.  The 
value  of  d  was  found  to  vary  between  O03  and  0-0315.  In  all  the 
author's  recent  experiments  d  was  taken  as  0-031. 

When  e  —  3  c.c.  the  reaction  seems  to  be  complete  in  five  minutes, 
but,  to  be  on  the  safe  side,  it  is  better  to  fix  the  minimum  at  fifteen 
minutes. 

The  use  of  coal-gas  is  recommended  by  the  author  without 
passing  it  over  alkaline  pyrogallol  or  otherwise  treating  it  before 
allowing  it  to  pass  through  the  apparatus. 

The  results  obtained,  however,  can  be  made  to  vary,  the  extreme 
limit  being  less  than  0-5  milligram  of  oxygen  per  litre  of  water, 
using  250  c.c.  for  the  determination.  To  quote  an  extreme  case. 
In  one  experiment  (1),  after  the  air  has  been  wholly  expelled  from 
the  bottle  A,  no  more  gas  was  passed  through,  and  the  titration 
was  effected  in  the  closed  apparatus,  the  volumetric  solution  being 
run  in  as  rapidly  as  possible.  The  end-reaction  was  not  well 
defined.  In  the  second  experiment  (2),  the  volumetric  solution 
was  run  in  very  slowly  drop  by  drop,  and  a  brisk  current  of  gas 
was  kept  passing  through  the  apparatus.  End-reaction  well 
defined. 

Volume  of  water.        Thiosulphate.  Oxygen  per  litre. 

(1)   322  c.c.  15-35  c.c.  9'14  milligrams. 

(2)  ....     322    „  14-9    „  8-80 

The  difference  is  probably  due  to  nearly  all  the  oxygen  dissolved 
in  thiosulphate  being  used  up  in  the  first  case,  and  being  lost  by 
diffusion  in  the  second. 

In  the  examination  of  waters  from  various  sources,  and  making 
the  experiments  in  pairs,  using  tubes  of  different  sizes,  the  author 
found  that  exceedingly  concordant  results  could  easily  be  obtained. 

In  determining  the  oxygen  in  distilled  water  saturated  with  air, 
the  author  found  that  the  results  at  25°  and  30°  C.  were  higher 
than  those  obtained  by  Roscoe  and  Lunt,  whilst  at  the  lower 
temperatures  they  were  almost  identical,  and  it  occurred  to  him 
that  the  difference  was  probably  due  to  the  mode  of  saturation. 
The  agitation  in  a  couple  of  Winchesters  was  done  as  directed  by 
them,  but  the  water  used  had  been  previously  saturated  at  the 
lower  temperatures,  and  probably  were  slightly  super-saturated. 
A  further  series  of  experiments  were  then  made  with  freshly- 
distilled  water,  which  was  not  agitated  with  air  until  it  had  attained 
the  desired  temperature.  The  results  proved  that  this  surmise 
was  correct.  Probably  some  such  explanation  accounts  for  the 
uniformly  higher  results  obtained  by  Dittinar. 

No  doubt  there  will  be  exceptional  cases  in  which  the  process 
cannot  be  used,  and  others  in  which  some  modification  may  be 
required.  A  water  containing  nitrites  will  require  the  amount  of 
the  nitrous  acid  to  be  determined  if  the  utmost  accuracy  is  required. 
(A  water  containing  one  part  of  HNOa  in  1,000,000,  will  affect  the 
results +0' 17  milligram  of  oxygen  per  litre,  94  parts  of  the  acid 


302 


DISSOLVED    OXYGEN. 


corresponding  to  16  of  oxygen.)  Where  nitrites  are  present  in 
sufficient  quantity  to  interfere,  the  amount  may  be  determined  by 
any  of  the  ordinary  processes,  but  the  author  prefers  the  following 
method  : — 

To  250  c.c.  of  the  water  to  be  examined,  rendered  faintly  alkaline 
if  not  already  so,  add  a  few  drops  of  strong  solution  of  potassium 
iodide,  and  boil  vigorously  for  a  few  minutes.  Then  transfer  to 
the  bottle  A  used  in  the  oxygen  determination,  and  allow  to  get 
quite  cold  in  a  slow  current  of  coal  gas.  Then  add  a  few  drops 
of  dilute  sulphuric  acid  and  solution  of  starch,  and  titrate  with  the 
thiosulphate.  The  correction  to  be  made  in  the  oxygen  determi- 
nation is  thus  ascertained.  One  or  two  experimental  results  may 
be  quoted. 


Quantity 
of  water 

Thiosulphate 
used. 

Corrected. 

Milligrams 
of  oxygen 
per  litre. 

1 

Tap  water     .           .  . 

232-5 

13-2 

9-7 

10-43 

Tap     water  +  5     milli- 

2] 

grams      commercial 

(  232-5 

15-95 

9-55 

10-27 

(^ 

sodium  nitrite    —  . 

j 

•1 

Tap    water  +10    milli- 
grams sodium  nitrite 

j  232-5 

18-6 

9-48 

10-19 

In  number  2,  the  thiosulphate  used  by  250  c.c.  of  the  boiled  water 

was  2*8  c.c. 
In  number  3,  the  thiosulphate  used  by  250  c.c.  of  the  boiled  water 

was  5*45  c.c. 

The  results  are  fairly  satisfactory,  even  with  such  large  proportions 
of  nitrite,  proportions  far  larger  than  are  likely  to  be  met  with  in 
practice. 

Nitrates  do  not  interfere,  even  when  present  in  large  quantities  ; 
but  fresh  urine,  when  present  to  the  extent  of  1  per  cent.,  has 
a  small  but  very  appreciable  effect. 

The  following  is  an  example  of  the  method  at  ordinary 
temperature  : — 

Temperature  15°  C. 


Quantity 
of  water 
taken. 

Thiosulphate 
used. 

c—b—ed. 

Milligrams 
of  Oxygen 
per  litre. 

Difference 
from  mean. 

1.. 

2.. 
3.. 
4.. 

322-0 
322-0 
232-5 
232-5 

15-45 
15-55 
11-90 
11-70 

12-87 
12-97 
9-43 
9-23 

Mean 

9-99 
10-07 
10-14 
9-92 

-0-04 
+  0-04 
+  0-11 
-0-11 

10-03 

Barometer  reading  760  mm. 
10-03  milligrams  =7 -02  c.c.  at  N.P.T. 
R  o  s  c  o  e  and  L  u  n  t  found  6-96  Difference  +  OO6. 


DISSOLVED   OXYGEN.  303 

Determination  of  the  dissolved  Oxygen  in  Waters  and  Effluents 
by  Winkler's  Method  as  modified  by  Rideal  and  Stewart.* 

The  principle  on  which  this  method  depends  is  the  oxidation  in 
an  alkaline  liquid  of  manganous  oxide  to  a  higher  oxide  of  manganese, 
the  subsequent  liberation  of  iodine  from  potassium  iodide  by  this 
in  the  acidified  solution,  and  the  titration  of  the  liberated  iodine 
by  thiosulphate. 

The  following  solutions  are  required  : — 

1.  Decinormal  permanganate  of  potash. 

2.  A  2  %  solution  of  potassium  oxalate. 

3.  A  33  %  solution  of  manganous  chloride. 

4.  A  mixed  solution  containing  50  %  caustic  soda  and  10  % 
potassium  iodide,  f 

5.  N/20  sodium  thiosulphate    (1    c.c.  =0*006346    gm.  iodine 

=0'0004  gm.  oxygen). 

METHOD  OF  PROCEDURE  :  The  temperature  having  been  noted,  a  stoppered 
bottle  of  known  capacity  (300-350  c.c.  when  full)  is  completely  filled  with  the 
sample  of  water  or  effluent  (filtered  or  unfiltered,  as  may  have  been  decided), 
avoiding  any  appreciable  aeration  in  doing  this,  and  1  c.c.  sulphuric  acid  is  added, 
together  with  enough  of  the  permanganate  solution  to  leave  a  slight  pink  colour 
after  the  whole  has  been  mixed  and  has  stood  for  10  minutes — the  object  being 
to  oxidize  any  nitrite  present  to  nitrate.  If  more  than  10  c.c.  of  decinormal 
permanganate  are  required  for  this  purpose,  2  c.c.  of  acid  must  be  added  instead 
of  1  c.c.  The  proper  amount  of  permanganate  to  be  added  is  best  determined 
by  a  preliminary  trial  on  50  c.c.  of  the  original  liquid  ;  this  having  been  done,  the 
amount  required  for  the  bottle-full  is  calculated  and  about  O'l  c.c.  in  excess  of  the 
calculated  amount  is  added.  The  contents  of  the  bottle  are  mixed  by  rotation 
and  allowed  to  stand  10  minutes,  after  which  any  excess  of  permanganate  is 
destroyed  by  the  addition  of  0-5  to  1*0  c.c.  of  the  oxalate  solution  by  means  of 
a  pipette,  the  neck  filled  up  with  the  sample  under  examination,  the  stopper 
inserted,  and  the  bottle  rotated  as  before.  The  colour  quickly  disappears,  and 
when  decolorized  1  c.c.  of  the  manganous  chloride  is  passed  to  the  bottom  of  the 
liquid  from  a  long  pipette,  and  immediately  afterwards  3'0  c.c.  of  the  mixed 
soda  and  iodide  solution  in  the  same  manner.  The  stopper  is  inserted  without 
air  bubbles  and  the  contents  mixed  by  inversion  and  rotation.  The  liberated 
manganous  hydroxide  absorbs  the  free  oxygen.  On  standing  a  few  minutes  the 
precipitated  oxides  of  manganese  settle  ;  the  stopper  is  then  removed  for  a  second 
and  3  c.c.  of  pure  concentrated  HC1  (free  from  chlorine)  are  passed  to  the  bottom 
by  a  pipette.  The  bottle  is  then  closed  and  again  rotated,  and  kept  in  the  dark 
for  5  to  10  minutes,  with  frequent  shaking,  till  clear.  The  liquid  is  then  trans- 
ferred to  a  porcelain  dish  and  the  iodine  determined  by  thiosulphate  and  starch. 
The  oxygen-equivalent  of  the  latter  being  known,  the  amount  of  dissolved  oxygen 
present  in  the  sample  is  readily  calculated.  The  reagents  being  used  in  the  con- 
centrated form  with  a  comparatively  large  volume  of  water  the  correction  for  the 
additions  is  small  and  can  usually  be  neglected.  When,  however,  the  oxygen  is 
low,  the  reagents  being  presumably  saturated  with  oxygen  under  atmospheric 
conditions,  will  make  the  result  too  high.  The  correction  then  to  be  applied  is : 

1000  «-Rn 
V-. 

*  The  Analyst,  28,  1901,  141. 

t  D  r.  Mo .  G  o  w  an  has  for  a  long  time  past  (1910)  used  a  solution  of  700  gm.  KOH 
and  100  gm.  KI  made  up  to  1  litre,  about  4  c.c.  being  used  in  each  determination. 


304  DISSOLVED    OXYGEN. 

Where  x  =no.  of  c.c.  of  oxygen  at  N.  T.  P.  per  litre  of  the  liquid. 
a  =the  amount  of  oxygen  in  c.c.  found  by  titration. 
V  =the  volume  of  the  bottle. 
n  =the  volume  of  the  reagents. 
R  =no.  of  c.c.  of  oxygen  contained  in  a  litre  of  saturated  water  at  the 

temperature  of  the  experiment. 

(R  can  be  obtained  from  Rose oe  and  Lunt's  table,  or,  preferably,  is  actually 
determined.) 

Results  have  usually  been  given  in  c.c.  of  oxygen  at  N.T.P. 
per  litre,  with  the  temperature  of  the  water  ;  but  in  the  Fifth 
Report  of  the  Royal  Commission  on  Sewage  Disposal  (see  Water 
Section)  the  absorption  of  dissolved  or  atmospheric  oxygen  by  an 
effluent  is  expressed  in  parts  by  weight  per  100,000. 

NOTE. — 1  gram  of  oxygen  occupies  699'79  c.c.  at  N.T.P. 

Rideal  and  Burgess*  have  recently  modified  Winkler's 
process  into  a  colorimetric  method,  which  will  give  fair  results 
even  in  the  presence  of  nitrites,  and  is  applicable  to  final  sewage 
effluents  in  most  cases. 

METHOD  OF  PROCEDURE  :  A  series  of  colour  standards  are  first  constructed 
corresponding  to  various  proportions  of  oxygen  from  0  to  1*5  parts  per  100,000, 
proceeding  by  increments  of  O'l  part  as  follows.  Square-shouldered  stoppered 
bottles  of  nearly  colourless  glass  are  carefully  selected  of  a  capacity  of  about  130 
c.c.  apiece  when  completely  full — those  actually  selected  not  varying  inter  se  by 
more  than  about  1*5  c.c.  Into  15  of  these  bottles  are  run  90  c.c.  of  distilled  water 
(free  from  organic  matter),  1*5  c.c.  of  10  %  KI  and  0*15  c.c.  cone.  HC1.  The 
iodine  tints  can  very  easily  be  obtained  by  running  into  the  series  of  bottles  the 
calculated  quantities  of  a  standard  solution  of  K2Mn208  (0'395  gram  per  litre, 
same  as  used  in  water  analysis).  For  example,  a  bottle  holding  131*5  c.c.  requires 
13*15  c.c.  of  the  solution  to  produce  an  iodine  tint  corresponding  to  1  part  of 
dissolved  oxygen  per  100,000.  The  bottles  are  then  filled  up  with  distilled  water 
except  for  a  minute  bubble  of  air,  the  stoppers  tightly  inserted,  and  the  contents 
mixed  by  agitation.  The  iodine  standards  should  be  kept  in  a  closed  box  and 
comparisons  should  only  be  made  in  diffused  light,  the  standards  being  put  away 
immediately  after  use  ;  in  this  way  they  may  be  relied  on  for  over  a  month. 

The  water  to  be  examined  is  siphoned  with  care  into  one  of  the  test-bottles,  0*5 
c.c.  of  nearly  saturated  MnCl2  solution  added,  and  then  1\5  c.c.  of  a  solution  con 
taining  30  %  NaOH  and  10  %  KI.  The  stopper  is  inserted,  and  the  contents 
well  mixed.  After  the  oxidation  of  the  manganous  hydrate  has  taken  place, 
and  the  precipitate  has  settled  down  somewhat,  the  stopper  is  withdrawn, 
and  I'Sc.c.  cone.  HC1  added.  The  stopper  is  then  reinserted,  and  the  contents 
of  the  bottle  well  mixed.  As  the  reaction  with  the  acid  causes  a  slight 
elevation  of  temperature  and  consequent  expansion  of  the  liquid,  the  stopper 
has  a  tendency  to  get  loose,  and  it  is  convenient  to  plunge  the  test-bottle  into 
a  large  basin  of  cold  water  and  to  turn  it  about  until  the  stopper  is  tight  again. 
After  wiping  the  bottle,  the  tint  of  the  liberated  iodine  is  compared  with  the  stand- 
ards, and  it  is  best  to  do  this  by  holding  the  bottles  in  an  inclined  position  a  few 
inches  above  a  white  card  and  looking  down  through  their  shoulders.  After 
a  little  practice  there  is  no  difficulty  in  estimating  the  fractions  between  the 
standards  to  within  0*03  part  of  oxygen.  The  results  obtained  compare  well 
with  the  titrations  by  Winkler's  method. 

In  this  method,  when  a  water  contains  nitrite,  there  is  no  trouble  through 
NO  acting  as  an  oxygen-carrier,  since  the  reaction  of  the  nitrite,  iodide,  and  acid 
takes  place  in  a  solution  from  which  the  free  oxygen  has  already  been  removed 

*  Analyst.  1909.34,193. 


HYDEOGEN   PEROXIDE.  305 

by  the  manganese  hydrate.     The  iodine  liberated  on  account  of  nitrite  is  coin,  ^ 
sequently  that  of  the  ordinary  equation — 

2NaN02  +  2KI  +4HC1  =2NaCl  +2KC1  +2NO  +I2  +2H20. 

Hence,  for  two  atoms  of  nitrous  nitrogen  we  have  to  allow  for  one  atom  of 
oxygen ;  so  that  if  a  water  contained  1  '4  parts  of  nitrous  nitrogen,  the  iodine  liberated 
would  be  equivalent  to  0'8  part  of  oxygen.  Experiments  have  proved  that  when 
nitrites  are  determined  and  thus  allowed  for  the  colorimetric  process  still  gives 
good  results. 

Effluents  that  are  very  turbid  or  have  much  dark-coloured  suspended  matter 
are  best  titrated. 

The  rate  of  absorption  of  dissolved  oxygen  by  a  sample  of  sewage 
or  effluent  affords  a  measure  of  the  readiness  with  which  it  will 
deprive  a  stream  of  its  dissolved  oxygen  when  the  two  are  mixed. 
Scudder*  makes  this  determination  as  follows: — 

100  c.c.  of  the  sewage  effluent  are  added  to  900  c.c.  of  tap  water  in  a  half 
Winchester  quart  bottle,  mixed  by  shaking,  and  allowed  to  stand  for  at  least  half 
an  hour  to  allow  small  bubbles  to  rise  to  the  surface.  The  dissolved  oxygen  in 
the  diluted  effluent  is  then  determined.  Two  10-oz.  stoppered  bottles  are  com- 
pletely filled  with  the  diluted  effluent  and  placed  in  an  incubator  at  a  temperature 
of  75°  Fah.f  The  dissolved  oxygen  is  determined  in  one  of  these  after  the 
expiration  of  24  hours,  and  in  the  other  after  48  hours. 

HYDROGEN  PEROXIDE,  HYDROGEN  DIOXIDE,  OR  HYDROXYL. 

H2O2= 34-02. 

This  substance  is  largely  used  in  commerce  and  is  commonly 
sold  as  of  "  10  volume  "  and  "  20  volume  "  strength,  meaning 
thereby  that  the  solution,  when  fully  decomposed,  yields  ten 
times  and  twenty  times  its  own  volume  respectively  of  oxygen. 
A  still  stronger  preparation  ("  Perhydrol ")  can  now  be  obtained 
which  yields  100  times  its  own  volume  of  oxygen  :  it  contains 
30  per  cent,  of  H202. 

Kingzettf  has  shown  that  the  best  method  of  determining 
hydrogen  peroxide  is  to  add  to  its  solution,  strongly  acidified  with 
sulphuric  acid,  some  potassium  iodide,  and  to  titrate  the  iodine  set 
free  with  sodium  thiosulphate  and  starch  solution.  The  reaction 
is  as  follows  :— 

2HI  +  H2O2=2H2O  +  I2. 

The  function  performed  by  the  sulphuric  acid  is  difficult  of 
explanation,  but  the  want  of  uniformity  in  the  reaction  experienced 
by  many  operators  no  doubt  has  arisen  from  the  use  of  insufficient 
acid. 

METHOD  OF  PROCEDURE  :  Mix  10  c.c.  of  the  peroxide  solution  to  be  examined 
with  about  30  c.c.  of  dilute  sulphuric  acid  (1  :  2)  in  a  beaker,  add  crystals  of 
potassium  iodide  in  sufficient  quantity,  and  after  standing  five  minutes  titrate  the 
liberated  iodine  with  N/io  thiosulphate  and  starch.  The  peroxide  solution  should 
not  exceed  the  strength  of  2  volumes;  if  stronger,  it  must  be  diluted  proportion- 
ately before  the  titration. 

*  Fowler  "Sewage  Works  Analyses,"  p.  83. 
f  Many  chemists  incubate  at  80°  F.  %  J.  C.  S.  1880,  792. 

X 


306  HYDROGEN   PEROXIDE. 

In  the  case  of  a  very  weak  solution  it  will  be  advisable  to  titrate  with  N/ioo 
thiosulphate. 

1  c.c.  N/i0  thiosulphate  =0-001 7  gin.  H202  or  O'OOOS  gm.  O. 

Carpenter  and  Nicholson*  report  a  series  of  experiments 
on  the  analysis  of  hydrogen  peroxide,  both  by  the  iodine  and 
permanganate  methods. 

The  conclusion  they  arrive  at  is  that  the  process  ofKingzettis 
accurate,  but  in  their  hands  somewhat  tedious,  owing  to  slow 
decomposition  towards  the  end.  Kingzett  however  states  that 
if  a  volume  of  strong  sulphuric  acid  equal  to  the  peroxide  taken  be 
used,  and  especially  if  the  dilute  solution  be  slightly  warmed,  the 
reaction  is  complete  in  a  few  minutes,  and  this  is  my  own 
experience. 

A  number  of  experiments  have  been  made  by  C.  Smitht  as 
to  the  value  of  titrimetric  and  gasometric  methods  of  ascertaining 
the  amount  of  oxygen  in  H202,  if  it  contains  any  preservative 
such  as  glycerin,  boric  acid,  boroglycerin,  salicylic  acid,  etc.  The 
result  was  to  show  that  the  iodine  and  thiosulphate  method  gives 
accurate  results  with  any  of  the  preservatives  tried,  and  in  the 
presence  of  large  proportions  of  glycerin,  whereas  the  permanganate 
methods  both  titrimetric  and  gasometric  were  valueless. 

The  free  acid  in  hydrogen  peroxide  solutions  can  be  determined 
with  sufficient  accuracy  for  all  practical  purposes  by  direct  titration 
in  the  cold  with  standard  caustic  alkali,  using  phenolphthalein 
as  indicator.^ 


Sodium  Peroxide. 
Na2O2=78. 

L.  Archbuttj|  gives  the  results  of  some  experiments  on  the 
determination  of  the  available  oxygen  in  this  substance,  and 
found  that  a  near  approximation  to  the  truth  could  be  obtained 
by  simple  titration  with  permanganate,  the  peroxide  (one  or  two 
decigrams)  being  added  to  cold  water  acidified  with  H2S04  contained 
in  a  white  dish,  and  N/10  permanganate  dropped  in  with  stirring 
until  the  colour  became  permanent ;  but  a  more  exact  method 
would  be  to  add  a  known  weight  of  the  peroxide  to  an  excess  of 
N/io  permanganate,  previously  mixed  with  dilute  H2SO4,  and 
titrate  for  the  excess  of  permanganate  with  N/10  oxalic  acid. 
Archbutt,  however,  prefers  to  use  the  nitrometer,  and  recommends 
the  following  procedure  :  about  0*25  gm.  of  the  substance  is  placed 
in  the  dry  tube  of  the  nitrometer  flask,  and  in  the  flask  itself  about 
5  c.c.  of  pure  water,  containing  in  suspension  a  few  milligrams  of 
precipitated  cobalt  sesqui-oxide ;  this  latter  reagent  brings  about 

«  Analyst,  9,  36.    :*   *  f  C.  N.  80,1194.  I  J  Brown,  The.Analyst,  35,  1910,  497. 

||  Analyst,  20,  5. 


PHOSPHATES.  307 

a  rapid  and  complete  decomposition  of  the  peroxide,  the  volume 
of  oxygen  evolved  being  the  available  oxygen  in  the  sample. 

PHOSPHORIC    ACID    AND    PHOSPHATES. 
P205= 142-08. 

THE  determination  of  phosphoric  acid  volumetrically  may  be 
done  with  more  or  less  accuracy  by  a  variety  of  processes,  among 
which  may  be  mentioned  that  of  Mohr  as  lead  phosphate ;  the 
indirect  method  as  silver  phosphate  (the  excess  of  silver  being 
found  by  thiocyanate)  ;  by  standard  uranium  nitrate  or  acetate  ; 
byPemberton's  method  as  phospho-molybdate ;  or,  when  existing 
only  as  monocalcic  phosphate,  by  standard  alkali,  as  recommended 
byMollendaorEmmerling.  These  processes  are  mainly  useful 
in  the  case  of  fertilizers,  or  the  raw  phosphates  from  which  manures 
are  manufactured,  and  for  P205  in  urine,  etc.  For  the  purpose 
mentioned,  that  is  to  say,  when  in  combination  with  alkali  or* 
alkaline  earthy  bases  and  moderate  quantities  of  iron  or  alumina, 
phosphoric  acid  may  be  determined  volumetrically  with  very  fair 
accuracy,  and  with  much  greater  rapidity  than  by  gravimetric 
means  as  usually  carried  out. 

1.    Precipitation  as  Uranium  Phosphate  in  Acetic  Acid 
Solution. 

This  method  is  based  on  the  fact  that  when  uranium  acetate  or 
nitrate  is  added  to  a  neutral  solution  of  tribasic  phosphoric  acid, 
such,  for  instance,  as  sodium  orthophosphate,  the  whole  of  the 
phosphoric  acid  is  thrown  down  as  yellow  uranium  phosphate 
Ur2O3,  P2O5+Aq.  Should  the  solution,  however,  contain  free 
mineral  acid,  it  must  be  neutralized  with  an  alkali,  and  an  alkaline 
acetate  added,  together  with  excess  of  free  acetic  acid.  In  case  of 
using  ammonia  and  ammonium  acetate,  the  whole  of  the  phosphoric 
acid  is  thrown  down  as  double  phosphate  of  uranium  and  ammonia, 
having  a  light  lemon  colour,  and  the  composition  Ur203  2(NH40), 
P205  +  Aq.  When  this  precipitate  is  washed  with  hot  water, 
dried  and  ignited,  the  ammonia  is  entirely  dissipated  leaving 
uranium  phosphate,  which  possesses  the  formula  Ur2O3,  P205,  and 
contains  in  100  parts  78-71  of  uranium  oxide  and  21-29  of  phosphoric 
acid.  In  the  presence  of  fixed  alkalies,  instead  of  ammonia,  the 
precipitate  consists  simply  of  uranium  phosphate.  By  this  method 
phosphoric  acid  may  be  completely  removed  from  all  the  alkalies 
and  alkaline  earths  ;  also,  with  a  slight  modification,  from  iron ; 
not,  however,  satisfactorily  from  alumina  when  present  in  any 
quantity. 

The  details  of  the  gravimetric  process  were  fully  described  by 
me,*  and  immediately  after  the  publication  of  that  article,  while 
employed  in  further  investigation  of  the  subject,  I  devised  the 

*  c.  N.  i,  97, 122. 

x  2 


308  PHOSPHATES. 

volumetric  method  now  to  be  described.  Since  that  time  it  has 
come  to  my  knowledge  that  Neubauer*  and  Pincusf  had 
independently  of.  each  other  and  of  myself  arrived  at  the  same 
process.  This  is  not  to  be  wondered  at,  if  it  be  considered  how 
easy  the  step  is  from  the  ordinary  determination  by  weight  to 
that  by  measure,  when  the  delicate  reaction  between  uranium 
and  potassium  ferrocyanide  is  known.  Moreover,  the  great  want 
of  a  really  good  volumetric  process  for  phosphoric  acid  determination 
in  place  of  those  hitherto  used  has  been  felt  by  all  who  have  had 
anything  to  do  with  the  subject,  and  consequently  the  most  would 
be  made  of  any  new  method  possessing  so  great  a  claim  to  accuracy 
as  the  gravimetric  determination  of  phosphoric  acid  by  uranium 
undoubtedly  does. 

Conditions  under  which  approximate  accuracy  may  be  ensured. — 

Objections  have  been  urged,  not  without  reason,  that  this  process 
is  inaccurate,  because  varying  amounts  of  saline  substances  have 
'an  influence  upon  the  production  of  colour  with  the  indicator. 
Again,  that  very  different  shades  of  colour  occur  with  lapse  of 
time.  This  is  all  true,  and  the  method  is  unfortunately  one  of 
that  class  which  requires  uniform  conditions  :  but  when  the  source 
of  irregularities  is  known,  it  is  not  difficult  to  obviate  them. 
Therefore,  it  is  absolutely  essential  that  the  standardizing  of  the 
uranium  solution  should  be  done  under  the  same  conditions  as 
the  analysis.  For  instance,  a  different  volume  of  uranium  solution 
will  be  required  to  give  the  colour  in  the  presence  of  salts  of  ammonia 
from  that  which  would  be  necessary  with  the  salts  of  the  fixed 
alkalies  or  alkaline  earths.  But  if  the  standard  solution  is  purposely 
adjusted  with  ammonia  salts  present  in  about  the  same  proportion, 
the  difficulties  are  less.  Fortunately  this  can  easily  be  done,  and 
as  the  chief  substances  requiring  analysis  are  more  or  less  ammoniacal 
in  their  composition,  such  as  urine,  manures,  etc.,  no  practical 
difficulty  need  occur. 

Excessive  quantities  of  alkaline  or  earthy  salts  modify  the  colour, 
but  especially  is  it  so  with  acetate  or  citrate  of  ammonia.  For 
this  reason  it  is  necessary  to  ensure  the  complete  washing  of  the 
citromagnesium  precipitate,  where  that  method  of  separating  P2O5 
is  adopted  previous  to  titration.  With  all  my  experience  of  this 
method  I  cannot  contend  that  it  is  an  absolutely  accurate  one,  but 
it  is  nevertheless  a  very  rapid  and  convenient  one  for  manure 
manufacturers  in  testing  superphosphates  and  other  phosphatic 
fertilizers. 

2.     Determination     of  'Phosphoric     Acid    in     combination     with 
Alkali    Bases,    or    in    presence    of    small    quantities    of 
Alkaline  Earths. 
The  necessary  materials  are — 
(a)     A  standard  solution  of  uranium  1  c.c.=  0-005  gm.  P2O5. 

*Archiv.  fur  wissenscM/iliehe  Heilkunde.,  4,  228.        f  Journal  fur  Prakl.  Chem.  76,  104. 


PHOSPHATES.  309 

(b)  A  standard  solution  of  tribasic  phosphoric  acid. 

(c)  A  solution  of  sodium  acetate  in  dilute  acetic  acid,  made  by 
dissolving  100  gm.  of  sodium  acetate  in  water,  adding  50  c.c.  of 
glacial  acetic  acid,  and  diluting  to  1  litre.     Exact  quantities  are 
not  necessary. 

(d)  A  freshly  prepared  solution  of  potassium  ferrocyanide,  or 
some  finely  powdered  pure  crystals  of  the  same  salt. 

Standard  Solution  of  Uranium. — This  solution  may  consist  either 
of  uranium  nitrate  or  acetate.*  An  approximate  solution  is 
obtained  by  using  about  35  gm.  of  either  salt  to  the  litre.  In  using 
uranium  nitrate  it  is  imperative  that  the  sodium  acetate  should  be 
added  in  order  to  avoid  the  possible  occurrence  of  free  nitric  acid 
in  the  solution.  With  acetate,  however,  it  may  be  omitted  at  the 
discretion  of  the  operator,  but  it  is  important  that  the  method 
used  in  standardizing  the  uranium  be  invariably  adhered  to  in 
the  actual  analysis.  The  solution  should  be  perfectly  clear  and 
free  from  basic  salt.  Whether  made  from  acetate  or  nitrate,  it  is 
advisable  to  include  about  50  c.c.  of  pure  glacial  acetic,  or 
a  corresponding  quantity  of  weaker  acid,  to  each  litre  of  solution ; 
exposure  to  light  has  then  less  reducing  action. 

My  own  practice  is  to  use  in  all  cases  acetate  solution,  and  dispense 
entirely  with  the  addition  of  sodium  acetate. 

3.     Titration  of  the  Uranium  Solution. 

Standard  Phosphoric  Acid. — WTien  the  uranium  solution  is  not 
required  for  phosphate  of  lime,  it  may  be  titrated  upon  ammonio- 
sodium  phosphate  (microcosmic  salt)  as  follows  : — 5*88  gm.  of 
the  crystallized,  non-effloresced  salt  (previously  powdered  and 
pressed  between  bibulous  paper  to  remove  any  adhering  moisture) 
are  weighed,  dissolved  in  water,  and  diluted  to  1  litre.  50  c.c.  of 
this  solution  will  represent  0*1  gm.  of  P205.t 

METHOD  OF  PROCEDURE  :  50  c.c.  of  this  solution  are  measured  into  a  small 
beaker  5  c.c.  sodium  acetate  solution  added  if  uranium  nitrate  is  to  be  used,  and 
the  mixture  heated  to  90°  or  100°  C.  The  uranium  solution  is  then  delivered  in 
from  a  burette,  divided  into  TV  c.c.,  until  a  test  taken  shall  show  the  slight  pre- 
dominance of  uranium.  This  is  done  by  spreading  a  drop  or  two  of  the  hot 
mixture  upon  a  clean  white  level  plate,  and  bringing  in  contact  with  the  middle 

*  Some  operators  object  to  the  use  of  acetate,  the  reason  for  which  I  cannot 
understand.  It  stands  to  reason  that,  as  with  the  use  of  nitrate  there  has  to  be 
a  considerable  quantity  of  sodium  acetate  used  to  prevent  the  formation  of  free 
HNOa,  the  same  conditions  practically  occur  as  if  uranium  acetate  was  used.  The 
real  reason,  I  believe  is,  that  it  is  rather  difficult  to  procure  pure  acetate.  In  the 
course  of  some  thousands  of  titrations,  1  have  found  no  advantage  in  using  nitrate, 
and  acetate  needs  no  corrector  to  complicate  the  process  as  is  the  case  with  nitrate. 

t  W.  B.  Giles,  who  has  had  great  experience  in  the  determination  of  phosphoric 
acid  in  various  forms,  has  called  my  attention  to  dihydric  potassium  phosphate, 
KH2PO4,  as  an  excellent  form  of  salt  for  a  standard  solution,  The  sample  sent  to  me 
was  in  beautifully  formed  crystals  which  do  not  alter  on  exposure  to  the  air,  and 
makes  a  solution  which  keeps  clear.  Kvery  one  knows  how  unsatisfactory  sodium 
phosphate  is,  both  as  to  its  state  of  hydration  and  its  keeping  qualities  in  solution  ; 
the  microcosmic  salt  is  better,  but  is  open  to  objection  on  the  score  of  indefinite 
hydration.  ]f  the  potassium  salt  is  used,  a  standard  solution  of  the  proper  strength 
is  made  by  dissolving  3'83G  gm.  in  a  litre. 


310  PHOSPHATES. 

of  the  drop  a  thin  glass  rod  moistened  with  the  freshly  made  solution  of  ferro 
cyanide,  or  a  dust  of  the  powdered  salt.  The  production  of  a  faint  brown  tinge 
shows  an  excess  of  uranium,  the  slightest  amount  of  which  produces  a  brown 
precipitate  of  uranium  ferrocyanide. 

A  second  or  third  titration  is  then  made  in  the  same  way,  so  as 
to  arrive  at  the  exact  strength  of  the  uranium  solution,  which  is 
then  diluted  and  re-titrated,  until  exactly  20  c.c.  are  required  to 
produce  the  necessary  reaction  with  50  c.c.  of  phosphate. 

Suppose  18'7  c.c.  of  the  uranium  solution  have  been  required  to 
produce  the  colour  with  50  c.c.  of  phosphate  solution,  then  every 
18'7  c.c.  will  have  to  be  diluted  to  20  c.c.  in  order  to  be  of  the 
proper  strength,  or  935  to  1000.  After  dilution,  two  or  three  fresh 
trials  must  be  made  to  ensure  accuracy. 

It  is  of  considerable  importance  that  the  actual  experiment  for 
determining  phosphoric  acid  by  means  of  the  uranium  solution 
should  be  made  with  about  the  same  bulk  of  fluid  that  has  been 
used  in  standardizing  the  solution,  and  the  same  depth  of  colour 
aimed  at  in  each  case.  Hence  the  proportions  here  recommended 
have  been  chosen,  so  that  50  c.c.  of  liquid  shall  contain  0*1  gm. 
of  P206. 

Standard  Phosphoric  Acid  corresponding  volume  for  volume  with 
Standard  Uranium. — This  solution  is  obtained  by  dissolving 
14*720  gm.  of  microcosmic  salt  in  a  litre,  and  is  two  and  a  half 
times  the  strength  of  the  solution  before  described  ;  it  is  used  for 
residual  titration  in  case  the  required  volume  of  uranium  is  over- 
stepped in  any  given  analysis. 

A  little  practice  enables  the  operator  to  tell  very  quickly  the 
precise  point ;  but  it  must  be  remembered  that  when  the  two  drops 
are  brought  together  for  the  production  of  the  chocolate  colour, 
however  faint  it  seems  at  first,  if  left  for  some  little  time  the  colour 
increases  considerably ;  but  this  has  no  effect  upon  the  accuracy  of 
the  process,  since  the  original  standard  of  the  solution  has  been 
based  on  an  experiment  conducted  in  precisely  the  same  way. 

METHOD  OF  PROCEDURE  :  In  determining  unknown  quantities  of  P206  it  is 
necessary  to  have  an  approximate  knowledge  of  the  amount  in  any  given  material, 
so  as  to  fulfil  as  nearly  as  possible  the  conditions  laid  down  above ;  that  is  to 
say,  50  c.c.  of  solution  shall  contain  about  O'l  gm.  P205,  or  whatever  other  propor- 
tion may  have  been  used  in  standardizing  the  uranium. 

The  compound  containing  the  P205  to  be  determined  is  dissolved  in  water ;  if 
no  ammonia  is  present,  1  c.c.  of  10  per  cent,  solution  is  dropped  in  and  neutralized 
with  the  least  possible  quantity  of  acetic  acid  (also  5  c.c.  of  sodium  acetate  if 
uranium  nitrate  has  to  be  used),  and  the  volume  made  up  to  about  50  c.c.,  then 
heated  to  about  90°  C.  on  the  water  bath,  and  the  uranium  solution  delivered  in 
cautiously,  with  frequent  testing  as  above  described,  until  the  faint  brown  tinge 
appears. 

The  first  trial  will  give  roughly  the  amount  of  solution  required  and  taking 
that  as  a  guide,  the  operator  can  vary  the  amount  of  liquid  for  the  final  titration, 
should  the  proportions  be  found  widely  differing  from  those  under  which  the 
strength  of  the  uranium  was  originally  fixed. 

Each  c.c.  of  uranium  solution  =0'005  gm.  P206. 


PHOSPHATES.  311 

4.  Determination  of  Phosphoric  Acid  in  combination  with  Lime 
and  Magnesia  (Bones,  Bone  Ash,  Soluble  Phosphates,  and 
other  Phosphatic  Materials  free  from  Iron  and  Alumina). 

The  procedure  in  these  cases  differs  from  the  foregoing  in  two 
respects  only  ;  that  is  to  say,  the  uranium  solution  is  preferably 
standardized  by  tribasic  calcium  phosphate  ;  and  in  the  process  of 
titration  it  is  necessary  to  add  nearly  the  full  amount  of  uranium 
required  before  heating  the  mixture,  so  as  to  prevent  the  precipita- 
tion of  calcium  phosphate,  which  is  apt  to  occur  in  acetic  acid 
solution  when  heated;  or  the  modification  adopted  byFresenius 
Neubauer,  and  Luck,  may  be  used,  which  consists  in  reversing 
the  process  by  taking  a  measured  volume  of  uranium,  and  delivering 
into  it  the  solution  of  phosphate  until  a  drop  of  the  mixture  ceases 
to  give  a  brown  colour  with  ferrocyanide.  This  plan  gives,  how- 
ever, much  more  trouble,  and  possesses  no  advantage  on  the  score 
of  accuracy,  because  in  any  case  at  least  two  titrations  must  be 
performed,  and  the  first  being  made  somewhat  roughly,  in  the 
ordinary  way,  shows  within  1  or  2  c.c.  the  volume  of  standard 
uranium  required ;  and  in  the  final  trial  it  is  only  necessary  to  add 
at  once  nearly  the  full  quantity,  then  heat  the  mixture,  and  finish 
the  titration  by  adding  a  drop  or  two  of  uranium  at  a  time  until 
the  required  colour  is  obtained. 

This  reversed  process  is  strongly  advocated  by  many  operators, 
but  except  in  rare  instances  I  fail  to  see  its  superiority  to  the  direct 
method  for  general  use.  The  best  modification  to  adopt  in  the 
reverse  process  is  to  use  invariably  an  excess  of  uranium,  and  to 
titrate  back  with  standard  phosphate  solution  till  the  colour 
disappears ;  this  avoids  all  the  trouble  of  preparing  and  cleaning 
a  burette  for  the  solution  to  be  analysed,  and  if  a  standard  phosphate 
is  made  to  correspond  volume  for  volume  with  the  uranium,  an 
analysis  may  always  be  brought  into  order  at  any  stage. 

Standard  Calcium  Phosphate. — It  is  not  safe  to  depend  upon 
the  usual  preparations  of  tricalcium  phosphate  by  weighing  any 
given  quantity  direct,  owing  to  uncertainty  as  to  the  state  in  which 
the  phosphoric  acid  may  exist ;  therefore,  in  order  to  titrate  the 
uranium  solution  with  calcium  phosphate,  it  is  only  necessary  to 
take  rather  more  than  5  gm.  of  precipitated  pure  tricalcium 
phosphate  such  as  is  obtained  in  commerce,  dissolve  it  in  a  slight 
excess  of  dilute  hydrochloric  acid,  precipitate  again  with  a  slight 
excess  of  ammonia,  re-dissolve  in  a  moderate  excess  of  acetic  acid, 
then  dilute  to  a  litre  ;  by  this  means  is  obtained  a  solution  of  acid 
monocalcium  phosphate,  existing  under  the  same  conditions  as 
in  the  actual  analysis.  In  order  to  ascertain  the  exact  amount  of 
tribasic  phosphoric  acid  present  in  a  given  measure  of  this  solution, 
two  portions  of  50  c.c.  each  are  measured  into  two  beakers,  each 
holding  about  half  a  litre.  A  slight  excess  of  solution  of  uranium 
acetate  or  nitrate  is  then  added  to  each,  together  with  about  10  c.c. 


312  "PHOSPHATES. 

of  the  acetic  solution  of  sodium  acetate  ;  they  are  then  heated  to 
actual  boiling  on  a  hot-plate  or  sand-bath,  the  beakers .  filled  up 
with  boiling  distilled  water,  and  then  set  aside  to  settie,  which 
occurs  very  speedily.  The  supernatant  fluid  should  be  faintly 
yellow  from  excess  of  uranium.  When  perfectly  settled,  the  clear 
liquid  is  poured  off  as  closely  as  possible  without  disturbing  the 
precipitate,  and  the  beakers  again  filled  up  with  boiling  water. 
The  same  should  be  done  a  third  time,  when  the  precipitates  may 
be  brought  on  two  niters,  and  need  very  little  further  washing. 

When  the  filtration  is  complete,  the  filters  are  dried  and  ignited 
apart  from  the  precipitate,  taking  care  to  burn  off  all  carbon. 
Before  being  weighed,  however,  the  uranium-phosphate  must  be 
moistened  with  strong  nitric  acid,  dried  perfectly  in  the  water-bath 
or  oven,  and  again  ignited  ;  at  first  very  gently,  then  strongly, 
so  as  to  leave  a  residue  of  a  pure  light  lemon  colour  when  cold. 
This  is  uranium  phosphate  Ur203,  P2O5,  the  percentage  composition 
of  which  is  78*71  of  uranium  oxide,  and  21-29  of  phosphoric  acid. 

The  two  precipitates  are  accurately  weighed,  and  should  agree 
to  within  a  trifle.  If  they  differ,  the  mean  is  taken  to  represent  the 
amount  of  P205  in  the  given  quantity  of  tricalcium  phosphate,  from 
which  may  be  calculated  the  strength  of  the  solution  to  be  used  as 
a  standard.  Of  course  any  other  accurate  method  of  determining 
the  P2O5  may  be  used  in  place  of  this. 

The  actual  standard  required  is  5  gm.  of  pure  tricalcium  phosphate 
per  litre  ;  and  it  should  be  adjusted  to  this  strength  by  dilution, 
after  the  actual  strength  has  been  found.  In  this  way  is  obtained 
a  standard  which  agrees  exactly  with  the  analysis  of  a  super- 
phosphate or  other  similar  manure. 

Standard  Uranium  Solution. — This  is  best  adjusted  to  such 
strength  that  25  c.c.  are  required  to  give  the  faint  chocolate  colour 
with  ferrocyanide,  when  50  c.c.  of  the  standard  acetic  solution  of 
calcium  phosphate  are  taken  for  titration.  Working  in  this  manner 
each  c.c.  of  uranium  solution  represents  1  per  cent,  of  soluble 
tricalcium  phosphate,  when  1  gm.  of  a  fertilizer  is  taken  for  analysis, 
because  50  c.c.  of  the  calcium  phosphate  will  contain  monocalcium 
phosphate  equal  to  0'25  gm.  of  Ca3P208  and  will  require  25  c.c.  of 
uranium  solution  to  balance  it. 

These  standards  are  given  as  convenient  for  fertilizers,  but  they 
may  be  modified  to  suit  any  particular  purpose. 

THE  METHOD  WITH  SUPERPHOSPHATES  FREE  FROM  FE  AND  AL,  EXCEPT  IN  MERE 

TRACES,  is  as  follows : — 10  gm.  of  the  substance  are  weighed,  placed  in  a  small 
glass  mortar  and  gently  broken  down  by  the  pestle,  cold  water  being  used  to 
bring  it  to  a  smooth  cream.  The  material  should  not  be  ground  or  rubbed  hard, 
which  might  cause  the  solution  of  some  insoluble  phosphate  in  the  concentrated 
mixture.  The  creamy  substance  is  washed  gradually  without  loss  into  a  measuring 
flask  marked  at  503'5  c.c.,  the  3*5  c.c.  being  the  space  occupied  by  the  insoluble 
matters  in  an  ordinary  25  to  30  per  cent,  superphosphate.  The  flask  is  filled 
to  the  mark  with  cold  water,  and  shaken  every  few  minutes  during  about  half  an 
hour.  A  portion  is  then  filtered  through  a  dry  filter  into  a  dry  beaker,  and  50  c.c. 
( =  1  gm.  of  fertilizer)  measured  into  a  beaker  holding  about  100  c.c.  Sufficient 


PHOSPHATES.  313 

10  per  cent,  ammonia  is  then  added  to  precipitate  the  monocalcium  phosphate  in 
the  form  of  Ca3P208  (in  all  ordinary  superphosphates  there  is  enough  Ca  present 
as  sulphate  to  ensure  this,  and  four  or  five  drops  of  ammonia  generally  suffice 
to  effect  the  precipitation).  Acetic  acid  is  then  added  in  just  sufficient  quantity 
to  render  the  liquid  clear.  Should  traces  of  gelatinous  A1P04  or  FeP04  appear 
at  this  stage,  the  liquid  will  be  slightly  opalescent ;  but  this  may  be  disregarded 
if  only  slight,  as  the  subsequent  heating  will  enable  the  uranium  to  decompose  it. 
If  more  than  traces  are  present,  the  method  will  not  be  accurate,  and  recourse 
must  be  had  to  separation  by  the  citro-magnesium  solution. 

While  the  liquid  is  still  cold,  a  measured  volume  of  the  standard  uranium  is 
run  in  with  stirring,  and  drops  are  occasionally  taken  out  with  a  glass  rod,  and 
brought  in  contact  with  some  ferrocyanide  indicator  spotted  on  a  white  plate 
until  a  faint  colour  appears.  The  beaker  is  then  placed  in  the  water-bath  for  a  few 
minutes,  and  again  the  mixture  tested  with  the  indicator ;  after  heating  in  this 
way  the  testing  ought  to  show  no  colour.  More  uranium  is  then  added  with 
stirring,  and  drop  by  drop  till  the  proper  reaction  occurs.  This  titration  is  only 
a  guide  for  a  second,  which  may  be  made  more  accurate  by  running  in  at  once 
very  nearly  the  requisite  volume  of  uranium. 

This  operation  may  be  reversed,  if  so  desired,  by  making  the 
clear  solution  of  phosphate  up  to  a  definite  volume  (say  60  c.c.),  and 
running  it  from  a  burette  into  a  measured  volume  of  uranium  until 

t> 

a  test  taken  shows  no  colour. 


5.     Determination  of  Phosphoric  Acid  in  Minerals  or  other  materials 
containing  Iron,  Alumina,  or  other  interfering  substances. 

In  order  to  make  use  of  any  volumetric  process  for  this  purpose 
the  phosphoric  acid  must  be  separated.  As  has  been  already 
described,  this  may  be  done  either  as  molybdenum  phosphate 
followed  by  solution  in  NH3,  and  again  precipitated  with  ordinary 
magnesia  mixture,  or  direct  separation  by  the  citro-magnesiuni 
mixture  described  below.  In  either  case  the  ammonio- magnesium 
salt  is  dissolved  in  the  least  possible  quantity  of  nitric  or  hydro- 
chloric acid,  neutralized  with  ammonia,  acidified  with  acetic  acid, 
and  the  titration  with  uranium  carried  out  as  before  described. 


6.     Pemberton's    Molybdic    Method. 

This  method*  is  one  which  requires  great  delicacy  of  manipulation, 
but  gives  excellent  results  with  all  the  alkali  or  earthy  phosphates. 

The  latest  form  of  the  method,  "Pemberton's  New  Molybdic 
Method  Modified,"  f  is  as  follows  :— 

The  solutions  required  are  : 

Molybdate  Solution.  Dissolve  100  gm.  of  molybdic  acid  in  144  c.c.  of  ammonia, 
sp.  gr.  0'90,  and  271  c.c.  of  water  ;  slowly,  and  with  constant  stirring,  pour  the 
solution  thus  obtained  into  489  c.c.  of  nitric  acid  (sp.  gr.  1  '42),  and  1 148  c.c.  of  water. 
Keep  the  mixture  in  a  warm  place  for  several  days,  or  until  a  portion  heated  to 
40°  C.  deposits  no  yellow  precipitate  of  ammonium  phosphomolybdate.  Decant 
the  solution  from  any  sediment  and  preserve  in  glass -stoppered  vessels. 

For  use  add  to  100  c.c.  of  this  solution  5  c.c.  of  nitric  acid,  sp.  gr.  1*42.  Filter 
each  time  before  using. 

*  J.  Am.  C.  S.  1894,  278.  t  Bulletin  No.  107,  U.  S.  Dept.  Agric. 


314  PHOSPHATES. 

Standard  Potassium  Hydroxide  Solution.  This  solution  contains  18-17106  gm. 
of  potassium  hydroxide  to  the  litre.  It  is  prepared  by  diluting  323 '81  c.c.  of 
normal  potash  (which  has  be,en  freed  from  carbonates  by  barium  hydroxide)  to 
one  litre.  100  c.c.  of  the  solution  should  neutralize  32'38  c.c.  of  normal  acid. 
One  c.c.  of  this  is  equal  to  O'OOl  gm.  of  P20S,  or  1  per  cent,  if  O'l  gm.  of  the  sub- 
stance is  taken  for  analysis.  Normal  soda  may  be  used  instead  of  potash. 

Standard  Nitric  Acid  Solution.  This  solution  should  correspond  in  strength  to 
the  standard  alkali  solution,  or  may  be  one  half  that  strength.  It  is  standardized 
by  titrating  against  that  solution,  using  phenolphthalein  as  indicator.  Any 
mineral  acid  may  be  used. 

If  a  soluble  phosphate  is  to  be  analysed,  dissolve  1  gm.  in  water  to^250  c.c. 
25  c.c.  of  this  solution,  representing  O'l  gm.  of  the  substance,  is  taken  for  analysis. 
If  the  phosphate  is  in  an  insoluble  compound  or  organic  substance,  2  gm.  are  treated 
by  one  of  the  methods  given  below.  After  solution,  cool,  dilute  to  200  or  250  c.c., 
mix  and  pour  on  a  dry  filter. 

Total  Phosphoric  Acid,  (a)  Dissolve  in  30  c.c.  of  concentrated  nitric  acid  and 
a  small  quantity  of  hydrochloric  acid  and  boil  until  organic  matter  is  destroyed. 

(6)  Evaporate  with  5  c.c.  of  magnesium  nitrate,  ignite,  and  dissolve  in  hydro- 
chloric acid. 

(c)  Add  30  c.c.  of  concentrated  hydrochloric  acid,  heat,  and  add  cautiously, 
in  small  quantities  at  a  time,  about  0*5  gm.  of  finely  pulverized  potassium  chlorate 
to  destroy  organic  matter. 

(d)  Dissolve  in  from  15  to  30  c.c.  of  strong  hydrochloric  acid  and  from  3  to 
10  c.c.  of  nitric  acid.     This  method  is  recommended  for  fertilizers  containing  much 
iron  or  aluminium  phosphate. 

DETERMINATION. — (1)  For  percentages  of  5  or  below  use  an  aliquot  volume 
corresponding  to  0'4  gm.  of  substance  ;  for  percentages  between  5  and  20  use 
a  volume  corresponding  to  0*2  gm.  of  substance,  and  for  percentages  above  20 
use  a  volume  corresponding  to  O'l  gm.  of  substance.  Add  from  5  to  10  c.c.  of 
nitric  acid,  depending  on  the  method  of  solution  (or  the  equivalent  in  ammonium 
nitrate),  nearly  neutralize  with  ammonia,  dilute  to  from  75  to  100  c.c.,  heat  in 
water-bath  to  from  60°  to  65°  C.,  and  for  percentages  below  5  add  from  20  to  25 
c.c.  of  freshly  filtered  molybdate  solution.  For  percentages  between  5  and  20 
add  from  30  to  35  c.c.  of  molybdate  solution  ;  stir,  let  stand  about  fifteen  minutes, 
filter  at  once  wash  once  or  twice  with  water  by  decantation,  using  from  25  to 
30  c.c.  each  time,  agitating  the  precipitate  thoroughly  and  allowing  to  settle  ; 
transfer  to  filter  and  wash  with  cold  water  until  two  fillings  of  the  filter  do  not 
greatly  diminish  the  colour  produced  with  phenolphthalein  by  one  drop  of  the 
standard  alkali.  Transfer  precipitate  and  filter  to  beaker  or  precipitating  vessel, 
dissolve  in  small  excess  of  standard  alkali,  add  a  few  drops  of  phenolphthalein 
solution,  and  titrate  with  standard  acid. 

(2)  Proceed  as  directed  in  (1),  with  this  exception:   Heat  in  a  water-bath 
at_45°  to  50*  C.,  add  the  molybdate  solution,  and  allow  to  remain  in  the  bath  with 
occasional  stirring  for  thirty  minutes. 

(3)  Proceed  as  in  (1)  to  the  point  where  the  solution  is  ready  to  place  in  the 
water-bath.     Then   cool  solution  to  room  temperature,  add  molybdate  solution 
at  the  rate  of  75  c.c.  for  each  O'l  gm.  of  phosphoric  acid  present,  place  the 
stoppered  flask  containing  the  solution  in  a  shaking  apparatus  and  shake  for 
thirty  minutes  at  room  temperature,  filter  at  once,  wash,  and  titrate  as  in  preceding 
method. 

Water-soluble  Phosphoric  Acid.  Place  2  gm.  of  the  sample  on  a  9-cm.  filter, 
wash  with  successive  small  portions  of  water,  allowing  each  portion  to  pass  through 
before  adding  more,  until  the  filtrate  measures  about  250  c.c.  If  the  filtrate  be 
turbid,  add  a  little  nitric  acid.  Make  up  to  any  convenient  definite  volume,  mix 
well,  use  an  aliquot  portion  of  the  solution  corresponding  to  0*2  or  0'4  gm.,  add 
10  c.c.  of  concentrated  nitric  acid  and  ammonia  until  a  slight  permanent  precipitate 
is  formed,  dilute  to  60  c.c.,  and  proceed  as  under  the  preceding  method  (1). 

Citrate-insoluble    Phosphoric    Acid.      Make    the    solution    according    to    the 


PHOSPHATES.  315 

directions  given  before  and  determine  the  phosphoric  acid  in  an  aliquot  volume 
corresponding  to  0'4  gm.,  as  directed  for  total  phosphates. 

Determination  in  Acidulated  Samples.  Heat  100  c.c.  of  strictly  neutral 
ammonium  citrate  solution  of  1'09  sp.  gr.  to  65°  C.  in  a  flask  placed  in  a  warm 
water-bath,  keeping  the  flask  loosely  stoppered  to  prevent  evaporation.  When 
the  citrate  solution  in  the  flask  has  reached  65°  C.  drop  into  it  the  filter  containing 
the  washed  residue  from  the  water-soluble  phosphate  determination,  close  tightly 
with  a  smooth  rubber  stopper,  and  shake  violently  until  the  filter  paper  is  reduced 
to  a  pulp.  Place  the  flask  in  the  bath  and  maintain  it  at  such  a  temperature 
that  the  contents  of  the  flask  will  stand  at  exactly  65°  C.  Shake  the  flask  every 
five  minutes. 

At  the  expiration  of  exactly  thirty  minutes  from  the  time  the  filter  and  residue 
are  introduced,  remove  the  flask  from  the  bath  and  immediately  filter  the  contents 
as  rapidly  as  possible ;  wash  thoroughly  with  water  at  65°  C.  Then  proceed 
as  under  (a)  or  (6). 

(a)  Transfer  the  filter  and  its  contents  to  a  crucible,  ignite  until  all  organic 
matter  is  destroyed,  add  from  10  to  15  c.c.  of  strong  hydrochloric  acid,  and  digest 
until  all  phosphate  is  dissolved,  or  (6)  return  the  filter  with  contents  to  digestion 
flask,  add  from  30  to  35  c.c.  of  strong  nitric  acid,  and  from  5  to  10  c.c.  of  strong 
hydrochloric  acid,  and  boil  until  all  phosphate  is  dissolved.  Dilute  to  200  c.c., 
mix  well,  and  pass  through  a  dry  filter.  Take  a  definite  portion  of  the  filtrate 
and  proceed  as  under  total  phosphoric  acid. 

Determination  of  Non-acidulated  Samples.  Treat  2  gm.  of  the  phosphatic 
material  without  previous  washing  with  water,  precisely  in  the  way  above  described, 
except  that  in  case  the  substance  contains  much  animal  nxatter  (bone,  fish,  etc.), 
the  residue,  insoluble  in  ammonium  citrate,  is  to  be  dissolved  by  the  treatment 
described  under  (&),  or  by  digestion  with  concentrated  sulphuric  acid  in  the 
presence  of  a  small  quantity  of  sodium  or  potassium  nitrate. 

Citrate-soluble  Phosphoric  Acid.  The  sum  of  the  water-soluble  and  citrate- 
insoluble  subtracted  from  the  total  phosphoric  acid,  gives  the  citrate-soluble 
phosphoric  acid. 

Richardson*  states  that  when  phosphoric  acid  is  determined  in  acid  phosphate 
by  the  Pemberton  volumetric  method  or  it  usual  modifications,  the  results 
do  not  agree  with  those  obtained  gravimetrically  by  the  A.  0.  A.  C.  method,  and 
the  error  frequently  amounts  to  +  1  per  cent.  P2O5. 

The  disturbing  substance  is  probably  sulphuric  acid,  and  if  this  be  removed 
by  barium  chloride,  the  volumetric  method  may  be  applied  and  accurate  results 
obtained. 

Richardson  recommends  the  following  procedure : 

Weigh  2  gm.  into  a  250  c.c.  flask,  digest  by  boiling  with  30  c.c.  of  concentrated 
nitric  acid  and  5  c.c.  concentrated  hydrochloric  acid,  then  add  10  c.c.  water  and 
boil  for  five  minutes.  Add  25  to  30  c.c.  of  10  per  cent,  barium  chloride,  cool,  and 
make  up  to  volume.  Filter  through  a  dry  filter,  rejecting  the  first  portion  of  the 
filtrate,  and  take  25  c.c.  for  the  determination.  From  this  point  on  follow  the 
Pemberton  Method  as  above. 

7.     Determination  of  Phosphoric  Acid  by  Silver  Nitrate 

(H  o  1 1  e  m  a  n).f 

J.  M.  WilkieJ  has  modified  Holleman's  method,  in  which 
phosphoric  acid  or  an  alkali-metal  phosphate  is  converted  to  the 
di-alkali  metal  salt  by  means  of  caustic  soda,  the  silver  phosphate 
precipitated  in  presence  of  sodium  acetate,  and  the  residual  silver 
determined  in  the  filtrate  by  Volhard's  method.  This  method 
usually  gives  high  results,  but  the  following  mode  of  operating 
gives  accurate  results  : 

*J.  A.  C.S.29,  1314,  and  So  hi  mpf,  "  Volumetric  Analysis,"  p.  320. 

\Z.  a.  Chem.,  83,  185,  and  J.  S.  C.  I.  1894,  763  and  843. 

iJ.S.  C.I.  1910,  794. 


316  PHOSPHATES. 

Phenolphthalein  is  added  to  the  solution  containing  the  phosphate,  then  strong 
caustic  soda  till  the  liquid  is  just  pink,  when  the  colour  is  discharged  by  dilute 
nitric  acid,  added  drop  by  drop.  In  presence  of  calcium  the  precipitated  phosphate 
is  the  best  indicator,  nitric  acid  being  added  until  the  precipitate  is  just  dissolved. 
Excess  of  silver  solution  is  next  added,  followed  by  10  c.c.  of  approximately  N/io 
sodium  acetate  ;  then  approximately  N/io  caustic  soda  is  run  in,  while  shaking, 
till  the  liquid  is  faintly  alkaline  to  phenolphthalein.  Two  c.c.  of  N/io  sulphuric 
acid  are  added  to  this  solution,  which  is  diluted  to  150  c.c.,  mixed,  filtered,  and 
the  silver  in  the  solution  determined  by  Volhard's  method.  The  method  is 
not  available  in  the  presence  of  appreciable  amounts  of  aluminium  or  iron. 
Chlorides  must  be  allowed  for,  but  may  be  got  rid  of  by  adding  excess  of  sulphuric 
acid  to  the  phosphate  solution  and  evaporating  at  100°  C.  The  determination  is 
made  as  above,  after  addition  of  nitric  acid. 

8.     Other  Volumetric  Methods  for  the  determination  of 
Phosphoric  Acid. 

Several  methods  depending  upon  alkalimetry  have  been  sug- 
gested, but  beyond  those  quoted  under  Phosphoric  Acid,  p.  114, 
I  have  found  none  easy  or  reliable  in  practice. 

A.  G  r  e  t  e  *  titrates  phosphoric  acid  in  acid  solution  with  alkaline  molybdate 
solution  in  the  presence  of  glue.  It  is  claimed  that  this  method  has  been  thoroughly 
tested  in  practice  since  1888,  and  that  20  determinations  can  be  made  in  an  hour, 
with  results  equal  hi  accuracy  to  those  obtained  by  the  gravimetric  process. 

Two  iodimetric  methods  recently  described  depend  upon  the  interaction  of 
standard  sodium  hypobromite  solution  in  the  presence  of  potassium  iodide  (1) 
with  ammonium  phospho-molybdate  (P.  Artmann)f  and  (2)  with  ammonium 
magnesium  phosphate  (R.  Brandis).$ 

SILVER. 

Ag= 107-88. 

1  c.c.  (or  1  dm.)  N/10  sodium  chloride =0-010788  gm.  (or  0-10788 
grn.)  Silver  ;  also  0-016989  gm.  (or  0-16989  grn.)  Silver  nitrate. 

1.  Precipitation  with  N/10  Sodium  Chloride  (Gay  L  us  sac). 

THE  determination  of  silver  is  precisely  the  converse  of  the 
operations  described  under  Chlorine  (p.  175),  and  the  process  may 
either  be  concluded  by  adding  the  sodium  chloride  till  no  further 
precipitate  is  produced,  or  potassium  chromate  may  be  used  as  an 
indicator.  In  the  latter  case,  however,  it  is  advisable  to  add  the 
salt  solution  in  excess,  then  a  drop  or  two  of  chromate,  and  titrate 
residually  with  N/10  silver,  till  the  red  colour  produced,  is  for  the 
excess  of  sodium  chloride. 

2.     By  Ammonium  Thiocyanate. 

The  principle  of  this  method  is  fully  described  on  page  145, 
et.  seq.,  and  need  not  further  be  alluded  to  here.  The  author  of 

*  Ber.  1909,  42,  3106,  and  J.  8.  C.  /.,  1909,  1105. 

t  Z.  a.  Chem.  1910,  49, 1,  and  J.  S.  O.  I.  1910,  455. 

%Z.a.  Chem.  1910,  49,  152,  and  J.  S.  C.  I.  1910,  455. 


SILVER.  317 

the  method  (Volhard)  states  that  comparative  tests  made  by  this 
method  and  by  that  ofGayLussac  gave  equally  exact  results, 
both  being  controlled  by  cupellation,  but  claims  for  this  process 
that  the  end  of  the  reaction  is  more  easily  distinguished,  and 
that  there  is  no  labour  of  shaking,  or  danger  of  decomposition  by 
light,  as  in  the  case  of  chloride.  My  own  experience  fully  confirms 
this.  The  method  is  now  adopted  largely  in  place  of  Gay 
Lussac's  for  silver  assays. 

3.    Determination  of  Silver,  in  Ores  and  Alloys,  by  Starch 
Iodide  (Method  of  Pisani  and  F.  Field). 

If  a  solution  of  blue  starch  iodide  be  added  to  a  neutral  solution 
of  silver  nitrate,  while  any  of  the  latter  is  in  excess  the  blue  colour 
disappears,  the  iodine  entering  into  combination  with  the  silver ;  as 
soon  as  all  the  silver  is  thus  saturated,  the  blue  colour  remains 
permanent,  and  marks  the  end  of  the  process.  The  reaction  is  very 
delicate,  and  the  process  is  more  especially  applicable  to  the  analysis 
of  ores  and  alloys  of  silver  containing  lead  and  copper,  but  not 
mercury,  tin,  iron,  manganese,  antimony,  arsenic,  or  gold  in  solution. 

The  solution  of  starch  iodide,  devised  by  Pisani,  is  made  by 
rubbing  together  in  a  mortar  2  gm.  of  iodine  with  15  gm.  of  starch 
and  about  6  or  8  drops  of  water,  putting  the  moist  mixture  into 
a  stoppered  flask,  and  digesting  in  a  water-bath  for  about  an  hour, 
or  until  it  has  assumed  a  dark  bluish-grey  colour ;  water  is  then 
added  till  all  is  dissolved.  The  strength  of  the  solution  is  then 
ascertained  by  titrating  it  with  10  c.c.  of  a  solution  of  silver  con- 
taining 1  gm.  in  the  litre,  to  which  a  portion  of  pure  precipitated 
calcium  carbonate  is  added  ;  the  addition  of  this  latter  removes  all 
excess  of  acid,  and  at  the  same  time  enables  the  operator  to 
distinguish  the  end  of  the  reaction  more  accurately.  The  starch 
iodide  solution  should  be  of  such  a  strength  that  about  50  c.c.  are 
required  for  10  c.c.  of  the  silver  solution  (=0*01  gm.  silver). 

F.  Field*,  who  discovered  the  principle  of  this  method  simul- 
taneously with  Pisani,  used  a  solution  of  iodine  in  potassium  iodide 
with  starch.  Those  who  desire  to  make  use  of  this  plan  can  use 
the  N/10  and  N/i0o  solutions  of  iodine  described  on  p.  129. 

In  the  analysis  of  silver  containing  copper,  the  solution  must  be 
considerably  diluted  in  order  to  weaken  the  colour  of  the  copper  ; 
a  small  measured  portion  is  then  taken,  calcium  carbonate  added, 
and  starch  iodide  till  the  colour  is  permanent.  It  is  best  to  operate 
with  from  60  to  100  c.c.,  containing  not  more  than  0.02  gm.  silver  ; 
when  the  quantity  is  much  greater  than  this,  it  is  preferable  to 
precipitate  the  greater  portion  with  N/10  sodium  chloride,  and  to 
complete  with  starch  iodide  after  filtering  off  the  chloride.  When 
lead  is  present  with  silver  in  the  nitric  acid  solution,  add  sulphuric 
acid,  and  filter  off  the  lead  sulphate,  then  add  calcium  carbonate  to 
neutralize  excess  of  acid,  filter  again  if  necessary,  then  add  fresh 
carbonate  and  titrate  as  described  above. 

»C.  N.  2.  17. 


318  SILVER. 

4.    Assay  of  Commercial  Silver  (Plate,  Bullion,  Coin,  etc.). 
Gay    Lussac's  Method  modified  by  J.  G.  Mulder. 

For  more  than  thirty  years  Gay  Lussac's  method  of  determin- 
ing silver  in  its  alloys  has  been  practised  intact,  at  all  the  European 
mints,  under  the  name  of  the  "  humid  method,"  in  place  of  the  old 
system  of  cupellation.  During  that  time  it  has  been  regarded  as 
one  of  the  most  exact  methods  of  quantitative  analysis.  The 
exhaustive  researches  of  Mulder,  however,  have  shown  that  it 
is  capable  of  even  greater  accuracy  than  has  hitherto  been  supposed. 

The  principle  of  the  process  is  the  same  as  described  on  p.  141, 
depending  on  the  affinity  which  chlorine  has  for  silver  in  preference 
to  all  other  substances,  and  resulting  in  the  formation  of  silver 
chloride,  a  compound  insoluble  in  dilute  acids,  and  which  readily 
separates  itself  from  the  liquid  in  which  it  is  suspended. 

The  plan  originally  devised  by  the  illustrous  inventor  of  the 
process  for  assaying  silver,  which  is  still  followed,  is  to  consider 
the  weight  of  alloy  taken  for  examination  to  consist  of  1000  parts, 
and  the  question  is  to  find  how  many  of  these  parts  are  pure  silver. 
This  empirical  system  was  arranged  for  the  convenience  of  commerce, 
and  being  now  thoroughly  established,  it  is  the  best  plan  of  pro- 
cedure. If,  therefore,  a  standard  solution  of  salt  be  made  of  such 
strength  that  100  c.c.  will  exactly  precipitate  1  gm.  of  silver,  it  is 
manifest  that  each  T^  c.c.  will  precipitate  1  mgm.  or  TTrVo~  Pai>t  °f 
the  gram  taken  ;  and  consequently  in  the  analysis  of  1  gm.  of  any 
alloy  containing  silver,  the  number  of  y1^  c.c.  required  to  precipitate 
all  the  silver  out  of  it  would  be  the  number  of  thousandths  of  pure 
silver  contained  in  the  specimen. 

In  practice,  however,  it  would  not  do  to  follow  this  plan  precisely, 
inasmuch  as  neither  the  measurement  of  the  standard  solution  nor 
the  ending  of  the  process  would  be  gained  in  the  most  exact  manner  ; 
consequently,  a  decimal  solution  of  salt,  one-tenth  the  strength  of 
the  standard  solution,  is  prepared,  so  that  1000  c.c.  will  exactly 
precipitate  1  gm.  of  silver,  and,  therefore,  1  c.c.  1  mgm. 

The  silver  alloy  to  be  examined  (the  composition  of  which  must 
be  approximately  known)  is  weighed  so  that  about  1  gm.  of  pure 
silver  is  present :  it  is  then  dissolved  in  pure  nitric  acid  by  the  aid 
of  a  gentle  heat,  and  100  c.c.  of  standard  solution  of  salt  added 
from  a  pipette  in  order  to  precipitate  exactly  1  gm.  of  silver ;  the 
bottle  containing  the  mixture  is  then  well  shaken  until  the  silver 
chloride  has  curdled,  leaving  the  liquid  clear. 

The  question  is  now  :  Which  is  in  excess,  salt  or  silver  ?  A  drop 
of  decimal  salt  solution  is  added,  and  if  a  precipitate  be  produced 
1  c.c.  is  delivered  in,  and  after  clearing,  another,  and  so  on  as  long 
as  a  precipitate  is  produced.  If  on  the  other  hand,  the  one  drop 
of  salt  produced  no  precipitate,  showing  that  the  pure  silver  present 
was  less  than  1  gm.,  a  decimal  solution  of  silver  is  used,  prepared 
by  dissolving  1  gm.  pure  silver  in  pure  nitric  acid  and  diluting  to 
1  litre.  This  solution  is  added  after  the  same  manner  as  the  salt 


SILVER.  319 

solution  just  described,  until  no  further  precipitate  appears  ;  in 
either  case  the  quantity  of  decimal  solution  used  is  noted,  and  the 
results  calculated  in  thousandths  for  1  gm.  of  the  alloy. 

The  process  thus  shortly  described  is  that  originally  devised  by 
Gay  Lussac,  and  it  was  taken  for  granted  that,  when  equivalent 
chemical  proportions  of  silver  and  sodium  chloride  were  brought 
thus  in  contact,  every  trace  of  the  metal  was  precipitated  from 
the  solution,  leaving  sodium  nitrate  and  free  nitric  acid  only 
in  solution.  The  researches  of  Mulder,  however,  go  to  prove 
that  this  is  not  strictly  the  case,  but  that  when  the  most  exact 
chemical  proportions  of  silver  and  salt  are  made  to  react  on  each 
other,  and  the  chloride  has  subsided,  a  few  more  drops  of  either 
salt  or  silver  solution  will  produce  a  further  precipitate,  indicating 
the  presence  of  both  silver  nitrate  and  sodium  chloride  in  a  state 
of  equilibrium,  which  is  upset  on  the  addition  of  either  salt  or 
silver.  Mulder  decides,  and  no  doubt  rightly,  that  this  peculiarity 
is  owing  to  the  presence  of  sodium  nitrate,  and  varies  somewhat 
with  the  temperature  and  state  of  dilution  of  the  liquid. 

It  therefore  follows  that  when  a  silver  solution  is  carefully 
precipitated,  first  by  concentrated  and  then  by  dilute  salt  solution, 
until  no  further  precipitate  appears,  the  clear  liquid  will  at  this 
point  give  a  precipitate  with  dilute  silver  solution ;  and  if  it  be 
added  till  no  further  cloudiness  is  produced,  it  will  again  be 
precipitable  by  dilute  salt  solution. 

EXAMPLE  :  Suppose  that  in  a  given  silver  analysis  the  decimal  salt  solution  has 
been  added  so  long  as  a  precipitate  is  produced,  and  that  1  c.c.  (  =  20  drops  of 
Mulder's  dropping  apparatus)  of  decimal  silver  is  in  turn  required  to  precipitate 
the  apparent  excess,  it  would  be  found  that  when  this  had  been  done,  1  c.c.  more 
of  salt  solution  would  be  wanted  to  reach  the  point  at  which  no  further  cloudiness 
is  produced  by  it,  and  so  the  changes  might  be  rung  time  after  time  ;  if,  however, 
instead  of  the  last  1  c.c.  (  =20  drops)  of  salt,  half  the  quantity  be  added,  that  is 
to  say  10  drops  (  =£  c.c.),  Mulder's  so-called  neutral  point  is  reached;  namely, 
that  in  which,  if  the  liquid  be  divided  in  half,  both  salt  and  silver  will  produce 
the  same  amount  of  precipitate.  At  this  stage  the  solution  contains  silver  chloride 
dissolved  in  sodium  nitrate,  and  the  addition  of  either  salt  or  silver  expels  it  from 
solution. 

A  silver  analysis  may  therefore  be  concluded  in  three  ways — 

(1)  By  adding  decimal  salt  solution  until  it  just  ceases  to  produce 
a  cloudiness. 

(2)  By  adding  a  slight  excess  of  salt,  and  then  decimal  silver 
till  no  more  precipitate  appears. 

(3)  By  finding  the  neutral  point. 

According  to  Mulder  the  latter  is  the  only  correct  method,  and 
preserves  its  accuracy  at  all  temperatures  up  to  56°  C.  (  =  133° 
Fahr.),  while  the  difference  between  1  and  3  amounts  to  J  a  mgm., 
and  that  between  1  and  2  to  1  mgm.  on  1  gm.  of  silver  at  16°  C. 
(  =  60-8°  Fahr.),  and  is  seriously  increased  by  .variation  of 
temperature. 

It  will  readily  be  seen  that  much  more  trouble  and  care  is  required 
by  Mulder's  method  than  by  that  of  Gay  Lussac,  but  as 
a  compensation,  much  greater  accuracy  is  obtained. 


320  SILVER. 

On  the  whole  it  appears  to  me  preferable  to  weigh  the  alloy  so 
that  slightly  more  than  1  gm.  of  silver  is  present,  and  to  choose 
the  ending  *No.  1,  adding  drop  by  drop  the  decimal  salt  solution 
until  just  a  trace  of  the  precipitate  is  seen,  and  which,  after  some 
practice,  is  known  by  the  operator  to  be  final.  It  will  be  found 
that  the  quantity  of  salt  solution  used  will  slightly  exceed  that 
required  by  chemical  computation  ;  say  100- 1  c.c.  are  found  equal 
to  ]  gm.  of  silver,  the  operator  has  only  to  calculate  that  quantity 
of  the  salt  solution  in  question  for  every  1  gm.  of  silver  he  assays 
in  the  form  of  alloy,  and  the  error  produced  by  the  solubility  of 
silver  chloride  in  sodium  nitrate  is  removed. 

If  the  decimal  solution  has  been  cautiously  added,  and  the 
temperature  not  higher  than  17°  C.  (62°  Fahr.),  this  method  of 
conclusion  is  as  reliable  as  No.  3,  and  free  from  the  possible  errors 
of  experiment ;  for  it  requires  a  great  expenditure  of  time  and 
patience  to  reverse  an  assay  two  or  three  times,  each  time  cautiously 
adding  the  solutions  drop  by  drop,  then  shaking  and  waiting  for 
the  liquid  to  clear,  besides  the  risk  of  discolouring  the  silver  chloride, 
which  would  at  once  vitiate  the  results. 

The  decimal  silver  solution,  according  to  this  arrangement, 
would  seldom  be  required  ;  if  the  salt  has  been  incautiously  added, 
or  the  quantity  of  alloy  too  little  to  contain  1  gm.  pure  silver, 
then  it  is  best  to  add  once  for  all  2,  3,  or  5  c.c.,  according  to  circum- 
stances, and  finish  with  decimal  salt  as  No.  1,  deducting  the  silver 
added. 

The  Standard  Solutions  and  Apparatus. 

(a)  Standard  Salt  Solution. — Pure  sodium  chloride  is  readily  purchased.  It 
is  made  by  passing  HC1  gas  into  a  strong  solution  of  common  salt,  when  pure 
sodium  chloride  crystallizes  out.  The  crystals  are  slightly  washed  with  cold 
water,  dried,  and  heated  to  dull  redness.  When  cold,  5 '4 19  grams  are  weighed 
and  dissolved  in  1  litre  of  distilled  water  at  15°  C.  100  c.c.  of  this  solution  will 
precipitate  exactly  1  gram  of  silver.  It  is  preserved  in  a  well-stoppered  bottle, 
and  shaken  before  use. 

(6)  Decimal  Salt  Solution. — 100  c.c.  of  the  above  solution  are  diluted  to  exactly 
1  litre  with  distilled  water  at  15°  C.  1  c.c.  will  precipitate  0*001  gm.  of  silver. 

(c)  Decimal  Silver  Solution. — Pure  metallic  silver  is  best  prepared  by  galvanic 
action  from  pure  chloride  ;  and  as  clean  and  safe  a  method  as  any  is  to  wrap  a 
lump  of  clean  zinc,  into  which  a  silver  wire  is  melted,  with  a  piece  of  wetted  bladder 
or  calico,  so  as  to  keep  any  particles  of  impurity  contained  in  the  zinc  from  the 
silver.  The  chloride  is  placed  at  the  bottom  of  a  porcelain  dish,  covered  with 
dilute  sulphuric  acid,  and  the  zinc  laid  in  the  middle ;  the  silver  wire  is  bent  over 
so  as  to  be  immersed  in  the  chloride.  As  soon  as  the  acid  begins  to  act  upon  the 
zinc  the  reduction  of  the  chloride  commences,  and  proceeds  gradually  throughout 
the  mass ;  the  resulting  finely-divided  silver  is  well- washed,  first  with  dilute  acid, 
then  with  hot  water,  till  all  acid  and  soluble  zinc  salts  are  removed. 

The  moist  metal  is  then  mixed  with  a  little  sodium  carbonate,  saltpetre,  and 
borax,  say  about  an  eighth  part  of  each,  dried  perfectly,  then  melted.  Mulder 
recommends  that  the  melting  should  be  done  in  a  porcelain  crucible  immersed 
in  sand  contained  in  a  common  earthen  crucible  ;  borax  is  sprinkled  over  the 
surface  of  the  sand  so  that  it  may  be  somewhat  vitrified,  that  in  pouring  out  the 


SILVER.  321 

silver  when  melted  no  particles  of  dirt  or  sand  may  fall  into  it.  If  the  quantity 
of  metal  be  small  it  may  be  melted  in  a  porcelain  crucible  over  a  gas  blowpipe. 

The  molten  metal  obtained  in  either  case  can  be  poured  into  cold  water  and  so 
granulated,  or  upon  a  slab  of  pipe-clay,  into  which  a  glass  plate  has  been  pressed 
when  soft  so  as  to  form  a  shallow  mould.  The  metal  is  then  washed  well  with 
boiling  water  to  remove  accidental  surface  impurities,  and  rolled  into  thin  strips 
by  a  goldsmith's  mill,  in  order  that  it  may  readily  be  cut  for  weighing.  The 
granulated  metal  is,  of  course,  ready  for  use  at  once  without  any  rolling. 

1  gm.  of  this  silver  is  dissolved  in  pure  dilute  nitric  acid,  and  diluted  to  1  litre  ; 
each  c.c.  contains  O'OOl  gm.  of  silver.  It  should  bo  kept  from  the  light. 

(d)  Dropping  Apparatus   for   Concluding    the   Assay.  —  Mulder   constructs 
a  special  apparatus  for  this  purpose  consisting  of  a  pear-shaped  vessel  fixed  in 
a  stand,  with  special  arrangements  for  preventing  any  continued  flow  of  liquid. 
The  delivery  tube  has  an  opening  of  such  size  that  20  drops  measure  exactly 
1  c.c.     The  vessel  itself  is  not  graduated.     As  this  arrangement  is  of  more  service 
in  assay  than  in  general  laboratories,  it  need  not  be  further  described  here.     A  small 
burette  divided   in  TV  c.c.  with  a  convenient  dropping  tube   will  answer  every 
purpose,  and  possesses  the  further  advantage  of  recording  the  actual  volume  of 
fluid  delivered. 

The  100  c.c.  pipette,  for  delivering  the  concentrated  salt  solution,  must  be 
accurately  graduated,  and  should  deliver  exactly  100  gm.  of  distilled  water  at 
15°  C. 

The  test  bottles,  holding  about  200  c.c.,  should  have  their  stoppers  well  ground 
and  brought  to  a  point,  and  should  be  fitted  into  japanned  tin  tubes  reaching  as 
high  as  the  neck,  so  as  to  preserve  the  precipitated  chloride  from  the  action  of 
light,  and,  when  shaken,  a  piece  of  black  cloth  should  be  placed  over  the  stopper. 

(e)  Titration  of  the  Standard  Salt  Solution.  —  From  what  has  previously  been 
stated  as  to  the  principle  of  this  method,  it  will  be  seen  that  it  is  not  possible 
to  rely  absolutely  upon  a  standard  solution  of  salt  containing  5  '4  19  gm.  per  litre, 
although  this  is  chemically  correct  in  its  strength.     The  real  working  value  must 
be  found  by  experiment.     From  1'002  to  1'004  gm.  of  absolutely  pure  silver  is 
weighed  on  the  assay  balance,  put  into  a  test  bottle  with  about  5  c.c.  of  pure  nitric 
acid  of  about  1  '2  sp.  gr.,  gently  heated  in  the  water  or  sand  bath  till  it  is  all  dissolved. 
The  nitrous  vapours  are  then  blown  from  the  bottle,  and  it  is  set  aside  to  cool 
down  to  about  16°  C.  or  60°  Fahr. 

The  100-c.c.  pipette,  which  should  be  securely  fixed  in  a  support,  is  then 
carefully  filled  with  the  salt  solution,  and  delivered  into  the  test  bottle  contained 
in  its  case,  the  moistened  stopper  inserted,  covered  over  with  the  black  velvet  or 
cloth,  and  shaken  continuously  till  the  chloride  has  clotted  and  the  liquid  become 
clear  ;  the  stopper  is  then  slightly  lifted,  and  its  point  touched  against  the  neck 
of  the  bottle  to  remove  excess  of  liquid,  again  inserted,  and  any  particles  of  chloride 
washed  down  from  the  top  of  the  bottle  by  carefully  shaking  the  clear  liquid 
over  them.  The  bottle  is  then  brought  under  the  decimal  salt  burette,  and  -|  c.c. 
added,  the  mixture  shaken,  cleared,  another  -|  c.c.  put  in  and  the  bottle  fifted 
partly  out  of  its  case  to  see  if  the  precipitate  is  considerable  ;  lastly,  2  or  3  drops 
only  of  the  solution  are  added  at  a  time  until  no  further  opacity  is  produced  by 
the  final  drop.  Suppose,  for  instance,  that  in  titrating  the  salt  solution  it  is  found 
that  T003  gm.  of  silver  require  100  c.c.  concentrated,  and  4  c.c.  decimal  solution, 
altogether  equal  to  100  '4  c.c.  concentrated,  then  — 

1-003  silver  :   I'OOO  :  100  "4  salt  :  :  x.  x  =100  '0999. 


The  result  is  within  Tu^mr  of  lOO'l,  which  is  near  enough  for  the  purpose,  and 
may  be  more  conveniently  used.  The  operator  therefore  knows  that  100*1  c.c.  of 
the  concentrated  salt  solution  at  15°  C.  will  exactly  precipitate  1  gm.  silver,  and 
in  his  examination  of  alloys  calculates  accordingly. 

In  the  assay  of  coin  and  plate  of  the  English  standard,  namely,  11*1  silver  and 
0'9  copper,  the  weight  corresponding  to  1  gm.  of  silver  is  T081  gm.  therefore  in 
examining  this  alloy  1'085  gm.  may  be  weighed. 

When  the  quantity  of  silver  is  not  approximately  known,  a  preliminary 
analysis  is  necessary,  which  is  best  made  by  dissolving  J  or  1  gm.  of  the  alloy  in 
nitric  acid,  and  precipitating  very  carefully  with  the  concentrated  salt  solution 


322  SILVER. 

from  a  TV  c-c-  burette.     Suppose  that  in  this  manner  1  gm.  of  alloy  required 
45  c.c.  salt  solution, 

salt          salt  silver  silver 

100-1     :     45     :     :     1     :     x 
x  =0-4495 

And  0-4495  :  1'003  :     :  1  :  x  ' 
x  =2-231. 

2-231  gm.  of  this  particular  alloy  are  therefore  taken  for  the  assay. 

Where  alloys  of  silver  contain  sulphur  or  gold,  with  small  quantities  of  tin, 
lead,  or  antimony,  they  are  first  treated  with  a  small  quantity  of  nitric  acid  so 
long  as  red  vapours  are  disengaged,  'then  boiled  with  concentrated  sulphuric  acid 
till  the  gold  has  become  compact,  set  aside  to  cool,  diluted  with  water,  and  titrated 
as  above. 

Assaying  on  the  Grain  System. 

It  will  readily  be  seen  that  the  process  just  described  may  quite 
as  conveniently  be  arranged  on  the  grain  system  by  substituting 
10  grains  of  silver  as  the  unit  in  place  of  the  gram  ;  each  decem 
of  concentrated  salt  solution  would  then  be  equal  to  -f^  of  a  grain 
of  silver,  and  each  decem  of  decimal  solution  to  TJ  ^  of  a  grain. 


5.    Titration  of  the  Silver  Solutions  used  in  Photography. 

The  silver  bath  solutions  for  sensitizing  collodion  and  paper 
frequently  require  examination,  as  their  strength  is  constantly 
lessening.  To  save  calculation,  it  is  better  to  use  an  empirical 
solution  of  salt  than  the  systematic  one  described  above. 

This  is  best  prepared  by  dissolving  43  grains  of  pure  sodium 
chloride  in  10,000  grains  of  distilled  water.  Each  decem  (  =  10  grn.) 
of  this  solution  will  precipitate  0*125  grn.  (i.e.,  J  grn.)  of  pure  silver 
nitrate  ;  therefore  if  one  fluid  drachm  of  any  silver  solution  be 
taken  for  examination,  the  number  of  decems  of  salt  solution 
required  to  precipitate  all  the  silver  will  be  the  number  of  grains 
of  silver  nitrate  in  each  ounce  of  the  solution. 

EXAMPLE  :  One  fluid  drachm  of  an  old  nitrate  bath  was  carefully  measured 
into  a  stoppered  bottle,  10  or  15  drops  of  pure  nitric  acid  and  a  little  distilled 
water  added  ;  the  salt  solution  was  then  cautiously  added,  shaking  well  after  each 
addition  until  no  further  precipitate  was  produced.  The  quantity  required  was 
26  -5  dm  =26  \  grains  of  silver  nitrate  in  each  ounce  of  solution. 

Crystals  of  silver  nitrate  may  also  be  examined  in  the  same  way,  by  dissolving 
say  30  or  40  grn.  in  an  ounce  of  water,  taking  one  drachm  of  the  fluid  and  titrating 
as  above. 

In  consequence  of  the  rapidity  and  accuracy  with  which  silver  may  be 
determined  when  potassium  chromate  is  used  as  indicator,  some  may  prefer 
to  use  that  method.  It  is  then  necessary  to  have  a  standard  solution  of  silver, 
of  the  same  chemical  value  as  the  salt  solution  ;  this  is  made  by  dissolving  125 
grains  of  pure  and  dry  neutral  silver  nitrate  in  1000  dm.  of  distilled  water  ;  both 
solutions  will  then  be  equal,  volume  for  volume. 

Suppose,  therefore,  it  is  necessary  to  examine  a  silver  solution  used  for 
sensitizing  paper.  One  drachm  is  measured,  and,  if  any  free  acid  be  present, 
cautiously  neutralized  with  a  weak  solution  of  sodium  carbonate  ;  100  dm.  of  salt 
solution  are  then  added  with  a  pipette.  If  the  solution  is  under  100  grn.  to  the 
ounce,  the  quantity  will  be  sufficient.  3  or  4  drops  of  chromate  solution  are 
then  added,  and  the  silver  solution  delivered  from  the  burette  until  the  red  colour 


SUGARS.  323 

of  silver  chromate  is  just  visible.  If  25 '5  dm.  have  been  required,  that  number 
is  deducted  from  the  100  dm.  of  salt  solution,  which  leaves  74'5  dm.,  or  74^  grains 
to  the  ounce. 

This  method  is  much  more  likely  to  give  exact  results  in  the  hands  of  persons 
not  expert  in  analysis  than  the  ordinary  plan  by  precipitation,  inasmuch  as, 
with  collodion  baths,  containing  as  they  always  do  silver  iodide,  it  is  almost 
impossible  to  get  the  supernatant  liquid  clear  enough  to  distinguish  the  exact  end 
of  the  analysis. 

SUGARS. 

SUGARS  belong  to  the  large  class  of  organic  bodies  known  as 
"  carbo-hydrates,"  of  which  there  are  three  main  classes,  viz.  :— 

(1)     The  Hexoses,  C6HlSO6,  including 

(1)  Glucose,  Dextrose  or  Grape  Sugar,  which  is  found  in  large 
quantities  in  grapes  (whence  its  name),  and  also  in  the  urine  of 
persons  suffering  from  diabetes  mellitus.     It  is  the  ultimate  product 
of  the  hydrolysis  of  starch  by  a  dilute  acid. 

(ii)  Fructose,  Laevulose  or  Fruit  Sugar,  which,  associated  with 
dextrose,  is  found  in  the  juice  of  sweet  fruits  and  in  honey. 

(iii)  Galactose,  which  is  produced,  together  with  dextrose,  when 
lactose  is  hydrolyzed  by  dilute  mineral  acids. 

All  the  above  reduce  Fehling's  solution.  None  of  them  are 
affected  by  boiling  with  dilute  acids. 

(2)  The  Cane-sugar  group,  C12H22On,  including 

(i)  Sucrose  or  Cane-Sugar,  which  is  found  in  the  sugar  cane, 
beetroot  and  maple. 

(ii)  Lactose  or  Milk  Sugar  (cryst.  C12H22On,H2O),  which  is  found 
in  the  milk  of  mammals  and  in  various  pathological  secretions. 

(iii)  Maltose  or  Malt  sugar  (cryst.  C12H22On,H2O),  which, 
together  with  dextrin,  is  produced  by  the  action  of  diastase  on 
starch. 

(iv)  Raffinose,  C18H32016  (cryst.  C18H32O]6,  5H20),  which  is 
found  in  beet  molasses  and  in  eucalyptus  manna. 

Lactose  and  maltose  reduce  Fehling's  solution,  but  to  a  less  degree 
than  the  hexoses.  Cane  sugar  does  so  only  after  hydrolysis,  but 
raffinose  has  no  reducing  effect  on  this  solution. 

All  the  members  of  this  group  are  hydrolyzed  by  heating  with 
dilute  mineral  acids  or  by  the  action  of  soluble  ferments  (enzymes), 
such  as  diastase  (in  malt),  invertase  (in  yeast),  and  ptyalin  (in 
the  saliva).  Cane  sugar  in  this  way  becomes  converted  into  a 
mixture  of  dextrose  and  laevulose  in  equal  numbers  of  molecules, 
the  process  being  known  as  "inversion"  and  the  product  "invert 
sugar,"  as  the  dextro-rotatory  cane  sugar  solution  becomes  the 
laevo-rotatory  mixture.  By  hydrolysis,  lactose  is  converted  into 
dextrose  and  galactose,  maltose  into  dextrose  alone,  and  raffinose 
into  dextrose,  laevulose  and  galactose. 

(3)  The  Cellulose  group  (C6H1005)n,  including 

(i)  Cellulose,  which  forms  the  membrane  of  plant  cells  and  of 
which  cotton,  wood,  etc.,  mainly  consist. 

Y  2 


324  SUGARS. 

(ii)  Starch  or  Amylum,  which  is  present  in  the  grains  of  cereals, 
in  potatoes,  etc. 

(iii)  Glycogen  or  Animal  Starch,  which  is  found  in  the  liver  in 
mammalia. 

(iv)  The  Dextrins,  produced  by  heating  starch  to  about  210°, 
either  alone  or  with  a  little  dilute  sulphuric  acid  ;  they  are  inter- 
mediate products  in  the  conversion  of  starch  into  glucose.  The 
various  dextrins  are  distinguished  by  their  behaviour  with  iodine. 
Amylo-dextrin,  like  starch,  gives  a  blue  colour  with  iodine  ;  erythro- 
dextrin,  which  is  formed  by  the  partial  hydrolysis  of  amylo-dextrin, 
a  red  colour.  The  succeeding  compounds,  the  achroo-dextrins, 
give  no  colour  at  all. 

None  of  the  above  reduce  Fehling's  solution. 

By  the  action  of  dilute  sulphuric  acid  starch,  glycogen,  and 
cellulose  are  ultimately  hydrolyzed  to  dextrose. 

The  reducing  action  of  certain  sugars,  as  specified  above,  on 
alkaline  solutions  of  copper  has  been  the  subject  of  much  careful 
work  by  a  large  number  of  chemists,  both  gravimetric  and 
volumetric  processes  being  employed.  Both  methods  are  capable 
of  giving  good  results,  but  in  order  to  obtain  a  high  degree  of 
accuracy  it  is  essential  in  every  case  that  certain  details  of  procedure 
be  strictly  adhered  to. 

Kjeldahl  maintains  that  Fehling's  solution,  however  pure  its 
constituents,  always  undergoes  a  slight  reduction  on  prolonged 
heating,  especially  in  strong  solution,  and  he  fixes  the  limit  of  time 
for  which  the  liquid  should  be  exposed  to  the  temperature  of  boiling 
water  at  twenty  minutes. 

1.    The  conversion  of  various  Sugars  into  Glucose. 

The  inversion  of  cane  sugar  is  carried  out  as  follows  :  To 
a  solution  of  sugar  containing  not  more  than  25  grams  in  100  c.c. 
add  one-tenth  of  its  volume  of  fuming  HC1  and  heat  in  a  flask, 
placed  in  a  water-bath,  till  the  contents  have  acquired  the  tempera- 
ture of  68°  C.,  arranging  matters  so  that  the  operation  occupies 
about  10  minutes.  The  flask  is  then  removed  and  cooled.  Some 
operators  prefer  dilute  sulphuric  acid  and  boil  the  mixture  for 
5  to  10  minutes.  The  hydrolysis  of  lactose  requires  a  longer 
boiling  than  this. 

Maltose,  when  heated  with  dilute  acid,  gradually  becomes 
hydrolyzed  into  glucose,  the  process  requiring  about  3  or  4  hours 
at  the  ordinary  pressure.  3  c.c.  of  concentrated  sulphuric  acid  are 
added  to  each  100  c.c.  of  the  solution,  which  is  then  heated  in 
a  water-bath  for  3  or  4  hours.  If  any  dextrin  be  present,  this  will 
also  be  converted  into  glucose. 

The  hydrolysis  of  the  slowly  changing  sugars  may  be  hastened 
considerably  by  heating  at  increased  atmospheric  pressure,  although 
some  authorities  condemn  the  process.  O'Sullivan,*  however, 

*  See  Allen's  Commercial  Organic  Analysis,  Vol.  I. 


SUGARS.  325 

states  that  a  good  result  with  maltose  or  dextrin  is  obtained  by 
heating  30  gm.  of  the  substance  in  100  c.c.  of  water  containing 
1  c.c.  of  H2S04  for  20  minutes,  at  a  pressure  of  one  additional 
atmosphere.  Allen  also  gives  a  handy  means  of  carrying  out  this 
method,  which  consists  in  using  a  soda  water  bottle  with  rubber 
stopper  through  whicli  passes  a  long  glass  tube  bent  at  right- 
angles,  and  immersed  to  a  depth  of  30  inches  in  mercury  contained 
in  a  vertical  tube  of  glass  or  metal.  The  rubber  stopper  must  be 
secured  by  wire,  and  the  bottle  heated  to  boiling  in  a  saturated 
solution  of  sodium  nitrate,  which  gives  a  temperature  corresponding 
to  an  extra  atmosphere. 

Of  course  in  all  cases  where  acid  has  been  used  for  the  inversion 
of  sugar,  it  must  be  neutralized  before  the  copper  titration  takes 
place  ;  this  may  be  done  either  with  sodium  or  potassium  hydrate  or 
carbonate,  or  calcium  carbonate  may  be  used. 

Starch  from  various  sources  may  be  hydrolyzed  in  the  same  way 
as  the  sugars,  but  it  needs  a  prolonged  heating  with  acid.  For 
approximate  purpose  1  gm.  of  starch  should  be  mixed  to  a  smooth 
cream  with  about  30  c.c.  of  cold  water,  then  1  c.c.  of  strong 
hydrochloric  acid  added,  and  the  mixture  kept  at  a  boiling  tempera- 
ture in  an  obliquely  fixed  flask  for  8  or  10  hours,  replacing  the 
evaporated  water  from  time  to  time  to  avoid  charring  the  sugar, 
and  testing  with  iodine  to  ascertain  when  tne  inversion  is  complete. 
The  product  is  glucose. 

For  the  determination  of  the  starch  itself  a  number  of  processes 
were  tried  by  Ost,*  the  one  which  was  found  to  answer  best  being 
that  of  Sachsse,|  slightly  modified.  In  this  modification  3  gm. 
of  the  starch  are  heated  with  200  c.c.  of  water  and  20  c.c.  of  hydro- 
chloric acid,  specific  gravity  1-125  (  =  5-600  gm.  of  HC1),  for  two 
to  three  hours  in  a  boiling  water-bath,  using  the  factor  0*925  to 
calculate  the  glucose  found  in  the  starch.  Longer  heating  gives 
results  too  low,  and  two  hours  on  the  water-bath  are  not  sufficient. 
Slightly  higher  yields  of  glucose  (89-8  instead  of  89-5  per  cent.) 
can  be  obtained  by  heating  for  a  much  longer  period  with  less 
starch  and  acid,  but  there  is  no  advantage  to  be  gained  by  the 
alteration.  Oxalic  acid  gives  no  better  results.  Dextrin  may 
be  determined  in  the  same  manner ;  also  maltose,  if  1  gm.  of  the 
latter  be  heated  for  five  hours  with  100  c.c.  of  1  to  2  per  cent,  of 
hydrochloric  acid  as  before. 

100  parts  of  grape  sugar,  found  byFehling's  process,  represent 
90  parts  of  starch  or  dextrin.  When  dextrin  is  present  with  grape 
sugar,  care  must  be  taken  not  to  boil  the  mixture  too  long  with  the 
alkaline  copper  solution,  as  it  has  been  found  that  a  small  portion 
of  the  copper  is  precipitated  by  the  dextrin. J 

The  hydrolysi?of  starch  may*  be  brought  about  more  rapidly, 
and  at  lower  temperature,  by  using  some  form  of  diastase  in 
place  of  acid.  An  infusion  of  malt  is  best  suited  to  the  purpose, 

*  Chem.  Zeit.  1895, 19,  1501.  f  Chem.  CentralbL  8,  732. 

J  R  u  m  p  f  and  H  e  i  n  t  x  c  r  1  i  n  g,  Z.  a.  C.  9,  358. 


326  SUGARS. 

but  the  temperature  must  not  exceed  71°  C.  (160°  Fahr.).  The 
digestion  may  vary  from  fifteen  minutes  to  as  many  hours.  The 
presence  of  unchanged  starch  may  be  ascertained  by  occasionally 
testing  with  iodine.  If  the  digestion  is  carried  beyond  half  an 
hour,  a  like  quantity  of  the  same  malt  solution  must  be  digested 
alone,  at  the  same  temperature  and  for  the  same  time,  then  titrated 
for  its  amount  of  sugar,  which  is  deducted  from  the  total  quantity 
found  in  the  mixture.  O' Sullivan*  has,  however,  clearly  shown 
that  the  effect  of  the  diastase  is  to  produce  jjialtose,  which  has  only 
the  power  of  reducing  the  copper  solution  to  the  extent  of  about 
three-fifths  that  of  dextrose  or  grape  sugar,  the  rest  being  probably 
various  grades  of  dextrin.  Brown  and  Heron's  experiments 
clearly  demonstrate  that  no  dextrose  is  produced  from  starch  by 
even  prolonged  treatment  with  malt  extract  ;  the  only  product  is 
maltose.  Sulphuric  or  other  similar  acids  cause  complete 
hydrolysis. 

For  the  exact  determination  of  starch  in  grain  of  various  kinds 
O' Sullivan  gives  very  elaborate  directions,  involving  the  treat- 
ment of  the  substance  with  alcohol  and  ether,  to  remove  fatty 
and  other  constituents  previous  to  digestion  with  diastase.  The 
same  authority  also  gives  special  directions  for  the  preparation 
of  the^proper  kind  of  diastase,  all  of  which  may  be  found  in 
J.  C. '&  xlv.  1. 

For  a  rapid  determination  of  starch  in  barley  or  malt  a  method 
>n  described  by  H.  T.  Brown  and  J,  H.  Millar.t 

.^  jparation  of  the  Solution  of  Sugar. — For  all  the  processes  of 
tit  ration  this  must  be  so  diluted  as  to  contain  J  or  at  most  1  per 
^;nt.  of  sugar  ;  if  on  trial  it  is  found  to  be  stronger  than  this,  it 
B|ust  be  further  diluted  with  a  measured  quantity  of  distilled  water. 

If  the  sugar  solution  to  be  examined  is  of  dark  colour,  or  likely 
to  contain  extractive  matters  which  might  interfere  with  the 
distinct  ending  of  the  reaction,  it  is  advisable  to  heat  a  measured 
quantity  to  boiling,  and  add  a  few  drops  of  milk  of  lime,  allow  the 
precipitate  to  settle,  then  filter  through  purified  animal  charcoal, 
and  dilute  with  the  washings  to  a  definite  volume.  In  some 
instances  cream  of  alumina  or  basic  lead  acetate  may  be  used  to 
clarify  highly  coloured  or  impure  solution,  but  no  lead  must  be 
left  in  the  solution.  J 

From  thick  mucilaginous  liquids,  or  those  which  contain  a  large 
proportion  of  albuminous  or  extractive  matters,  the  sugar  is  best 
extracted  by  Graham's  dialyzer. 

*  J.  C.  S-  1872,  579.  f  Trans,  of  the  Guinness  Research  Lab.  1903,  1,79. 

J  Although  traces  of  lead  are  of  no  great  consequence  when  clarifying  sugars  for 
the  polanmeter  it  is  of  great  importance  to  remove  all  lead  when  using  the  volu- 
metric method.  In  order  to  do  this  it  is  best  to  treat  a  measTlred  quantity  of  the 
sugar  solution  which  has  been  clarified  by  lead  with  a  strong  solution  of  sulphurous 
acij  u,ntl1  no  further  precipitate  is  formed,  then  add  a  few  drops  of  aluminium 
hydrate  suspended  in  water,  dilute  to  a  definite  volume  and  filter.  In  many  cases 
concentrated  solution  of  sodium  carbonate  will  suffice  to  remove  all  lead.  These 
ethpds  of  clarification  are  highly  necessary  in  the  case  of  albuminous  or  gelatinous 
i9£l  il  as  otherwise  the  copper  oxide  will  not  settle  readily,  and  it  becomes 
difficult  to  tell  when  the  end-reaction  occurs. 


SUGARS.  327 

The  Fehling  method  may  be  applied  directly. to  fresh  diabetic 
urine  (see  Analysis  of  Urine),  as  also  to  brewer's  wort  or  distiller's 
mash.  Dextrin  does  not  interfere,  unless  the  boiling  of  the  liquid 
under  titration  is  long  continued. 


2.     Determination  of  Glucose  by  F  e  h  1  i  n  g  '  s  Solution. 

Preparation  o!  the  Standard  Solutions. — Fehling 's  standard 
copper  solution. — Crystals  of  pure  cupric  sulphate  are  powdered 
and  pressed  between  unsized  paper  to  remove  adhering  moisture ; 
69-28  gm.  are  weighed,  dissolved  in  water,  about  1  c.c.  of  pure 
sulphuric  acid  added,  and  the  solution  diluted  to  1  litre. 

Alkaline  tartrate  solution. — 350  gm.  of  Rochelle  salt  (sodium 
potassium  tartrate)  are  dissolved  in  about  700  c.c.  of  water,  and 
the  solution  filtered,  if  not  already  clear  ;  there  is  then  added  to  it 
a  clear  solution  of  100  gm.  of  caustic  soda  (prepared  by  alcohol) 
in  about  200  c.c.  of  water.  The  volume  is  made  up  to  1  litre  at 
60°  Fah. 

These  solutions  are  prepared  separately,  and  when  mixed  in 
exactly  equal  proportions  form  the  original  Fehling  solution, 
each  c.c.  of  which  should-  contain  0'03464  gm.  of  cupric  sulphate, 
and  represents  0*005  gm.  of  pure  anhydrous  grape  sugar,  if  the 
conditions  of  titration  laid  down  below  are  adhered  to.*  The 
method  is  based  on  the  fact  that  although  Feh ling's  solution  may 
be  heated  to  boiling  without  change,  the  introduction  into  it  of  the 
smallest  quantity  of  grape  sugar,  at  a  boiling  temperature,  at  once 
produces  a  precipitate  of  cuprous  oxide,  the  ratio  of  reduction  being 
uniform  if  the  conditions  of  experiment  are  always  the  same. 

THE  TITRATION  OF  GLUCOSE  WITH  FEHLING'S  SOLUTION. — 5  c.c.  each 
of  standard  capper  and  alkaline  tartrate  solutions  are  accurately  measured  into 
a  thin  white  porcelain  basin,  40  c.c.  of  water  added,  and  the  basin  quickly  heated 
to  boiling  on  a  sand-bath  or  by  a  small  flame.  No  reduction  or  change  of  colour 
should  occur  ;  if  it  does,  the  alkaline  tartrate  solution  is  probably  defective  from 
age.  This  may  probably  be  remedied  by  the  addition  of  a  little  fresh  caustic 
alkali  on  second  trial,  but  it  is  advisable  to  use  a  new  solution.  The  £  or  1  per 
cent,  sugar  solution  is  then  delivered  in  from  a  burettef  in  small  quantities  at 
a  time,  with  subsequent  boiling,  until  the  blue  colour  of  the  copper  solution  is 
just  discharged,  a  point  which  is  readily  detected  by  inclining  the  basin,  so  that 
the  colour  of  the  clear  supernatant  fluid  may  be  observed  against  the  white  sides 
of  the  basin.  Some  operators  use  a  small  thin  boiling  flask  instead  of  the  basin. 

It  is  almost  impossible  to  hit  the  exact  point  of  reduction  in 
the  first  titration,  but  it  affords  a  very  good  guide  for  a  more  rapid 

*  If  pure  cupric  sulphate  has  been  used,  and  the  solutions  mixed  only  at  the 
time  o£  titration,  there  need  be  very  little  fear  of  inaccuracy;  nevertheless  it  is 
advisable  to  verify  the  mixed  solutions  from  time  to  time.  This  may  be  done  by 
weighing  and  dissolving  0'95  gm.  of  pure  cane  sugar  in  about  50 'c.c.  of  water, 
adding  -J  c.c.  of  hydrochloric  acid,  and  heating  to  70°  C.  for  ten  minutes.  The 
acid  is  then  neutralized  with  sodium  carbonate  and  diluted  to  a  litre.  50  c.c.  of 
this  liquid  should  exactly  reduce  the  copper  in  10  c.c.  of  Fehling's  solution.  A 
standard  solution  of  invert  sugar,  which  will  keep  good  for  many  months,  may 
be  made  in  the  foregoing  manner;  it  should  be  of  about  23  per  cent,  strength,  and 
rendered  strongly  alkaline  with  soda  or  potash. 

f  The  instrument  should  be  arranged  as  described  on  page  12. 


328  SUGARS. 

and  exact  addition  of  the  sugar  solution  in  a  second  trial,  when  the 
sugar  may  be  added  with  more  boldness,  and  the  time  of  exposure 
of  the  copper  solution  to  the  air  lessened,  which  is  a  matter  of  great 
importance,  since  prolonged  boiling  has  undoubtedly  a  prejudicial 
effect  on  the  accuracy  of  the  process. 

The  volumetric  determination  of  reducing  sugars  is  dealt  with  by 
A.  R.  Ling  and  T.  Rendle,*  and  by  A.  R.  Ling  and  G.  C.  Jones,  t 
who  describe  a  modification  of  the  volumetric  method,  by  which 
results  are  obtained  equal  in  accuracy  to  those  of  the  gravimetric 
method  of  Brown,  Morris  and  Millar.f  The  average  error  is 
1  in  300.  The  principal  points  in  this  method  are  that  the  titratioii 
is  performed  in  a  boiling  flask  on  10  c.c.  of  undiluted  Fehling's 
solution,  and  that  ferrous  thiocyanate  is  used  as  indicator. 

The  indicator  is  prepared  by  dissolving  ammonium  thiocyanate  (1'5  grains),, 
ferrous  ammonium  sulphate  (1  gram),  in  concentrated  hydrochloric  acid  (2 -5  c.c.); 
and  water  (10  c.c.).  The  solution  so  obtained  has  invariably  a  red  colour,  due 
to  the  presence  of  ferric  salt  which  is  readily  reduced  by  addition  of  a  trace  of 
zinc  dust.  The  indicator  when  kept  for  some  hours  develops  the  red  coloration 
by  atmospheric  oxidation  and  must  again  be  reduced.  In  practice  it  is  found 
that  the  freshly  prepared  indicator  is  too  sensitive,  and  that  it  is  most  useful 
after  it  has  been  reduced  twice  with  zinc  dust.  The  method  of  titratioii  is  as 
follows: — Freshly  mixed  Fehling's  solution  (10  c.c.)  is  accurately  measured 
into  a  200  c.c.  boiling  flask  and  raised  to  boiling.  The  sugar  solution,  which  should 
be  adjusted  to  such  a  strength  that  20  to  30  c.c.  of  it  are  required  to  reduce  10  c.c. 
of  Fehling's  solution,  is  then  run  into  the  boiling  liquid  in  small  amounts,, 
commencing  with  5  c.c.  After  each  addition  of  sugar  solution,  the  mixture  is 
boiled,  the  liquid  being  kept  rotated.  About  a  dozen  drops  of  the  indicator  are 
placed  on  a  porcelain  or  opal  glass  slab,  and  when  it  is  judged  that  the  precipitation 
of  cuprous  oxide  is  complete,  a  drop  of  the  liquid  is  withdrawn  by  a  clean  glass 
rod  or  by  a  capillary  tube,  and  brought  in  contact  with  a  drop  of  the  indicator 
on  the  slab.  The  test  must  be  carried  out  rapidly.  It  is  also  essential  to  perform 
the  titration  as  rapidly  as  possible,  as  an  atmosphere  of  steam  is  then  kept  in  the 
neck  of  the  flask,  and  the  influence  of  atmospheric  oxygen  avoided.  At  the 
final  point,  the  liquid  is  boiled  for  about  ten  seconds.  Duplicate  titrations  should 
agree  to  O'l  c.c.  The  only  defect  of  this  indicator  is  that  it  cannot  be  used  with 
products  containing  ferric  iron.  The  titration  can  bo  carried  out  by  artificial 
light. 

To  standardize  Fehling's  solution,  Ling  and  Rendle  (fee.  cit.)  dissolve 
pure  sucrose  (0'95  gram)  in  water  (150  c.c.),  and  boil  for  one  minute  with  N/g 
hydrochloric  acid  (30  c.c.).  After  cooling  the  solution  is  neutralized  with  N/2 
sodium  hydroxide  (30  c.c.)  and  made  up  to  500  c.c.  It  is  then  titrated  as  above 
against  10  c.c.  of  Fehling's  solution.  The  relative  amounts  of  anhydrous 
invert  sugar,  dextrose,  and  maltose  required  to  reduce  a  fixed  volume  of  F  e  h  1  i  n  <r  "s 
solution  are  100  :  96  :  161  ;  but  in  the  case  of  the  first  two  sugars  the  ratio  varies. 
somewhat  with  concentration.  Ling  and  Jones||  give  a  table  to  correct  for 
this;  It  may  be  required  in  the  analysis  of  commercial  invert  sugar  when  the 
percentage  of  dextrose  and  of  laevulose  are  separately  reported. 

A  method  has  been  proposed  for  indicating  when  all  copper  has- 
been  precipitated  by  E.  F.  Harrison, §  and  is  very  sensitive.  It 
depends  upon  the  action  of  copper  salts  in  liberating  iodine  from 
iodide. 

?  Analyst,  1905,  30,  182;  1908,  33,  167.  t  Ibid,  1908,  33,  160. 

t  J-  Chem.  Soc.  Trans.,  1897,  71,  112.  !'  (loc.  cit.,  165). 

§  Pharm.  Journ.  1903,  170. 


SUGARS.  329 

The  indicator  is  prepared  by  boiling  0'05  gm.  of  starch  with  a  few  c.c.  of  water, 
adding  10  gm.  of  potassium  iodide  and  diluting  to  100  c.c.  These  quantities  need 
not  of  course  be  exactly  adhered  to,  but  too  much  starch  or  too  little  iodide 
lessens  the  delicacy  of  the  test ;  the  solution  should  be  prepared  as  required,  and 
not  used  after  it  has  been  made  more  than  two  or  three  hours.  In  use  about  0'5 
or  1  c.c.  of  this  solution  is  taken,  acidified  with  about  five  or  ten  drops  of  acetic 
acid,  and  one  drop  or  more  of  the  titration  liquid  added  ;  the  latter  need  not  be 
filtered.  As  long  as  unreduced  copper  is  present,  a  colour  is  produced,  varying 
from  red  to  blue,  and  of  greater  or  less  intensity,  according  as  the  end-point  is 
far  off  or  near.  The  production  of  no  colour  marks  the  end  of  the  reaction. 

This  indicator  gives  a  readily  observed  colour  with  one  drop  of  a  solution  of 
copper  sulphate  of  strength  1  in  20,000,  and  by  its  use  titration  of  Fehling's 
solution  with  a  suitable  sugar  solution  can  be  made  as  accurate  as  most  other 
volumetric  operations.  After  very  little  practice  one  titration  is  sufficient  for 
moderately  accurate  results,  but  greater  exactness  is  of  course  obtained  by 
repeating,  and  at  once  running  in  the  sugar  solution  almost  up  to  the  required 
amount  before  testing. 

When  the  exact  point  of  reduction  is  obtained,  it  is  assumed  that 
the  volume  of  sugar  solution  used  represents  0'05  gm.  of  grape  sugar 
or  glucose,  for  10  c.c.  Fehling's  solution  contain  0' 11  gm.  cupric 
oxide,  and  5  molecules  CuO  (397*85)  are  reduced  to  cuprous  oxide 
by  1  molecule  of  glucose  (180-1),  therefore  397'85  :  Ovll  =  180-1  :  0'05 
i.e.,  0-05  gm.  glucose  exactly  reduces  10  c.c.  Fehling's  solution. 

With  this  assumption,  however,  Soxhlet  does  not  agree,  but 
maintains  from  the  results  of  his  experiments  on  carefully  prepared 
standard  sugars  that  the  accuracy  of  the  reaction  is  interfered  with 
by  varying  concentration  of  the  solutions,  duration  of  the  experi- 
ment, and  the  character  of  the  sugar. 

For  example,  he  found  that  the  reducing  power  of  glucose,  invert 
sugar,  and  galactose  was  in  each  case  lowered  by  dilution  of  the 
Fehling's  solution,  whilst  that  of  maltose  was  raised,  and  that  of 
•milk  sugar  was  not  affected. 

The  remarks  which  Soxhlet  appends  to  his  experiments  are 
thus  classified  :— 

(!)  The  reducing  power  of  invert  sugar  for  alkaline  copper  solution  is- 
greatly  influenced  by  the  concentration  of  the  solutions:  a  smaller  quantity 
of  sugar  being  required  to  decompose  Fehling's  solution  in  the  undiluted  state 
than  when  it  is  diluted  with  1,  2,  3,  or  4  volumes  of  water.  It  is  immaterial 
whether  the  sugar  solution  be  added  to  the  cold  or  boiling  copper  reagent. 

(2)  If  invert  sugar  acts  on  a  larger  quantity  of  copper   solution   than   it   i» 
just  able  to  reduce,  its  reducing  power  will  be  increased,  the  increment  varying 
according  to  the  amount  of  copper  in  excess  and  the  concentration  of  the  cupric 
liquid  ;  in  the  previous  experiments  the  equivalents  varied  from  1  :    9'7  to  1  :  12-6, 
these  numbers  being  by  no  means  the  limit  of  possible  variation. 

(3)  In  a  volumetric  determination  of  invert  sugar  by  means  of  Fehling's 
solution,   the  amount  of  copper  reduced  by  each  successive  addition  of  sugar 
solution  is  a  decreasing  quantity  ;  the  results  obtained  are  therefore  perfectly 
empirical,  and  are  only  true  of  that  particular  set  of  conditions. 

(4)  The  statement  that  1  equivalent  of  invert  sugar  reduces  10  equivalents 
of   cupric   oxide  is  not  true,  the  hypothesis  that  Ov>  gm.  invert  sugar  reduces- 
100  c.c.  of  Fehling's  solution  being  shown  to  be  incorrect;  the  real  amount 
under  the   conditions  laid  down   by   Fehling   (1   volume   of  alkaline   copper 
solution,  4  volumes  of  water,  sugar  solution  -J-l  per  cent.)  being  97  c.c.  the  results 
obtained   under  this  hypothesis  are,   therefore,   3   per  cent,   too  low.     Where, 
however,   the  above  conditions  have  been  fulfilled,   the  results,   although  not 
absolutely,  arc  relatively  correct ;  not  so,  however,  those  obtained  by  gravimetric 


330  SUGARS. 

processes,  since  the  interference  of  concentration  and  excess  has  not  been  pre- 
viously recognized. 

These  facts,  however,  do  not  vitiate  the  process  as  carried  out 
under  the  well  recognized  conditions  insisted  on  in  the  directions 
for  titration  that  were  given  above.     If  these  are  adhered  to  it  is 
found  that  the  sugars  have  the  following  reducing  powers — 
10  c.c.  Fehling's  solution  are  completely  reduced  by 
0'05  gm.  glucose,  laevulose,  galactose 
0*0475  gm.  cane  sugar  (after  hydrolysis) 
0*0678  gm.  milk  sugar 
0-0807  gm.  maltose 
0*045  gm.  starch  (after  hydrolysis). 

Lowe  and,  more  recently,  Haines  have  advocated  the  substi- 
tution of  an  alkaline  solution  of  glycerine  for  the  alkaline  tartrate 
in  Fehling's  solution.  This  solution  is  said  to  keep  indefinitely, 
but  it  is  not  so  delicate  a  test  as  Fehling's. 

Determination  of  the  Cuprous  Oxide  by  Permanganate. — In  cases 
where  it  is  permissible  to  weigh  the  cuprous  oxide  produced  in  the 
Fehling  method,  R.  M.  Caven  and  A.  Hill*  have  devised  a  volu- 
metric method  by  which  the  amount  precipitated  can  be  determined 
in  a  shorter  time,  and  with  very  fair  accuracy. 

The  necessary  standard  solutions  are  potassium  permanganate 
about  N/5  strength,  the  exact  oxygen  value  of  which  is  known, 
and  an  oxalic  acid  solution  of  preferably  the  same  strength.  These 
must  be  titrated  together  in  the  same  way  as  in  the  actual  process. 

There  is  also  required  a  dilute  sulphuric  acid,  1  of  acid  to  3  of 
water. 

METHOD  or  PROCEDURE  :  The  cuprous  oxide,  whether  from  a  sugar  determination 
or  other  sources,  is  best  collected  on  an  asbestos  filter  connected  with  water  pump 
as  follows  : — Selected  fibrous  asbestos  is  cut  into  pieces  an  eighth  of  an  inch  in 
length,  digested  with  strong  sulphuric  acid  to  destroy  organic  matter,  then 
thoroughly  washed,  and  mixed  into  a  paste  with  water.  For  the  preparation  of 
the  filter  it  is  best  to  use  a  H  i  r  s  c  h '  s  porcelain  funnel  with  perforated  filter 
plate  ;  pouring  the  asbestos  cream  into  the  funnel,  and  applying  suction  by 
means  of  the  filter  pump  until  a  mat  of  asbestos,  suitable  to  receive  the  precipitated 
cuprous  oxide,  is  obtained.  After  the  removal  of  the  beaker  containing  the 
precipitated  cuprous  oxide  from  the  water-bath,  the  supernatant  liquid  is  at 
once  decanted  through  the  filter,  and  the  cuprous  oxide  remaining  in  the  beaker 
is  stirred  up  with  hot  water,  transferred  to  the  filter,  and  washed  until  free  from 
alkali.  The  last  traces  of  cuprous  oxide  need  not  be  removed  from  the  beaker, 
as  these  can  be  dissolved  later  on  in  a  little  of  the  acidified  permanganate  solution. 
The  asbestos  containing  the  cuprous  oxide  is  transferred  by  means  of  a  glass  rod 
to  a  porcelain  dish  about  eight  inches  in  diameter,  and  the  mass  thoroughly 
broken  up  with  water. 

If  the  quantity  of  oxide  does  not  exceed  0"2  gm.,  20  or  25  c.c.  of  the  standard 
permanganate  are  mixed  with  80  or  100  c.c.  of  the  dilute  sulphuric  acid  and 
poured  over  the  cuprous  oxide,  and  the  mixture  well  stirred  till  dissolved. 
Boiling  water  is  then  added  so  as  to  bring  the  temperature  to  45°  or  50°  C.,  but 
not  more  than  the  latter.  It  is  now  ready  for  titration.  It  is  found  best  to  add 
excess  of  oxalic  acid  solution,  after  adjusting  the  temperature  of  the  liquid,  and 
then  to  titrate  back  with  the  permanganate.  This  process  is  very  rapid,  owing  to 
the  use  of  the  filter  pump,  and  it  gives  consistent  and  good  results. 
*  J.  S.  C- 1. 16,  981  and  17,  124. 


SUGARS.  331 

The  amount  of  cuprous  oxide  corresponding  to  the  volume  of 
permanganate  used,  is  calculated  by  multiplying  the  oxygen  value 

of  the  number  of  c.c.  used  by  the  factor  8-946  I  ~       *    j.     The 

authors  use  the  factor  O5045  for  the  conversion  of  weight  of  Cu2O 
into  dextrose,  laevulose,  or  invert  sugar.  The  most  important  appli- 
cation of  this  process  is  its  use  in  the  analysis  of  sugars  by  the 
determination  of  their  cuprie  reducing  power.  For  this  purpose 
the  hot 'solution  of  sugar  is  introduced  into  excess  of  Fehling's 
solution  contained  in  a  beaker  immersed  in  the  water-bath,  and  the 
reduction  allowed  to  proceed  for  14  minutes,  according  to  the 
method  recommended  by  C.  O' Sullivan  (Watts's  Diet.,  art. 
Sugar).  The  method  can  of  course  be  used  for  the  determination  of 
copper  as  cuprous  oxide  in  cases  other  than  sugar  analysis. 

The  cuprous  iodide  process  may  be  also  used  to  ascertain  the 
amount  of  copper  not  precipitated  by  the  sugar.  Several  operators 
have  experimented  on  the  method,  the  best  form  of  which  is  that 
given  by  Schoorl.*  Results  agreeing  with  the  gravimetric  determi- 
nations can  be  obtained  if  a  fair  excess  of  potassium  iodide  be  used, 
and  if  this  be  added  to  the  alkaline  liquid  prior  to  acidification.  The 
author  describes  the  following  modification  as  being  convenient. 

METHOD  OF  PROCEDURE:  10  c.c.  of  Fehling's  copper  solution  (10  c.c.  = 
27 "75  c.c.  N/io  thiosulphate)  are  mixed  with  10  c.c.  of  Soxhlet's  alkaline 
tartrate  solution  in  an  Erlenmeyer  flask  of  200  c.c.  capacity.  Water  is  added 
to  make  up  50  c.c.  and  the  contents  of  the  flask  are  boiled  for  2  minutes  on  wire 
gauze,  over  which  is  placed  an  asbestos  ring  having  a  hole  6  cm.  in  diameter. 
The  liquid  is  then  quickly  and  thoroughly  cooled  under  the  tap,  and  10  c.c.  of 
a  20  per  cent,  solution  of  potassium  iodide  with  10  c.c.  of  25  per  cent,  sulphuric 
acid  (1'5  of  concentrated  acid  with  8 '5  of  water  by  volume)  are  added.  The 
iodine  liberated  is  immediately  titrated  with  decinormal  thiosulphate  with  the 
addition  of  starch  until  the  blue  colour'  changes  to  cream.  After  this  blank 
•experiment,  a  similar  one  in  every  respect  is  made,  introducing  a  known  quantity 
•of  sugar  solution  in  place  of  some  of  the  water  making  up  to  50  c.c.  Not  more 
than  90  mgm.  of  glucose  or  invert  sugar  or  125  mgm.  of  lactose  should  be  taken, 
and  in  the  determination  of  lactose,  the  liquids  should  be  boiled  for  5  minutes 
instead  of  2.  When  the  sugar  is  impure  care  should  be  taken  to  determine 
whether  there  is  any  impurity  capable  of  combining  with  iodine. 

T.  B.  Wood  and  R.  A.  Berryf  have  devised  the  following 
method  for  use  where  a  polarimetric  determination  is  not  possible. 

The  saccharine  solution  is  clarified  by  means  of  basic  lead  acetate,  the  cane 
sugar  present  inverted  by  treatment  with  dilute  acid,  the  solution  neutralized 
and  diluted  till  it  contains  from  0'5  to  I'O  per  cent,  of  reducing  sugar.  10  c.c.  of 
the  sugar  solution  are  now  added  to  50  c.c.  of  a  boiling  copper  solution  (23 '5  gin. 
of  copper  sulphate,  250  gm.  of  potassium  carbonate  and  100  gin.  of  potassium 
bicarbonate  per  litre)  and  the  mixture  boiled  for  10  minutes.  The  cuprous  oxide 
precipitated  is  filtered  off  in  a  Gooch  crucible,  washed  with  boiling  water,  and 
transferred  to  a  flask  filled  with  carbon  dioxide.  Tt  is  then  shaken  vigorously 
for  a  few  moments  with  25  c.c.  of  2|  per  cent,  solution  of  ferric  sulphate  in  25 
per  cent,  sulphuric  acid,  whereby  the  cuprous  oxide  dissolves,  reducing  an 
•equivalent  amount  of  ferric  sulphate  to  the  ferrous  salt.  The  latter  is  titrated 
with  a  solution  of  potassium  permanganate  of  such  strength  that  1  c.c.  is  equivalent 
to  O'Ol  gm.  of  copper. 

*  Zeii.  angew  Chcm.  1899,  633.  t  Proc.  Cambridge  Phil.  Soc.,  1903,  12,  97-98. 


332  SUGARS. 

3,     Determination  of  Glucose  by  Mercury. 

K  n  a  p  p  '  s  Standard  Mercuric  cyanide. — 10  gm.  of  pure  dry 
mercuric  cyanide  are  dissolved  in  about  600  c.c.  of  water  ;  100  c.c. 
of  caustic  solution  (sp.  gr.  1/145)  are  added,  and  the  liquid 
diluted  to  1  litre. 

Sachsse's  Standard  Mercuric  iodide. — 18  gm.  of  pure  dry 
mercuric  iodide  and  25  gm.  of  potassium  iodide  are  dissolved  in 
water,  and  to  the  liquid  is  added  a  solution  of  80  gm.  of  caustic 
potash ;  the  mixture  is  finally  diluted  to  1  litre. 

These  solutions,  if  well  preserved,  will  hold  their  strength  unaltered 
for  a  long  period. 

These  solutions  are  very  nearly,  but  not  quite,  the  same  in 
mercurial  strength,  Knapp's  containing  7*9365  gm.  Hg  in  the 
litre,  Sachsse's  7'9295  gm.  100  c.c.  of  the  former  are  equal  to 
100-1  c.c.  of  the  latter. 

Indicators  for  the  Mercurial  Solutions. — In  the  case  of  Fehling's 
solution,  the  absence  of  blue  colour  acts  as  a  sufficient  indicator, 
but  with  mercury  solutions  the  end  of  reaction  must  be  found  by 
an  external  indicator.  In  the  case  of  Knapp's  solution  the  end 
of  the  reaction  is  found  by  placing  a  drop  of  the  clear  yellowish 
liquid  above  the  precipitate  on  pure  white  Swedish  filter  paper, 
then  holding  it  first  over  a  bottle  of  fuming  HC1,  then  over  strong 
sulphuretted  hydrogen  water  ;  the  slightest  trace  of  free  mercury 
shows  a  light  brown  or  yellowish-brown  stain.  The  indicator  best 
adapted  for  Sachsse's  solution  is  a  strongly  alkaline  solution  of 
stannous  chloride  spotted  on  a  porcelain  tile.  An  excess  of  mercury 
gives  a  brown  colour. 

METHOD  OF  PROCEDURE  :  40  c.c.  of  either  solution  arc  placed  in  a  porcelain 
basin  or  a  flask,  diluted  with  an  equal  bulk  of  water,  and  heated  to  boiling.  The 
solution  of  sugar  of  £  per  cent,  strength  is  then  delivered  in  until  all  the  mercury 
is  precipitated,  theory  indicating  that  in  either  case  40  c.c.  should  be  reduced 
by  O'l  gm.  of  dextrose. 

The  results  of  Sox h let's  experiments  show  that  this  estimate 
is  entirely  wrong*  ;  nevertheless,  it  does  not  follow  that  these 
mercurial  solutions  are  useless.  It  is  found  that,  using  them  by 
comparison  with  Fehling's  solution,  it  is  possible  to  define  to 
some  extent  the  nature  of  mixed  sugars,  on  the  principle  of  indirect 
analysis. 

Knapp's  solution  is  strongly  recommended  by  good  authorities 
for  the  determination  of  diabetic  sugar  in  urine.  The  method  of 
using  it  is  described  in  the  section  on  Urinary  Analysis. 

The  behaviour  of  the  sugars  with  alkaline  mercury  solutions  was  tested  by 
Soxhlet  both  with  Knapp's  solution  and  Sachsse's  solution. 

He  found  that  different  results  are  obtained  from  Knapp's  solutions,  according 
as  the  sugar  solution  is  added  gradually  or  all  at  once  ;  when  gradually  added 
more  sugar  is  required  ;  with  Sachsse's,  however,  the  reverse  is  the  case. 

*  Careful  experiment  shows  that  40  c.c.  of  Sachsse's  solution  is  reduced  by 
0-1342  gm.  dextrose  or  0'1072  gm.  invert  sugar. 


SUGARS.  333 

To  get  comparable  results  the  sugar  must  be  added  all  at  once,  the  solution 
boiled  for  two  or  three  minutes,  and  the  liquid  tested  for  mercury,  always  using 
the  same  indicator  ;  in  using  the  alkaline  tin  solution  as  indicator,  0*200-0'202 
gm.  of  grape  sugar  was  always  required  for  100  c.c.  Knapp,  this  being  proved 
by  a  large  number  of  experiments.  It  is  remarkable  that  these  two  solutions, 
although  containing  almost  exactly  the  same  amount  of  mercury,  require  very 
different  quantities  of  sugar  to  reduce  equal  volumes  of  them.  This  is  shown  to 
be  due,  to  a  great  extent,  to  the  different  amounts  of  alkali  present  in  them. 

The  various  sugars  have  different  reducing  powers  for  the  alkaline 
mercury  solutions,  and  there  is  no  definite  relation  between  the 
amount  of  Knapp 's  and  Sachsse's  solutions  required  by  them  ; 
the  amount  of  Sachsse's  solution,  to  which  100  c.c.  Knapp's 
correspond,  varying  from  54*7  c.c.  in  the  case  of  galactose  to  74*8  in 
the  case  of  invert  sugar. 

The  two  mercury  methods  have  no  advantage  in  point  of  accuracy 
or  convenience  over  Fehl  ing's  method,  the  latter  having  the 
preference  on  account  of  the  great  certainty  of  the  point  at  which 
the  reduction  is  finished. 

The  mercury  methods  are,  however,  of  great  importance,  both 
for  the  identification  of  a  sugar  and  for  the  determination  of  two 
sugars  in  presence  of  each  other,  as  proposed  by  Sachsse.  For 
instance,  in  the  determination  of  grape  and  invert  sugars  in  presence 
of  each  other  there  are  the  two  equations  :  ax  +  by  =  F,  cx  +  dy  =  S. 

Where— 

a  =  number  of  c.c.  Fehling  reduced  by  1  gm.  grape  sugar. 
6=  ,,  ,,  ,,  .,        invert  sugar. 

c=  ,,  Sachsse  ,,  ,,         grape  sugar. 

d=  „  „  ,,  ,,         invert  sugar. 

F=  ,,  Fehling  used  for  1  vol.  sugar  solution. 

'S=  „  Sachsse 

x  =  amount  of  grape  sugar  in  gms.  in  1  vol.  of  the  solution. 
y=         „  invert  sugar 

It  need  hardly  be  mentioned  that  the  above,  like  all  other  indirect 
methods,  leaves  room  for  increased  accuracy  ;  but  nevertheless  the 
combination  of  a  mercury  method  with  a  copper  method,  in  the 
determination  of  a  sugar  whose  nature  is  not  exactly  knows,  gives 
a  more  serviceable  result  than  the  hitherto  adopted  plan,  by  which 
a  solution  that  reduced  10  c.c.  Fehling  was  said  to  contain  0-05  gm. 
of  sugar.* 

Taking  the  reducing  power  of  grape  sugar  =  100,  the  reducing 
powers  of  the  other  sugars  are  : — 

Fehling  (undiluted).    Knapp.  Sachsse. 

Grape  Sugar   100  100  100 

Invert  sugar 96*2  99-0  124-5 

Laevulose  (calculated)  . .  92-4  102-2  148-6 

Milk  sugar 70-3  64-9  70-9 

Galactose     93-2  83-0  74-8 

Hydrolyzed  milk  sugar  96-2  90'0  85-5 

Maltose        61-0  63-8  65-0 

*  J.  C.  /S-  Abstracts,  1830,  758. 


331  SUGARS. 

4.     S  i  d  e  r  s  k  y  '  s  Method. 

This  process  has  found  great  favour  among  French  sugar  experts, 
and  is  based  on  the  use  of  Soldaini's  cupric  solution,  which  was 
devised  to  remedy  the  faults  common  to  Fell  ling's  and  other  copper 
solutions  containing  tartrated  and  caustic  or  carbonated  alkalies. 

This  liquid  is  prepared,  according  to  Degener,  in  the  following 
manner  :— 40  gm.  of  cupric  sulphate  are  dissolved  in  water,  and, 
in  another  vessel,  40  gm.  of  sodium  carbonate  are  also  dissolved  in 
water.  The  two  solutions  are  mixed,  and  the  copper  precipitated 
in  the  state  of  hydrated  basic  carbonate.  The  precipitate  is  washed 
with  cold  water  and  dried.  This  precipitate  is  added  to  a  very 
concentrated  and  boiling  solution  of  potassium  bicarbonate  (about 
415  gm.)  and  agitated  until  the  whole  is  completely  or  nearly 
dissolved,  water  is  added  to  make  up  the  volume  to  1400  c.c.,  and 
the  whole  mass  heated  for  twro  hours  upon  a  water-bath.  The 
insoluble  matter  is  filtered  off,  and  the  filtrate,  after  cooling,  is  of 
a  deep  blue  colour.  The  sensibility  of  this  liquid  is  so  great  that  it 
gives  a  decided  reaction  with  0*0014  gm.  of  invert  sugar.  The  presence 
of  sucrose  in  the  solution  increases  this  sensibility  still  more. 

Sidersky  has  recently  offered  a  new  volumetric  method,  based 
upon  the  use  of  Soldaini's  solution.  With  sugars  the  same 
method  as  is  now  in  use  with  Fehling's  solution  can  easily  be 
followed,  watching  the  disappearance  of  the  blue  colour,  and  testing 
the  end  with  ferrocyanide  and  acetic  acid.  This  process  offers  no 
serious  objections  common  to  Fehling's  solution,  but  is  in- 
applicable to  coloured  sugar  solutions,  such  as  molasses,  etc.  For 
the  last  the  following  is  recommended  :— 

METHOD  OF  PROCEDURE  :  25  gm.  of  molasses  are  dissolved  in  100  c.c.  of  water 
and  subacetate  of  lead  added  in  sufficient  quantities  to  precipitate  the  impurities, 
and  the  volume  raised  to  200  c.c.  and  filtered.  To  100  c.c.  of  the  filtrate  are 
added  25  c.c.  of  concentrated  solution  of  sodium  carbonate,  agitated,  and 
filtered  again.  100  c.c.  of  the  second  filtrate  with  excess  of  lead  removed  are 
taken  for  analysis.  On  the  other  hand,  100  c.c.  of  Soldaini's  solution  are 
placed  in  a  flask  and  heated  five  minutes  over  an  open  flame.  The  sugar  solution 
is  now  added  little  by  little,  and  the  heating  continued  for  five  minutes.  Finally, 
the  heat  is  withdrawn  and  cooled  by  turning  in  100  c.c.  of  cold  water,  and 
filtered  through  a  Swedish  filter,  washed  with  hot  water,  letting  each  washing 
run  off  before  another  addition.  Three  or^  four  washings  will  generally  remove 
completely  the  alkaline  reaction.  The  precipitate  is  then  washed  through  a  hole 
in  the  filter  into  a  flask,  removing  the  last  trace  of  copper.  25  c.c.  of  normal 
sulphuric  acid  are  added  with  two  or  three  crystals  of  potassium  chlorate,  and  the 
whole  gently  heated  to  dissolve  completely  the  oxide  of  copper,  which  is  trans- 
formed into  copper  sulphate.  The  excess  of  sulphuric  acid  is  determined  by 
a  standard  ammonia  solution  (semi-normal)  of  which  the  best  indicator  is  the 
sulphate  of  copper  itself.  When  the  deep  blue  colour  gives  place  to  a  greenish 
tinge  the  titration  is  completed.  The  method  of  titration.  is  performed  as 
follows : — Having  cooled  the  contents  of  the  flask,  a  quantity  of  ammonia 
equivalent  to  25  c.c.  of  normal  sulphuric  acid  is  added.  From  a  burette  graduated 
into  one-tenth  c.c.  standard  sulphuric  acid  is  dropped  in  drop  by  drop,  agitating 
after  each  addition.  The  blue  colour  disappears  with  each  addition,  to  reappear 
after  shaking.  When  the  last  trace  of  ammonia  is  saturated  the  titration  is 
complete,  which  is  known  by  a  very  feeble  greenish  tinge.  The  number  of  c.c. 
is  read  from  the  burette,  which  is  equivalent  to  the  copper  precipitated.  The 
equivalent  of  copper  being  taken  at  31  '78,  the  normal  acid  equivalent  is  0*03178 


SUGARS.  335 

of  copper.  Multiplying  the  copper  found  by  3546  the  invert  sugar  is  found. 
A  blank  tit-ration  is  needed  to  accurately  determine  the  slight  excess  which  gives 
the  pale  green  tinge.* 

5.     Pavy's  modified  F  e  h  1  i  n  g  Process. 

This  method  consists  in  adding  ammonia  to  the  ordinary  F  eh  ling 
solution,  by  which  means  the  precipitation  of  cuprous  oxide  is 
entirely  prevented,  the  end  of  the  reaction  being  shown  by  the 
disappearance  of  the  blue  colour  in  a  perfectly  clear  solution.! 

The  solution  recommended  by  Pavy  is  made  by  mixing  120  c.c. 
ordinary  Fehling  solution:]:  (see  page  327)  with  300  c.c.  of  strong 
ammonia  (sp.  gr.  G'880),  adding  100  c.c.  of  a  10  per  cent,  caustic 
soda  solution  or  of  a  14  per  cent,  solution  of  potash,  and  diluting  to 
a  litre.  If  Fehling 's  solution  is  not  available,  Pavy's  solution 
may  be  made  directly  by  adding  a  cooled  solution  of  21 '6  gm. 
Rochelle  salt  and  18'4  gm.  of  soda  (or  25'8  gm.  of  potash)  to  a, 
solution  of  4-157  gm.  pure  cupric  sulphate,  adding  300  c.c.  of  strong 
ammonia  and  making  up  to  a  litre.  100  c.c.  Pavy's  solution 
=  10  c.c.  Fehling's  solution=0'05  gm.  of  glucose. 

As  ammoniacal  cuprous  solutions  are  readily  oxidized,  it  is 
important  to  exclude  air  from  the  liquid  during  titration.  The 
titration  should  be  made  in  a  small  boiling  flask,  through  the  cork 
of  which  the  elongated  end  of  the  burette  is  passed.  A  small  escape 
tube,  preferably  with  a  valve,  also  passes  through  the  same  cork, 
and  leads  into  a  vessel  containing  water  or  weak  acid,  to  condense  the 
ammonia.  Allen  has  found  a  layer  of  paraffin  over  the  liquid  an 
effective  means  of  excluding  air. 

In  carrying  out  the  titration  (100  c.c.  of  the  Pavy's  solution  is 
a  convenient  quantity  to  take)  a  few  pieces  of  pumice  or  pipe-stem 
are  added,  the  liquid  brought  to  boiling,  and  kept  boiling  whilst 
the  sugar  solution  is  gradually  run  in.  The  end-point  is  very  sharp. 
Whilst  rapid  manipulation  is  desirable,  the  solution  must  not  be 
run  in  too  quickly,  because  reduction  takes  place  more  slowly  than 
with  Fehling's  solution. 

The  method  is  well  adapted  for  the  examination  of  diabetic  urine 
and  milk,  also  mixtures  of  milk  and  cane  sugars,  and  certainly  has 
the  advantage  over  the  ordinary  Fehling  method  by  its  definite 
end  point. 

Z.  Peska||  gives  the  following  method  for  the  volumetric  deter- 
mination of  sugar  by  means  of  ammoniacal  copper  solution.  In 
order  to  avoid  the  oxidation  of  the  copper  oxide  in  solution,  a  layer 
of  vaseline  is  used  instead  of  the  usual  current  of  hydrogen.  Two 
solutions  are  prepared  :  6'927  gm.  of  the  purest  crystallized  copper 
sulphate  are  dissolved  in  water,  160  c.c.  of  25  per  cent,  ammonia 
added,  and  the  whole  made  up  to  500  c.c.  ;  34\5  gm.  of  Rochelle 

*  Report  of  Proceedings  of  Fifth  Annual  Convention  of  the  American  Association  of 
Official  Agricultural  Chemists  (1888).  t  C.  N.  49,  77. 

J  In  ammoniacal  solution  only  5  molecules  CuO  are  reduced  by  1  molecule  glucose 
instead  of  6  CuO,  as  in  Fehling's  solution,  hence  120  c.c.  of  the  latter  are  used  in 
making  Pavy's  solution  and  not  100  c.c.  ||  Chem  Zeit.  Rep.  1895,  257. 


336 


SUGARS. 


salt  and  10  gin.  of  caustic  soda  are  also  dissolved  and  diluted  to 
500  c.c. 

METHOD  OF  PROCEDURE  :  A  mixture  of  50  c.c.  of  each  liquid  is  heated  in  a 
beaker  under  a  layer  of  vaseline  oil  5  mm.  thick,  to  a  temperature  of  80°  C. 
The  sugar  solution  is  run  in  1  c.c.  at  a  time  for  the  first  test,  but  on  a  repetition 
the  whole  amount  may  be  added  at  once.  Towards  the  end  of  the  titration,  the 
temperature  must  be  raised  to  85°,  and  the  heating  continued  for  two  minutes 
when  working  on  either  glucose  or  invert  sugar,  four  minutes  for  maltose,  and 
six  minutes  for  milk  sugar.  Dextrin  increases  the  reducing  power  of  the  sugar 
in  this  solution  less  than  in  the  one  prepared  with  potash,  and  as  the  ammonia 
has  no  injurious  action,  the  whole  process  is  both  exact  and  convenient.  When 
sucrose  is  present,  1  gm.  of  it  has  a  reducing  action  equivalent  to  0'026  gin.  of 
invert  sugar.  In  the  determination  of  lactose  in  milk  the  albuminoids  should 
be  precipitated  with  lead  acetate  and  the  excess  of  lead  removed  by  sodium 
sulphate.  The  following  table  gives  directly  the  number  of  milligrams  of  each 
sugar  in  100  c.c.  of  solution. 


c.c. 

Glucose 

Invert 

Milk  Maltose. 

c.c 

Glucose. 

Invert 

Milk  Maltose. 

used 

sugar. 

sugar. 

used 

sugar. 

sugar. 

8 

'  997-8 

1049-2 

— 



50 

'  163-0 

173-2 

318-1 

360-0 

9 

889-4 

935-1 

— 



51 

159-8 

169-8 

311-9 

353-0 

10 

802-3 

844-6 





52 

156-8 

166-5 

306-0 

346-3 

11 

730-7 

770-0 



—      !  53 

153-9 

163-4 

300-3 

339-9 

12 

670-8 

.  707-6 





54 

151-1 

160-4 

294-8 

333-8 

13 

620-0 

654-5 





55 

148-4 

157-5 

289-4 

;V27*9 

14 

576-3 

608-7 





56 

145-7 

154-7 

2842 

322*2 

15 

538-4 

568-9 

1033-9 



57 

143-1 

152-0 

279-3 

316'7 

16 

505-2 

534-2 

971-4 

—        58 

140-6 

149-4 

274-5 

311*4 

17 

475-8 

503-3 

916-0 

1023-0     59 

138-2 

146-9 

209  •!> 

306-3 

18 

449-7 

475-7 

866-5 

968-8     60 

135-9 

144-5 

265-4 

301-3 

19 

426-3 

451-2 

822-3 

920-3  !  61 

133-7 

142-2 

261-1 

296-1 

20 

405-2 

429-0 

782-4 

876-3 

62 

131-5 

139-9 

256-9 

291-0 

21 

386-0 

408-8 

746-0 

836-4     63 

129-4 

137-7 

252-9 

287-0 

22 

368-7 

390-6 

7130 

800-0     64 

127-4 

135-5 

249-0 

282-0 

23 

352-8 

373-8 

682-7 

766-5 

65 

125-4 

133-4 

245-2 

278-3 

24 

338-2 

358-4 

654-8 

735-8 

66 

123-5 

131-4 

241-5 

274-1 

25 

324-8 

344-3 

629-2 

707-5 

67 

121-7 

129-5 

237-9 

270-0 

26 

312-4 

331-2 

605-5 

681-3 

68 

119-9 

127-6 

234-4 

206-1 

27 

300-9 

319-3 

583-5 

656-8 

09 

118-2 

125-7 

231-0 

262-3 

28 

290-3 

307-8 

563-1 

634-1 

70 

116-5 

123-9 

227-7 

258-0 

29 

280-3 

297-3 

544-1 

613-0 

71 

114-9 

122'2 

224-0 

255-0 

30 

271-1 

287-5 

526-2 

593-2 

72 

113-3 

120-5 

221-5 

251-5 

31 

262-4 

278-2 

509-5 

574-5 

73 

111-8 

118-9 

218-5 

248-1 

32 

254-2 

269-6 

493-8 

557-1 

74 

110-3 

117-3 

215-0 

244-S 

33 

246-6 

261-6 

479-1 

540-8 

75 

108-8 

115-8 

212-8 

241-0 

34 

239-3 

253-9 

465-3 

525-3 

76 

107-4 

114-3 

210-0 

238-1 

35 

232-6 

246-7 

452-2 

510-7 

77 

106-0 

112-8 

207-3 

235-3 

36 

226-1 

240-0 

439-8 

496-8 

78 

104-6 

111-4 

204-7 

232-3 

37 

220-0 

233-5 

428-1 

483-7 

79 

103-3 

110-0 

202-1 

229-1 

38 

214-3 

227-4 

417-0 

471-3 

80 

102-0 

108-6 

199-0 

226-0 

39 

208-8 

221-7 

406-5 

459-5 

81 

100-8 

107-2 



223-9 

40 

203-6 

216-2 

396-5 

448-3 

82 

99-6 

105-9 



2212 

41 

198-7 

211-0 

387-0 

437-6 

83 



104-6 



218-0 

42 

194-1 

206-0 

377-8 

427-4 

84 



103-4 



216-0 

43 

189-7 

201-3 

369-2 

417-7 

85 



102-2 



213-5 

44 

185-4 

196-7 

360-9 

408-4 

86 



101-1 



211-1 

45 

181-2 

192-3 

353-0 

399-5 

87 

^_ 



208-7 

46 

177-3 

188-1 

345-4 

391-0 

88 







206-1 

47 

173-5 

184-1 

338-1 

382-8 

89 







204-1 

48 

169-9 

180-3 

331-2 

374-9 

90 



*    

' 

201-i) 

49 

166-4 

176-7 

324-5 

367-3 

91 



— 

— 

199-7 

SUGARS.  337 

6.     Gerrard's    Cyano-cupric    Process. 

This  process,  as  improved  by  Gerrard*  and  A.  H.  Allen,  has 
proved  a  valuable  addition  to  the  processes  of  titration  based  on 
the  reducing  power  of  glucose.  It  has  the  advantage  over  Pa  vy '  s 
method  in  causing  no  evolution  of  ammonia  ;  moreover,  the  reduced 
solution  is  reoxidized  so  slowly  that  titration  may  with  reasonable 
expedition  even  be  conducted  in  an  open  dish.  The  process  is 
based  on  the  following  facts  : — When  a  solution  of  potassium 
cyanide  is  added  to  a  solution  of  copper  sulphate  a  colourless 
stable  double  cyanide  of  copper  and  potassium  is  formed,  thus  :• — 

CuSO4  +  4KCy  -  CuCy2, 2KCy + K2SO^ 

This  salt  is  not  decomposed  by  alkalies,  hydrogen  sulphide,  or 
ammonium  sulphide.  If  potassium  cyanide  be  added  to  F  e  h  1  i  n  g '  s 
solution  the  latter  is  decolourized,  the  above  double  salt  being 
formed  at  the  same  time,  and  if  the  colourless  solution  be  boiled 
with  glucose  no  cuprous  oxide  is  precipitated.  If  there  be  present 
•excess  of  Fehling's  solution  over  the  amount  capable  of  being 
decolorized  by  the  potassium  cyanide,  the  mixture  is  blue,  and 
when  it  is  boiled  with  a  reducing  sugar  the  extra  portion  is  reduced, 
but  no  cuprous  oxide  is  precipitated,  the  progress  of  the  reduction 
being  marked  by  the  gradual  and  final  disappearance  of  the  colour 
of  the  solution,  just  as  in  Pavy's  process. 

PROCESS  OF  TITRATION  :  10  c.c.  of  freshly  made  Fehling's  solution,  or  5  c.c.  of 
oach  of  the  constituent  solutions,  are  diluted  with  40  c.c.  of  water  in  a  porcelain 
<lish  and  heated  to  boiling.  An  approximately  5  per  cent,  solution  of  potassium 
cyanide  is  added  very  cautiously  from  a  burette  or  pipette  to  the  still  boiling  and 
well  agitated  blue  liquid  till  the  colour  is  just  about  to  disappear.  Excess  of 
cyanide  must  be  carefully  a  voided,  f 

10  c.c.  of  Fehling's  solution  are  now  accurately  measured  into  the  dish,  and 
the  sugar  solution  (of  about  £  per  cent,  strength  glucose)  run  in  slowly  from 
n  burette,  with  constant  stirring  and  ebullition,  till  the  blue  colour  disappears. 
Only  the  second  measure  of  Fehling's  solution  suffers  reduction.  The  volume 
of  sugar  solution  run  in  contains  0'05  gm.  of  glucose. 

Some  technical  applications  of  the  Solutions  to 
mixtures  of  various  Sugars. 

It  cannot  be  claimed  for  these  determinations  that  they  are 
absolutely  exact  ;  but  with  care  and  practice,  accompanied  with 
uniform  conditions,  they  are  probably  capable  of  the  best  possible 
results  whatever  methods  may  be  used. 

Cane-Sugar,  Grape-Sugar,  and  Dextrin.  J  The  solution  containing  these  three 
forms  is  first  titrated  with  the  usual  F  e  hi  i  n  g  solution  for  grape  sugar.  A  second 
portion  is  boiled  with  acetic  acid  (which  only  inverts  cane  sugar)  and  titrated. 
Finally,  a  third  portion  is  completely  hydrolyzed  with  sulphuric  acid  and  titrated. 

*  Year  Book  Pharm.   1892,  400. 

t  As  the  double  cyanide  solution  keeps  for  some  time,  a  stock  may  be  made  up,  so 
that  50  c.c.  contain  10  c.c.  of  Fehling's  solution,  and  that  volume  taken  for  each 
titration,  instead  of  going  through,  the  process  of  exact  dceolourization  every  time. 

J  Ward  cC-  Pcllctt,  Z  a.  C.  24,  27") 


338  SUGARS. 

The  difference  between  the  first  and  second  titratiens  gives  the"  cane  sugar,  and 
that  between  the  second  and  third  the  dextrin. 

Another  method  'might  be  suggested  as  follows  : — First  determine  the  grape- 
sugar  by  Fehling's  solution.  Next,  convert  the  sucrose  into  invert-sugar  by 
the  action  of  invertase — the  other  constituents  being  unaffected — and  determine 
the  grape-  and  invert-sugars  together  by  a  second  titration  with  Fehling. 
Finally,  the  solution  is  heated  for  some  hours  with  dilute  sulphuric  acid,  whereby 
the  cane-sugar  is  inverted  and  the  dextrin  hydrolyzcd  to  dextrose  :  the  solution 
is  then  titrated  for  the  third  time.  The  difference  between  the  third  and  second 
titrations  gives  the  dextrose-equivalent  of  the  dextrin  present.  (Or  the  dextrin 
may  be  determined  gravimetrically  by  pouring  the  aqueous  solution  into  a  large 
excess  of  rectified  spirit  in  a  weighed  beaker.  After  standing  till  the  precipitate 
is  completely  settled  the  liquid  is  poured  off,  and  the  dextrin  weighed). 

Milk-Sugar  and  Cane-Sugar. — If  the  determination  of  milk  sugar  is  alone 
required,  and  by  the  usual  Fehling  solution,  the  casein  and  albumen  must 
first  be  removed.  Acidify  the  liquid  with  a  few  drops  of  acetic  acid,  warm  until 
coagulation  is  effected,  and  filter.  Boil  the  filtrate  to  coagulate  the  albumen. 
Filter  again,  and  neutralize  with  soda  previous  to  treatment  for  sugar  by  the 
copper  test.  The  number  of  c.c.  of  Fehling's  solution  required,  multiplied  by 
0 '006786,  will  give  the  weight  of  milk  sugar  in  grams.  Direct  determination  by 
Pavy-Fehling  is  preferable  to  this  method.  Cane  sugar  in  presence  of  milk 
sugar  may  be  determined  as  follows  : — Dilute  the  milk  to  ten  times  its  bulk, 
having  previously  coagulated  it  with  a  little  citric  acid,  filter,  and  make  up  to 
a  definite  volume,  titrate  a  portion  with  Pavy-Fehling  solution,  and  note 
the  result.  Then  take  100  c.c.  of  the  filtrate,  add  2  gm.  of  citric  acid,  and  boil 
for  10  minutes,  cool,  neutralize,  make  up  to  200  c.c.,  and  titrate  with  copper 
solution  as  before.  The  difference  between  the  reducing  powers  of  the  solutions 
before  and  after  inversion  is  due  to  the  cane  sugar,  the  milk  sugar  not  being 
affected  by  citric  acid. 

Stokes  and  Bodmer*  have  experimented  largely  on  this  method,  and  with 
satisfactory  results.  The  plan  adopted  by  them  is  to  use  40  c.c.  of  P  a  v  y-  F  e  h  1  i  n  g 
liquid  (  =0'02  gm.  glucose),  and  to  dilute  the  sugar  solution  (without  previous 
coagulation),  so  that  from  6  to  12  c.c.  are  required  for  reduction.  By  using 
a  screw-clamp  on  the  rubber  burette  tube,  the  sugar  solution  is  allowed  to  drop 
into  the  boiling  liquid  at  a  moderate  rate.  If  Cu,O  should  be  precipitated  before 
the  colour  disappears,  a  fresh  trial  must  be  made,  adding  the  bulk  of  the  sugar 
at  once,  then  finishing  by  drops.  If,  on  the  other  hand,  the  sugar  has  been  run 
in  to  excess,  which  owing  to  the  rather  slow  reaction  is  easily  done,  fresh  trial 
must  be  again  made  until  the  proper  point  is  reached  ;  this  gives  the  milk  sugar. 
Meanwhile  a  measured  portion  of  the  mixed  sugar  solution  is  boiled  with  2  per 
cent,  of  citric  acid  for  at  least  30  minutes. f  For  example,  suppose  that  20  c.c. 
of  milk  had. been  diluted  to  200  c.c.,  50  c.c.  of  the  latter  should  be  boiled  with 
1  gram  of  citric  acid.  The  liquid  is  then  neutralized  with  ammonia,  made  up  to 
double  its  original  volume,  and  titrated  as  before. 

These  operators  have  determined  the  reducing  action  of  milk-, 
cane-,  and  grape-sugar  on  the  Pavy-Fehling  liquid,  the  result 
being  that  100  lactose  represents  respectively  52  glucose  or  49*4 
sucrose. 

F.  W.  Richardson  and  A.  Jaff  ej  state  that  the  copper  method 
of  determining  mixtures  of  sugars  in  milk  are  practically  valueless, 
having  obtained  far  more  consistent  results  by  polarimetric 
methods. 

*  Analyst  10,  62. 

t  The  authors  specified  10  minutes,  but  Watts  and  Temp  any  (Analyst.  1905, 
30,  119)  have  shown  that  30-40  minutes'  boiling  is  required. 

%  J,  S.  C.  I.  23,  309. 


SULPHUR.  33$ 

SULPHUR. 

8  =  32-07. 

Determination  in  Pyrites,  Ores,  Residues,  etc. 
1.     Alkalimetric  Method  (P  e  1  o  u  z  e). 

THIS  process,  designed  for  the  rapid  determination  of  sulphur  in 
iron  and  copper  pyrites,  has  hitherto  been  thought  tolerably 
accurate,  but  experience  has  shown  that  it  cannot  be  relied  upon 
except  for  rough  technical  purposes. 

The  process  is  based  on  the  fact  that  when  a  sulphide  is  ignited 
with  potassium  chlorate  and  sodium  carbonate  the  sulphur  is 
converted  entirely  into  sulphuric  acid,  which  expels  its  equivalent 
proportion  of  carbonic  acid  from  the  soda,  forming  neutral  sodium 
sulphate  ;  if  therefore  an  accurately  weighed  quantity  of-  the 
substance  be  fused  with  a  known  Aveight  of  pure  sodium  carbonate 
in  excess,  and  the  resulting  mass  titrated  with  normal  acid  in 
order  to  find  the  quantity  of  unaltered  carbonate,  the  proportion 
of  sulphur  is  readily  calculated  from  the  difference  between  the 
volume  of  normal  acid  required  to  saturate  the  original  carbonate 
and  that  actually  required  after  the  ignition. 

It  is  advisable  to  take  1  gm.  of  the  finely  levigated  pyrites  and 
5'3  gm.  of  pure  sodium  carbonate  for  each  assay  ;  and  as  5*3  gm. 
of  sodium  carbonate  represent  100  c.c,  of  normal  sulphuric  acid,  it 
is  only  necessary  to  subtract  the  number  of  c.c.  used  after  the 
ignition  from  100,  and  multiply  the  remainder  by  0*016,  in  order 
to  arrive  at  the  weight  of  sulphur  in  the  1  gm.  of  pyrites,  and  by 
moving  the  decimal  point  two  places  to  the  right  the  percentage 
is  obtained. 

EXAMPLE  :  1  gm.  of  finely  ground  FeS2  was  mixed  intimately  with  5 '3  gm. 
sodium  carbonate,  and  about  7  gm.  each  of  potassium  chlorate  and  decrepitated 
sodium  chloride  in  powder ;  then  introduced  into  a  platinum  crucible,  and 
gradually  exposed  to  a  dull  red  heat  for  ten  minutes ;  the  crucible  allowed  to 
cool,  and  warm  water  added  ;  the  solution  so  obtained  was  brought  on  a  moistened 
filter,  the  residue  emptied  into  a  beaker  and  boiled  with  a  large  quantity  of  water, 
brought  on  the  filter,  and  washed  with  boiling  water  till  all  soluble  matter  was 
removed  ;  the  filtrate  coloured  with  methyl  orange,  and  titrated.  67  c.c.  of  normal 
acid  were  required,  which  deducted  from  100,  left  33  c.c.  ;  this  multiplied  by 
0-016  gave  0"528  gm.  or  52'8  per  cent.  S. 

Burnt  Pyrites. — The  only  satisfactory  volumetric  method  of 
determining  the  sulphur  in  the  residual  ores  of  pyrites  is  that 
described  by  Watson,*  which  is  in  daily  use  in  large  alkali  works. 
In  order  to  avoid  calculation,  Watson  adopts  the  following 
method  : — 

Standard  hydrochloric  acid. — 1  c.c.  =0*02  gm.  Na20. 

Sodium  bicarbonate. — This  may  be  the  ordinary  commercial 
salt,  but  its  exact  alkalinity  must  be  ascertained  by  the  standard 

*  J.  S.  C.  7.  7,  305. 

7    2 


340  SULPHUR. 

acid.  Where  a  number  of  analyses  are  being  made,  a  good  quantity 
of  the  salt  should  be  well  mixed,  and  kept  in  a  stoppered  bottle. 
Its  exact  alkalinity  having  been  once  determined  it  will  not  alter, 
though  daily  opened. 

METHOD  OF  PROCEDURE  :  2  gm.  of  bicarbonate  is  placed  in  a  crucible  which 
may  be  either  of  platinum,  porcelain,  or  nickel,  and  to  it  .is  added  5'16  gm.  of  the 
finely  powdered  ore,  then  intimately  mixed  with  a  flattened  glass  rod.  Heat 
gently  over  a  Bun  sen  burner  for  5  or  10  minutes,  and  break  up  the  mass  with 
a  stout  copper  wire.  After  stirring,  the  heat  is  increased  and  continued  for 
10  or  15  minutes.  The  crucible  is  then  washed  out  with  hot  water  into  a  beaker. 
The  mixture  is  boiled  for  15  minutes,  filtered  into  a  flask,  the  residue  washed 
repeatedly  with  hot  water,  then  cooled  and  titrated  with  the  standard  acid,  using 
methyl  orange  as  indicator. 

JEXAMPLE  :  2  gm.  of  the  bicarbonate  originally  required  37 '5  c.c.  of  acid.  After 
ignition  with  the  ore,  28  c.c.  were  required.  The  difference  =9'5  c.c.,  divided  by 
5  will  give  I'D,  which  is  the  percentage  of  total  sulphur  in  the  ore. 

This  total  sulphur  includes  that  which  exists  as  soluble  sulphide, 
and  which  is  not  available  for  acid  making.  In  order  to  find  the 
amount. of  this  soluble  sulphur,  Watson  boils  5*16  gm.  of  the 
ore  with  5  c.c.  of  standard  sodium  carbonate  (1  c.c.  —  0*05  gm.  Na20) 
diluted  with  water,  for  15  minutes.  After  filtering  and  washing, 
the  filtrate  is  titrated  with  the  standard  hydrochloric  acid,  and  the 
difference  between  the  volume  used  and  that  which  was  originally 
required  for  5  c.c.  of  the  soda  solution  is  divided  by  5,  as  in  the 
case  of  the  former  process,  which  gives  at  once  the  percentage  of 
sulphur  existing  in  the  ore  in  a  soluble  form.  The  results  are  not 
absolutely  exact,  but  quite  near  enough  to  guide  a  manufacturer  in 
the  working  of  the  furnaces. 

This  method  is  not  available  for  unburnt  pyrites. 


2.    Determination  of  Sulphur  in  Coal  Gas. 

A  most  convenient  and  accurate  process  for  this  determination 
is  that  of  Wildenstein  (see  p.  3*50).  The  liquid  produced  by 
burning  the  measured  gas  in  a  Lethebyor  Vernon  Harcoujrt 
apparatus  is  well  mixed,  and  brought  to  a  definite  volume  ;  a  portion 
representing  a  known  number  of  cubic  feet  of  gas  is  then  poured 
ij$o  a  glass,  porcelain,  or  platinum  basin,  acidified  slightly  with 
HC1,  heated  to  boiling,  and  a  measured  excess  of  standard  barium 
chloride  added  ;  the  excess  of  acid  is  then  cautiously  neutralized 
with  ammonia  (free  from  carbonate),  and  the  excess  ,of  barium 
ascertained  by  standard  potassium  chromate  exactly  as  described 
on  p.  350. 

The  usual  method  of  stating  results  is  in  grains  of  sulphur  per 
100  cubic  feet  of  gas.  This  may  be  done  very  readily  by  using 
semi-normal  solutions  of  barium. chloride  and  potassium  chromate 
on  the  metric  system,  and  multiplying  the  number  of  c.c.  of  barium 
solution  required  by  the  factor  0-1234,  which  at  once  gives  the 
amount  of  sulphur  in  grains. 


SULPHIDES.  341 

3.     Determination  of  Sulphur  in  Sulphides  decomposable  by 
Hydrochloric  or  Sulphuric  Acid  (Weil). 

This  process,  communicated  to  me  by  M.  Weil,  is  based  on  the 
fact  that,  in  the  case  of  sulphides  where  the  whole  of  the  sulphur  is 
given  off  as  H2S  by  heating  with  HC1  or  H2SO4,  the  H2S  may  be 
evolved  into  an  excess  of  a  standard  alkaline  copper  solution. 
After  the  action  is  complete,  the  amount  of  Cu  left  unreduced  is 
determined  by  standard  stannous  chloride.  The  method  is  available 
for  the  sulphides  of  lead,  antimony,  zinc,  iron,  etc.  Operators 
should  consult  and  practise  the  methods  described  on  p.  198,  in 
order  to  become  accustomed  to  the  special  reaction  involved. 

METHOD  OF  PROCEDURE  :  From  1  to  10  gm.  of  material  (according  to  its 
richness  in  sulphur)  in  the  finest  state  of  division,  are  put  into  a  long-necked  flask 
of  about  200  c.c.  capacity,  to  which  is  fitted  a  bent  delivery  tube,  so  arranged  as 
to  dip  to  the  bottom  of  a  tall  cylinder,  containing  50  or  100  c.c.  of  standard 
copper  solution  made  by  dissolving  39 "523  gm.  of  cupric  sulphate,  200  gin.  of 
Rochelle  salt,  and  125  gm.  of  pure  caustic  soda  in  water,  and  diluting  to  1  litre 
(10  c.c.  — O'l  gm.  Cu).  When  this  is  ready,  a  few  pieces  of  granulated  zinc  are 
added  to  the  sulphide.  75  c.c.  of  strong  HC1  are  then  poured  over  them,  the 
cork  with  delivery  tube  immediately  inserted,  connected  with  the  copper  solution, 
and  the  flask  heated  on  a  sand-bath  until  all  evolution  of  H2S  is  ended.  The 
blue  solution  and  black  precipitate  are  then  brought  on  a  filter,  filtrate  and 
washings  collected  in  a  200  or  250  c.c.  flask,  and  diluted  to  the  mark ;  20  c.c.  of 
the  clear  blue  liquid  are  then  measured  into  a  boiling  flask,  and  evaporated  to 
10  or  15  c.c.  25  to  50  c.c.  of  strong  HC1  are  then  added,  and  the  standard  tin 
solution  dropped  in  while  boiling,  until  the  blue  gives  place  to  a  clear  pure 
yellow. 

Each  c.c.  of  standard  copper  solution  represents  0-5045  gm.  of 
sulphur.  The  addition  of  the  granulated  zinc  facilitates  the 
liberation  of  the  H2S,  and  sweeps  it  out  of  the  flask  ;  moreover,  in 
the  case  of  dealing  with  lead  sulphide,  which  forms  insoluble 
lead  chloride,  it  materially  assists  the  decomposition.  Alkaline 
tartrate  solution  of  copper  may  be  used  in  place  of  ammoniacal 
solution  if  so  desired. 

EXAMPLES  (Weil):  1  gin.  of  galena  was  taken,  and  the  gas  delivered  into 
T>0  c.c.  of  standard  copper  solution  (  =0'5  gm.  Cu).  After  complete  precipitation 
the  blue  liquid  was  diluted  to  200  c.c.  20  c.c.  of  this  required  12'5  c.c.  of 
stannous  chloride,  the  titre  of  which  was  16*5  c.c.  for  0*04  gm.  Cu.  Therefore 
16-5  :  0-04  :  :  12*5  :  0'0303.  Thus  200  c.c.  (  =1  gm.  galena)  represent  0'303  gm.  Cu. 
Then  0'5  gm.  Cu,  less  0'303  =0'197  gm.  for  1  gm.  galena  or  19'7  for  100  gm. 
Consequently  19 '7  xO '5045  =9 '94  per  cent.  S.  Determination  by  weight  gave 
9 '85  per  cent.  Again,  1  gm.  zinc  sulphide  was  taken  with  100  c.c.  copper  solution 
and  made  up  to  250  c.c.,  25  c.c.  of  which  required  14*3  c.c.  of  same  stannous 
chloride,  or  143  c.c.  for  the  1  gm.  sulphide.  This  represents  0'347  gm.  Cu.  Thus 
1 — 0'347  =0'653  gm.  Cu  (precipitated  as  CuS)  or  65 '3  per  100.  Consequently, 
65 -3  xO '5045  =32 '9  per  cent.  S.  Control  determination  by  weight  gave  33  per 
cent. 

Tiie  process  has  given  me  good  technical  results  with  Sb2S3,  but 
the  proportion  of  sulphur  to  copper  is  too  great  to  expect  strict 
accuracy. 


342  SULPHIDES    AND    SULPHITES. 

4.     Determination  of  Alkali  Sulphides  by  Standard 
Zinc  Solution. 

This  method,  which  is  simply  the  converse  of  that  described 
under  Zinc,  is  especially  applicable  for  the  technical  determination 
of  alkaline  sulphides  in  impure  alkalies,  mother-liquors,  etc. 

If  the  zinc  solution  be  made  by  dissolving  3*268  gm.  of  pure 
metallic  zinc  in  hydrochloric  acid,  supersaturating  with  ammonia, 
and  diluting  to  1  litre,  1  c.c.  will  respectively  indicate — 

0-0016  gm.  Sulphur 
0*0039     „    Sodium  sulphide 
0'00551  ,,    Potassium  sulphide 
0-0034     .,     Ammonium  sulphide. 

The  zinc  solution  is  added  from  a  burette  until  no  dark  colour  is 
shown  when  a  drop  is  brought  in  contact  with  solution  of  nickel 
sulphate  spread  in  drops  on  a  white  porcelain  tile. 

5.    Sulphurous  Acid  and  Sulphites. 

The  difficulties  formerly  presented  in  the  iodimetric  analyses  of 
these  substances  are  now  fortunately  quite  overcome  by  the  modifi- 
cation devised  by  Giles  and  Shearer.*  A  valuable  series  of 
experiments  on  the  determination  of  SO2,  either  free  or  combined, 
is  detailed  in  these  papers.  The  modification  is  both  simple  and 
exact,  and  consists  in  adding  the  weighed  SO2  or  the  sulphite  in 
powder  to  a  measured  excess  of  N/10  iodine  without  dilution  with 
water,  and  when  the  decomposition  is  complete,  titrating  back 
with  N/10  thiosulphate.  Very  concentrated  solutions  of  SO2  are 
cooled  by  a  freezing  mixture,  and  enclosed  in  thin  bulbs,  which 
can  be  broken  under  the  iodine  solution  :  this  is,  however,  not 
required  with  the  ordinary  preparations.  Sulphites  and  bisulphites 
of  the  alkalies  and  alkaline  earths,  also  of  zinc  and  aluminium,  may 
all  be  titrated  in  this  way  with  accuracy  ;  the  less  soluble  salts,  of 
course,  requiring  more  time  and  agitation  to  ensure  their 
decomposition.  A  preliminary  titration  is  first  made  with  a 
considerable  excess  of  iodine,  and  a  second  with  a  more  moderate 
excess  as  indicated  by  the  first  trial.  1  c.c.  N/10  iodine  =0-0032 
gm.  SO2. 

The  authors  found  that  when  perfectly  pure  iodine  and  neutral 
potassium  iodide  were  used  for  the  standard  solution,  its  strength 
remained  intact  for  a  long  period  ;  and  the  same  with  the  thio- 
sulphate, if  the  addition  of  about  2  gm.  of  potassium  bicarbonate 
to  the  litre  was  made,  and  the  stock  solution  kept  in  the  dark. 

From  a  large  number  of  experiments  they  also  deduced  the 
simple  law  of  the  ratio  between  any  given  percentage  of  S02  in 
aqueous  solution  at  15-4°  and  760  mm.,  and  the  specific  gravity  ; 
namely,  the  percentage  found  by  titration  multiplied  by  0-005 
and  added  to  unity  gives  the  sp.  gr. 

*  J.  S.  C.  7.  3,  197  and  4,  303. 


SULPHIDES,  SULPHITES,  ETC.  343 

In  cases  where  the  iodine  method  may  not  be  suitable,  W.  B. 
Giles  recommends  the  use  of  a  standard  ammoniacal  silver  nitrate. 
This  process  is  applicable  alike  to  SO2,  sulphites  and  bisulphites. 
The  silver  solution  may  conveniently  be  of  N/10  strength,  but 
before  use  ammonia  is  added  in  sufficient  quantity,  first  to  produce 
a  precipitate  of  silver  oxide,  then  to  dissolve  it  to  a  clear  solution. 
A  known  excess  of  this  solution  is  digested  in  a  closed  bottle,  with 
the  substance,  in  a  water-bath  for  some  hours,  the  result  of  which 
is  the  reduction  of  the  silver  as  a  bright  mirror  on  the  sides  of  the 
vessel.  The  filtered  liquid  and  washings  may  then  be  titrated  by 
thiocyanate  for  the  excess  of  silver,  or  the  mirror  together  with 
any  collected  on  the  filter  after  washing  and  burning  to  ash  may 
be  dissolved  in  nitric  acid  and  determined  by  the  same  process 
(p.  145).  1  c.c.  N/10  silver =00032  gm.  of  SOa. 

EXAMPLE  :  0'1974  gm.  of  chemically  pure  potassium  metabisulphite  was  weighed 
out  and  treated  as  above  described,  the  mirror  of  silver  and  a  little  on  the  filter 
determined  gave  O1918  gm.  of  metallic  silver,  which  multiplied  by  the  factor 
L-028  gives  0'19717  of  metabisulphite  or  99'9  %. 

This  method  is  very  useful  in  determining  the  percentage  of  the 

502  in  liquefied   sulphurous   acid,   which  is  now  found  in  large 
quantities  in  commerce.     By  cooling  down  this  substance  to  a  point 
where  it  has  no  tension,  small  bulbs  can  be  filled  with  facility  and 
sealed  up.     After  weighing  they  are  introduced  into  a  well-stoppered 
bottle  containing  an  excess  of  the  ammoniacal  silver,  and  the  stopper 
firmly  secured  by  a  clamp.     By  shaking  the  bottle  vigorously  the 
bulb  is  broken,  and  the  determination  is  then  conducted  as  above 
described. 

Ag2ON2O5  +  SO2  -f  xNH3  =  Aga  +  S08 + N2O5  -f  xNH3. 

6.     Analysis  of  Mixtures  o£  Alkali  Sulphides,  Sulphites, 
Thiosulphates,  and  Sulphates. 

The  determination  of  the  above-mentioned  substances  when 
existing  together  in  any  given  solution  presents  great  difficulty. 
Richardson  and  Aykroyd*  have,  however,  published  a  method 
which  seems  to  give  fairly  accurate  results. 

The  determination  of  the  SO3  in  such  a  mixture  cannot  be  done 
volume trically,  but  by  the  addition  of  about  5  gm.  of  tartaric  acid 
to  such  a  quantity  of  solution  of  mixed  thiosulphate,  sulphate,  and 
sulphite  as  would  be  usually  taken  for  analysis,  the  SO3  may  be 
precipitated  with  barium  chloride  in  the  cold.  The  precipitate  of 
BaSO4  contains  some  barium  sulphite,  but  this  is  easily  removed  by 
hot  dilute  HC1  and  boiling  waiter.  The  thiosulphate  produces  no 

503  whatever  under  these  circumstances,  whereas  in  the  presence 
of  a  mineral  acid,  sulphate  is  always  produced. 

The  sulphides  are  determined  by  standard  ammoniacal  zinc 
solution,  which  may  conveniently  be  of  such  strength  that  1  c.c.  = 
0-0016  of  S,  using  nickel  sulphate  solution  as  an  external  indicator. 

*  j.  s.  c.  i.  is,  171 


.4U4  SULPHIDES,    SULPHITES, 

This  zinc  solution  is  easily  made  from  pure  metallic  zinc  dissolved 
in  HC1,  and  the  precipitate  which  is  formed  by  adding  ammonia 
is  brought  into  clear  solution  by  a  moderate  excess  of  the  same 
re-agent. 

The  zinc  solution  is  also  used  for  removing  sulphides  from  a 
mixtures  of  these  with  thiosulphates,  sulphites,  and  sulphates  prior 
to  the  determination  of  the  latter  bodies.  In  this  case  it  is  onl  y 
necessary  to  add  a  slight  excess  of  the  zinc  solution,  and  filter  off 
the  precipitated  sulphide. 

The  authors  of  this  method  after  pointing  out  the  value  of 
Giles  and  Shearer's  method  of  determining  sulphites  by  iodine 
just  described,  mention  a  method  devised  by  themselves,  which 
enables  them  to  determine  not  only  sulphites  but  free  SO2,  not  only 
in  a  pure  state  but  in  mixtures  with  sulphates,  thiosulphates,  and 
sulphides.  They  avail  themselves  of  the  well-known  reaction  that 
when  iodine  is  added  to  a  neutral  sulphite,  neutral  sulphate  and 
an  equivalent  amount  of  hydriodic  acid  are  formed. 

NaaS03  +  I2  +  H20  -  Na2S04  +  2HI, 

and  the  acidity  of  the  solution  may  be  accurately  measured  by 
standard  alkali  and  methyl  orange. 

The  authors  state  that  the  best  plan  is  to  convert  all  sulphites 
to  bisulphites,  i.e.,  to  the  hydrogen  sulphite  of  the  base  :  this  is 
necessary  because  a  sulphite  may  be  alkaline  or  it  may  be 
exclusively  acid.  Sodium  bisulphite  is  quite  neutral  to  methyl 
orange,  and  by  titrating  the  solution  of  a  neutral  sulphite  with.  N/10 
sulphuric  acid,  using  methyl  orange,  a  point  is  reached  when  all 
the  sulphite  is  converted  into  the  acid  sulphite.  The  reason  for 
this  is  patent  when  the  reaction  which  takes  place  when  an  acid 
sulphite  acts  upon  iodine  is  considered— 


Here  is  a  new  factor,  inasmuch  as  the  titration  with,  alkali  and  with 
methyl  orange  as  indicator  is  concerned  ;  although  the  acid  sodium 
sulphite  is  neutral  to  methyl  orange,  the  acid  sodium  sulphate  is- 
acid  to  the  full  and  exact  extent  of  its  combining  power. 

Thus  one  molecule  of  sodium  bisulphite,  on  titration  with  N/10 
iodine,  liberates  acid  equivalent  to  three  molecules  of  sodium  or 
potassium  hydrate. 

EXAMPLE:  A  solution  containing  1'62  per  cent,  of  Na2SOs.7Aq  was  titrated. 
Iodine  solution  equivalent  to  9  '5  c.c.  N/io  I  ;  29  '9  c.c.  were  required  ;  the  mixture 
required  14'6  c.c.  of  N/1O  NaHO.  Now  9'5  c.c.  N/iO  I  and  14-6  c.c.  N/iO 
NaHO  are  in  the  ratio  of  2  ;  3  almost  exactly  ;  by  using  0'0126  as  the  factor  for 
the  c.c.  of  N/10  I  and  0'084  for  the  N/IO  NaHO,  both  results  give  1'64  per  cent. 
of  Na2S03.7Aq.  (Of  course  the  sulphite  solution  had  been  previously  titrated 
with  N/10  H2SO4  in  the  presence  of  methyl  orange.) 

As  the  details  of  calculation  may  be  somewhat  obscure  to  those  who  have  not 
experimented  in  this  direction,  the  working  out  of  an  actual  analysis  is  of  interest. 
A  solution  containing  one  per  cent,  of  pure  sodium  thiosulphate,  and  0'78  per 
cent,  of  sodium  sulphite,  was  titrated  upon  20  c.c.  of  iodine  ;  19'3  c.c.  were  required 
to  decolorize  ;  to  neutralize  with  methyl  orange  as  indicator  17'9  c.c.  of  N/IQ 
soda  were  required  ;  therefore  100  c.c.  of  the  mixture  required  103  -6  c.c.  iodine 


WITH   THIOSULPHATES   AND    SULPHATES.  345 

and  92*7  c.c.  of  N/io  soda  respectively  ;  the  c.c.  of  soda  x  0*0084  give  0*7787  as 
the  percentage  of  Na2SO3.7Aq,  and  this  figure-*- 0*0 126  (the  factor  for  1  c.c.  iodine 
in  Na,SO3.7Aq)  gives  61*8  c.c.,  and  this  subtracted  from  103*6  c.c.  of  total  iodine 
required  gives  41*8  c.c.,  and  thisxO'0248  gives  1*036  instead  of  1  per  cent,  of 
Xa2S,0>5Aq. 

The  advantage  of  this  method  is  better  seen  in  the  case  of 
a  complex  mixture,  where  one  must  remove  sulphides  or  other 
bodies  by  the  addition  of  an  alkaline  solution  of  zinc  or  other 
precipitating  agent.  The  alkaline  filtrate  is  speedily  brought  into 
a  condition  suitable  for  iodimetric  and  alkalimetric  titration  by 
the  method  proposed. 


:  A  solution  of  known  amounts  of  sodium  thiosulphate  and  sulphite 
was  treated  with  10  c.c.  of  a  strongly  ammoniacal  zinc-chloride  solution,  and  the 
mixture  was  titrated  with  it  until  it  gave  a  neutral  reaction  with  methyl  orange  ; 
it  was  now  made  to  1000  c.c.,  and  was  titrated  upon  a  known  volume  of  N/1O 
iodine,  using  starch  to  find  the  end-reaction  (which  is  otherwise  somewhat 
obscured  by  the  methyl  orange).  The  disappearance  of  the  blue  colour  and  the 
appearance  of  the  pinkish-purple  of  the  acidified  methyl  orange  is  both  interesting 
and  striking.  Titration  with  N/iO  NaHO  was  now  easily  accomplished.  The 
results  were  exact  in  the  case  of  thiosulphate.  and  very  slightly  in  excess  in  the 
case  of  sulphite. 

After  the  sulphite  and  thiosulphate  solution  lias  been  titrated 
upon  a  known  volume  of  N/i0  iodine,  the  sulphate  formed  is  de- 
termined by  barium  at  a  boiling  heat  in  the  presence  of  a  little 
dilute  HC1.  Any  sulphate  in  the  original  solution  is,  of  course, 
determined  by  the  tartaric  acid  method  and  deducted  from  the 
result.  Ammonium  tartrate  must  be  avoided  in  the  process, 
owing  to  its  solvent  action  on  barium  sulphate. 

The  process  is  only  strictly  applicable  in  the  absence  of  organic 
matter.  When  that  is  present  it  is  preferable  to  use  the  iodine 
process  as  follows  :— 

(1)  Total  Iodine  value  of  solution  =H2S  +H2S03  +  H2S203  determined  by 
running  known  volume  of  solution  into  excess  of  N/io  iodine  acidified  with 
hydrochloric  acid  and  bringing  back  with  N/1O  thiosulphate.  This  gives  A  =  H2S  + 
H2S00  +  H2S203  accurately  without  loss. 

"(2)  "  Iodine  value  of  H  S. — Add  excess  of  ammoniacal  zinc  chloride,  filter, 
wash  ;  wash  ZnS  into  N/io2  iodine  and  hydrochloric  acid  (as  described  on  page  81). 
This  gives  B  =H2S  accurately.  Then  A  -B  gives  H2S03  +H2S2O3  accurately. 

(3)  Iodine  value  of  H2S203. — To  filtrate  from  B  add  acid  to  exactly  neutralize- 
with  methyl  orange  as  indicator. 

Then  titrate  with  iodine  and  starch  C=H2S203  +H2S03   in   f2trate 

Then  with  N/iO  alkali     „  f  alkali  N/1Q  D  =     H2S03     „ 

The  f  N/io  alkali   used  =  iodine  equivalent  of   H2S03  in  the  filtrate  (not  in  the- 
original  solution,  as  oxidation  occurs  in  filtration)  accurately. 
And  G-D=H2S203  accurately. 

(4)  Iodine  value  of  H.,SO3.— Got  by  difference 

H2S  0^  +  H2S2O3  =  A  -  B  ace  urately 

H.,S,03=C-D 
H2S03  =  A-B-(C-D) 

The  procedure,  so  modified,  is  to  obtain  the  sulphurous  acid  by 
difference  in  place  of  direct  determinations.  Thiosulphate  suffers 
no  appreciable  oxidation  on  filtration  ;  sulphite  does.  Hence  by 


346  .        SULPHIDES    AND    SULPHITES 

determining  tliiosulphate  and  sulphide  accurately  sulphite  is  got 
by  difference  accurately.  This  difference  figure  is  always  rather 
higher  than  the  one  deduced  acidi metrically  (D,  above.) 

Another  series  of  processes  for  ascertaining  the  proportions  of 
mixtures  of  sulphuretted  hydrogen,  sulphurous  and  thiosulphuric 
acids  has  been  worked  out  by  W.  Feld*.  The  methods 
described  are  applicable  to  the  alkali  or  alkaline  earth  salts  of  the 
above  acids,  even  when  present  in  small  quantities. 

(1)  SULPHIDES. — Alkali  or  alkaline  earth  sulphides  evolve  the  whole  of  their 
sulphur  as  H2S  when  boiled  with  a  concentrated  solution  of  magnesium  chloride 
in  an  atmosphere  of  00^.     The  powdered  and  moistened  sample  is  placed  in 
a  300  c.c.   Erlenmeyer  flask  provided  with  a  doubly-bored  rubber  stopper. 
Through  one  hole  a  small  tap-funnel  passes  to  the  bottom  of  the  flask,  through 
the  other  a  glass  tube  leads  to  four  sets  of  potash-bulbs  in  series.     The  last  of 
these  is  connected  to  a  10  litre  bottle  acting  as  aspirator.     The  neck  of  the  tap- 
funnel  is  connected  to  a  supply  of  C02,  which  must  have  no  action  on  a  solution 
of  iodine.     The  first  set  of  potash-bulbs  is  empty,  the  second  and  third  contain 
rather  more  iodine  solution  than  will  suffice  to  absorb  all  the  H2S  evolved,  the 
fourth  contains  N/io  thiosulphate  solution,  to  take  up  any  iodine  carried  over  by 
the  C02.     About  1  litre  of  C02  is  first  passed,  in  order  to  displace  the  air  in  the 
apparatus,  the  tap  of  the  funnel  is  then  closed,  and  about  20  c.c.  of  25  per  cent, 
magnesium  chloride  solution  introduced.     Connection  is  now  made  to  the  supply 
of  COL,  and  the  magnesium  chloride  solution  run  into  the  flask,  the  contents  of 
which  are  slowly  heated  to  boiling  in  a  current  of  C02  passing  at  the  rate  of 
10  litres  in  three-quarters  of  an  hour.     The  operation  is  usually  ended  when 
5  litres  have  passed.     The  contents  of  the  potash-bulbs  are  finally  washed  out 
and  titrated  ;  the  reactions  are — 

BaS  +  MgCl2  +C02  +H2O  =BaCl2  +  MgC03  +H2S,  and  H2S  +I2  =2111  +  S. 

Test  analyses  with  BaSH.OH  +5H20  gave  good  results. 

(2)  SULPHITES  are  determined  in  the  same  apparatus  and  in  the  same  way, 
hydrochloric  acid  taking  the  place  of  magnesium  chloride. 

(3)  THIOSULPHATES  evolve  some  H2S  when  treated  with  hydrochloric  acid. 
The  following  method  is  found,  however,  to  give  accurate  results  : — The  thio- 
sulphate is  first  converted  (by  titration  with  iodine  solution)  into  tetrathionate. 
The  solution  of  the  tetrathionate,  diluted  with  50  c.c.  of  water,  is  placed  in  the 
flask  with  excess  of  aluminium  foil,  and  treated,  in  an  atmosphere  of  C02,  with 
dilute  hydrochloric  acid  in  the  cold.     The  reduction  to  H2S,  which  is  collected 
as  before,  takes  place  quantitatively  according  to  the  equation — 

Na2S40G  +20HC1  +3A12  =2NaCl  +  3A12C16  +  6H20  +4H2S. 

(4)  THIOSULPHATE  IN  PRESENCE  OF  SULPHITE. — This  determination  is  made 
by  method  (3).     The  titration  with  iodine  oxidizes  the  sulphite  into  sulphate, 
which  is  not  affected  by  nascent  hydrogen. 

(5)  SULPHITE  IN  PRESENCE  OF  THIOSULPHATE. — Excess  of  mercuric  chloride 
is  added  to  the  substance  ;   the  thiosulphate  is  thus  converted   into  mercuric 
sulphide — 

Na2S20.j  +HgCl2  +H20  =Na2S04  +HgS  +2HC1, 

whilst  the  sulphite  is  not  affected  and  is  determined  by  (2). 

(6)  SULPHIDE,  SULPHITE,  AND  THIOSULPHATE. — The  sample  is  first  distilled 
with  magnesium  chloride,   as  described  in  (1).     This  gives  the  sulphide.     The 
potash-bulbs  are  then  refilled,  excess  of  mercuric  chloride  added  to  the  cold 
contents  of  the  flask,  which  are  then  distilled  with  hydrochloric  acid  as  described 
under  (2).     This  gives  the  sulphite.     The  thiosulphate  is  determined  in  a  fresh 
sample  by  titrating  with  iodine,  by  which  the  sulphide  is  oxidized  to  sulphur  and 
the  sulphite  to  sulphate,  and  then  reducing  by  nascent  hydrogen  as  described 
under  (3). 

*  Die.  Chem.  Tnd.  1898,  372. 


IN   PRESENCE    OF   THIOSULPHATES.  347 

When,  in  addition  to  the  alkali  or  alkaline  earth  salts  of  the  acids  considered, 
the  substance  contains  polysulphides,  free  sulphur,  and  sulphides  of  the  heavy 
metals,  the  difficulties  are  much  greater,  and  the  author  is  working  for  further 
information.  In  the  meantime  he  has  obtained  satisfactory  results  as  follows : — 
Free  sulphur  is  extracted  by  carbon  disulphide,  and  weighed  after  evaporation  of 
the  solvent.  The  sulphur  present  as  sulphide  is  then  determined  by  method  (1). 
In  this  operation  the  sulphur  of  the  polysulphides  is  evolved  partly  as  H2S,  the 
remainder  separating  in  the  free  state.  The  latter  part  is  extracted  by  carbon 
disulphide.  If  a  sulphite  is  present,  however,  some  thiosulphate  is  formed.  The 
solution  is  now  titrated  with  iodine,  during  which  operation  the  sulphur  present 
as  ferrous  sulphide  separates  in  the  free  state  and  is  extracted  with  carbon 
disulphide.  The  solution  is  now  treated  by  method  (3)  to  determine  the 
thiosulphate.  The  presence  of  other  polythionic  acids  introduces  an  error  here. 
In  solid  substances  sulphites  may  occur  in  presence  of  polysulphides  :  in  this 
case  they  are  determined  by  treatment  with  mercuric  chloride  and  distillation 
with  hydrochloric  acid,  according  to  method  (2). 

Lunge  and  Smith's  methods  for  the  same  purpose  are  described 
in  J.  S.  C.  I.  ii.  463,  and  also  in  the  fifth  edition  of  this  book. 

SULPHURETTED    HYDROGEN. 

H2S  =  34-09. 

1  c.c.  N/10  arsenious  solution— 0*002557  gm.  H2S. 
1.     By  Arsenious  Acid  (M  o  h  r). 

THIS  residual  process  is  far  preferable  to  the  direct  titration  of 
sulphuretted  hydrogen  by  iodine.  The  principle  is  based  on  the 
fact  that  when  H2S  is  brought  into  contact  with  an  excess  of 
arsenious  acid  in  hydrochloric  acid  solution,  arsenic  sulphide  is 
formed  ;  1  eq.  of  arsenious  acid  and  3  eq.  of  sulphuretted  hydrogen 
produce  1  eq.  of  arsenic  sulphide  and  3  eq.  of  water, 

As2O3 + 3H2S  -  As2S3  +  3H2O . 

The  excess  of  arsenious  acid  is  found  by  N/IO  iodine  and  starch, 
as  on  p.  139.  In  determining  the  strength  of  sulphuretted  hydrogen 
water  the  following  plan  may  be  pursued. 

METHOD  OF  PROCEDURE  :  A  measured  quantity,  say  10  c.c.,  of  N/1O  arsenious 
solution  is  put  into  a  300  c.c.  flask,  and  20  c.c.  of  sulphuretted  hydrogen  water 
added,  well  mixed,  and  sufficient  HC1  added  to  produce  a  distinct  acid  reaction  ; 
this  produces  a  precipitate  of  arsenic  sulphide,  and  the  liquid  itself  is  colourless. 
The  whole  is  then  diluted  to  300  c.c.,  filtered  through  a  dry  filter  into  a  dry 
vessel,  100  c.c.  of  the  filtrate  taken  out  and  neutralized  with  sodium  bicarbonate, 
then  titrated  with  N/io  iodine  and  starch.  The  quantity  of  arsenious  acid  so 
found  is  deducted  from  the  original  10  c.c.,  and  the  remainder  multiplied  by  the 
requisite  factor  for  H2S. 

The  determination  of  H2S  contained  in  coal  gas  may  by  this 
method  be  made  very  accurately  by  leading  the  gas  very  slowly 
through  the  arsenious  solution,  or  still  better,  through  a  dilute 
solution  of  caustic  alkali,  then  adding  arsenious  solution,  and 
titrating  as  before  described.  The  apparatus  devised  by  Mohr 
for  this  purpose  is  arranged  as  follows  : — 

The  gas  from  a  common  burner  is  led  by  means  of  a  vulcanized  tube  into  two 
successive  small  wash-bottles  containing  the  alkaline  solution  ;  from  the  last  of 


348  SULPHURETTED  HYDROGEN. 

these  it  is  led  into  a  large  Wo u Iff 's  bottle  filled  with  water.  The  bottle  has 
two  necks,  and  a  tap  at  the  bottom  ;  one  of  the  necks  contains  the  cork  through 
which  the  tube  carrying  the  gas  is  passed  ;  the  other,  a  cork  through  which  a 
good-sized  funnel  with  a  tube  reaching  to  the  bottom  of  the  bottle  is  passed. 
When  the  gas  begins  to  bubble  through  the  flask,  the  tap  is  opened  so  as  to  allow 
the  water  to  drop  rapidly ;  if  the  pressure  of  gas  is  strong,  the  funnel  tube  acts 
as  a  safety  valve,  and  allows  the  water  to  rise  up  into  the  cup  of  the  funnel. 
When  a  sufficient  quantity  of  gas  has  passed  into  the  bottle,  say  six  or  eight 
pints,  the  water  which  has  issued  from  the  tap  into  some  convenient  "vessel  is 
measured  in  cubic  inches  or  litres,  and  gives  the  quantity  of  gas  which  has 
displaced  it.  In  order  to  ensure  accurate  measurement,  all  parts  of  the  apparatus 
must  be  tight. 

The  flasks  are  then  separated,  and  into  the  second  5  c.c.  of  arsenious  solution  are 
placed,  and  acidified  slightly  with  HC1.  If  any  traces  of  a  precipitate  occur  it  is 
set  aside  for  titration  with  the  contents  of  the  first  flask,  into  which  10  c.c.  or  so 
of  arsenious  solution  are  put,  acidified  as  before,  both  mixed  together,  diluted  to 
a  given  measure,  filtered,  and  a  measured  quantity  titrated  as  before  described. 

This  method  does  not  answer  for  very  crude  gas  containing  large 
quantities  of  H2S  unless  the  absorbing  surface  is  largely  increased. 

2.    By  Permanganate    (M  o  ft  i)1 

If  a  solution  of  H2S  is  added  to  a  dilute  solution  of  ferric  sulphate, 
the  ferric  salt  is  reduced  to  the  ferrous  state,  and  free  sulphur 
separates.  The  ferrous  salt  so  produced  may  be  measured  accurately 
by  permanganate  without  removing  the  separated  sulphur.  Ferric 
sulphate,  free  from  ferrous  compounds,  in  sulphuric  acid  solution, 
is  placed  in  a  stoppered  flask,  and  the  solution  of  H2S  added  to  it 
with  a  pipette  ;  the  mixture  is  allowed  to  stand  half  an  hour  or  so, 
then  diluted  considerably,  and  permanganate  added  until  the  rose 
colour  appears.  55.85  Fe  _  17-04  H2S 

or  each  c.c.  of  N/10  permanganate  represents  0*001704  gm.  of  H2S. 
The  process  is  considerably  hastened  by  placing  the  stoppered  flask 
containing  the  acid  ferric  liquid  into  hot  water  previous  to  the 
addition  of  H2S,  and  excluding  air  as  much  as  possible. 

3.     By  Iodine. 

Sulphuretted  hydrogen  in  mineral  waters  may  be  accurately 
determined  by  iodine  in  the  following  manner  : — 

METHOD  OF  PROCEDURE  :  10  c.c.  or  any  other  necessary  volume  of  N/ioo 
iodine  solution  are  measured  into  a  500  c.c.  flask,  and  the  water  to  be  examined 
added  until  the  colour  disappears.  5  c.c.  of  starch  indicator  are  then  added,  and 
N/1OO  iodine  until  the  blue  colour  appears  ;  the  flask  is  then  filled  to  the  mark 
with  pure  distilled  water.  The  respective  volumes  of  iodine  and  starch  solution, 
together  with  the  added  water,  deducted  from  the  500  c.c.,  will  show  the  volume 
of  water  actually  titrated  by  the  iodine.  A  correction  should  be  made  for  the 
excess  of  iodine  necessary  to  produce  the  blue  colour. 

Fresenius*  examined  the  sulphur  water  of  the  Grindbrunnen, 
in  Frankfurt  a.  M.,  both  volume  trie  ally  and  gravimetrically  for 
H2S  with  very  concordant  results.  361*44  gm.  of  water  (correction 
for  blue  colour  being  allowed)  required  20'14  c.o.  of  iodine,  20'52 
c.c.  of  which  contained  0-02527  gm.  of  free  iodine =H2S  0-0092  gm. 

"     *'Z    a.  C.  14,  321. 


SULPHATES.  349 

per  million.  444-65  gm.  •  of  the  same  water  required,  under  the 
same  conditions,  25*05  c.c.  of  the  same  iodine  solution =H2S 
0-0092  gm.  per  million.  Gravi  metrically  the  H2S  was  found  to 
be  0*0094  gm.  per  million. 

SULPHURIC    ACID    AND    SULPHATES. 

Monohydrated  Sulphuric  Acid. 

H2SO4-98*086. 
Sulphuric  Anhydride. 

S03  =  80-07. 
1.     Mohr's    Method. 

IN  my  opinion  the  determination  of  sulphuric  acid  in  most  cases 
is  more  easily  obtained  by  gravimetric  than  by  volumetric  methods, 
but  there  are  circumstances  in  which  the  latter  are  useful.  The 
indirect  process  devised  by  C.  Mohr*  consists  in  adding  a  known 
volume  of  barium  solution  to  the  compound,  more  than  sufficient 
to  precipitate  the  S03.  The  excess  of  barium  is  converted  into 
carbonate,  and  titrated  with  .normal  acid  and  alkali. 

Normal  barium  chloride  is  made  by  dissolving  122*161  gm.  of 
pure  crystals  of  chloride  in  the  litre  ;  this  solution  likewise  suffices 
for  the  determination  of  SO3  by  the  direct  method. 

METHOD  OF  PROCEDURE  :  If  the  substance  contains  a  considerable  quantity 
of  free  acid,  it  must  be  brought  nearly  to  neutrality  by  adding  pure  sodium 
carbonate  ;  if  alkaline,  slightly  acidified  with  hydrochloric  acid  ;  a  round  number 
of  c.c.  of  barium  solution  in  excess  is  then  added,  and  the  whole  digested  in  a  warm 
place  for  some  minutes;  the  excess  of  barium  is  precipitated  by  a  mixture  of 
carbonate  and  caustic  ammonia  in  slight  excess  ;  if  a  piece  of  litmus  paper  be  thrown 
into  the  mixture,  a  great  excess  may  readily  be  avoided.  The  precipitate  contain- 
ing both  sulphate  and  carbonate  is  now  to  be  collected  on  a  filter,  thoroughly 
washed  with  boiling  water,  and  titrated. 

The  difference  between  the  number  of  c.c.  of  barium  solution 
added  and  that  of  normal  acid  required  for  the  carbonate  will  be 
the  measure  of  the  sulphuric  acid  present  ;  each  c.c.  of  barium 
solution  is  equal  0*040  gm.  SO3. 

EXAMPLE  :  2  gm.  of  pure  and  dry  barium  nitrate  and  1  gm.  of  pure  potassium 
sulphate  were  dissolved  separately,  mixed,  and  precipitated  hot  with  carbonate 
and  free  ammonia  ;  the  precipitate,  after  being  thoroughly  washed,  gave  1'002  gm. 
potassium  sulphate,  instead  of  1  gm. 

For  technical  purposes  this  process  may  be  considerably  shortened 
by  the  following  modification,  which  dispenses  with  the  washing  of 
the  precipitate. 

The  solution  containing  the  sulphates  or  sulphuric  acid  is  first  rendered 
neutral  ;  normal  barium  chloride  is  then  added  in  excess,  then  normal  sodium 
carbonate  in  excess  of  the  barium  chloride,  and  the  volume  of  both  solutions 
noted  ;  the  liquid  is  then  made  up  to  200  or  300  c.c.  in  a  flask,  and  an  aliquot 
portion  filtered  off  and  titrated  with  normal  acid.  The  difference  .between  the 
barium  chloride  and  sodium  carbonate  gives  the  sulphuric  acid. 
*jnn.  dei-  Chem.  u.  Pharm.  90,  16o. 


350  SULPHATES. 

The  solution  must  of  course  contain  no  substance  precipitable  by 
sodium  carbonate  except  barium  (or  if  so,  it  must  be  previously 
removed) ;  nor  must  it  contain  any  substance  precipitable  by  barium, 
such  as  phosphoric  or  oxalic  acid,  etc. 

2.     Titration  by  Barium  Chloride  and  Potassium 
Chromate  (W  i  1  d  e  n  s  t  e  i  n). 

To  the  hot  solution  containing  the  SO3  to  be  determined  (which 
must  be  neutral, — or  if  acid,  neutralized  with  ammonia  free  from 
carbonate),  a  standard  solution  of  barium  chloride  is  added  in 
slight  excess,  then  a  solution  of  potassium  chromate  of  known 
strength  is  cautiously  added  to  precipitate  the  excess  of  barium. 
So  long  as  any  barium  remains  in  excess,  the  supernatant  liquid  i& 
colourless  ;  when  it  is  all  precipitated  the  liquid  is  yellow,  from  the 
potassium  chromate  ;  a  few  drops  only  of  the  chromate  solution 
are  necessary  to  produce  a  distinct  colour. 

Wildenstein  uses  a  barium  solution,  of  which  1  c.c.=O015  gm. 
SO3,  and  chromate  1  c.c.  =O010  gm.  of  SO3.  I  prefer  to  use  ™/2 
solutions,  so  that  1  c.c.  of  each  is  equal  to  O02  gm.  of  SO3.  If  the 
chromate  solution  is  made  of  equal  value  to  the  barium  chloride, 
the  operator  has  simply  to  deduct  the  one  from  the  other,  in  order 
to  obtain  the  quantity  of  barium  solution  really  required  to  pre- 
cipitate all  the  SO3. 

METHOD  OF  PROCEDURE  :  The  substance  or  solution  containing  S03  is  brought 
into  a  small  flask,  diluted  to  about  50  c.c.,  acidified  if  necessary  with  IIC1,  heated 
to  boiling,  and  precipitated  with  a  slight  excess  of  standard  barium  chloride 
delivered  from  the  burette.  As  the  precipitate  rapidly  settles  from  a  boiling 
solution,  it  is  easy  to  avoid  any  great  excess  of  barium^  which  would  prevent 
the  liquid  from  clearing  so  speedily.  The  mixture  is  then  cautiously  neutralized 
with  ammonia  free  from  carbonic  acid  (to  be  certain  of  this,  it  is  well  to  add  to 
it  two  or  three  drops  of  calcium  chloride  or  acetate  solution). 

The  flask  is  then  heated  to  boiling,  and  the  chromate  solution  added  in  |  c.c. 
or  so,  each  time  removing  the  flask  from  the  heat  and  allowing  to  settle  until  the 
liquid  is  of  a  light  yellow  colour  ;  the  quantity  of  chromate  is  then  deducted 
from  the  barium  solution,  and  the  remainder  calculated  to  S03. 

Or  the  mixture  with  barium  in  excess  may  be  diluted  to  100  or  150  c.c.,  the 
precipitate  allowed  to  settle  thoroughly,  and  25  or  50  c.c.  of  the  clear  liquid 
heated  to  boiling,  after  neutralizing,  and  precipitated  with  chromate  until  all  the 
barium  is  carried  down  as  chromate,  leaving  the  liquid  of  a  light  yellow  colour  ; 
the  analysis  should  be  checked  by  a  second  titration.  The  process  has  yielded 
me  very  satisfactory  results  in  comparison  with  the  barium  method  by  weight  ; 
it  is  peculiarly  adapted  for  determining  sulphur  in  gas  when  burnt  in  the 
Lethe  by  sulphur  apparatus,  details  of  which  will  be  found  on  p.  340. 

The  presence  of  alkali  and  alkaline  earthy  salts  is  of  no 
consequence — Zn  and  Cd  do  not  interfere — Ni,  Co,  and  Cu  give 
coloured  solutions  which  prevent  the  yellow  chromate  being  seen, 
but  this  difficulty  can  be  overcome  by  the  use  of  an  external 
indicator  for  showing  the  excess  of  chromate.  This  indicator  is 
an  ammoniacal  lead  solution,  made  by  mixing  together,  at  the 
time  required,  one  volume  of  pure  ammonia  and  four  volumes  of 
lead  acetate  solution  (1  :  20).  The  liquid  has  an  opalescent  appear- 
ance. To  use  the  indicator,  a  large  drop  is  spread  upon  a  white 


SULPHATES.  351 

porcelain  plate,  and  one  or  two  drops  of  the  liquid  under  titration 
added  :  if  the  reddish-yellow  colour  of  lead  chromate  is  produced, 
there  is  an  excess  of  chromate,  which  can  be  cautiously  reduced  by 
adding  more  barium  until  the  exact  balance  is  reached. 

A  variation  of  the  chromate  method  has  been  devised  by 
Andrews,*  which  is  especially  serviceable  for  determining  the 
combined  SO3  in  alkali  salts.  The  method  is  strongly  recommended 
by  Reutert  as  simple  and  easy  of  execution. 

METHOD  or  PROCEDURE  :  3  or  4  gra.  of  pure  precipitated  barium  chromate 
are  dissolved  in  30  c.c.  of  strong  hydrochloric  acid,  and  the  whole  is  diluted  to 
1  litre.  The  liquid  to  be  tested,  which  should  contain  about  0'07  gm.  of  S03  as 
an  alkali  sulphate,  is  mixed  at  the  boiling  point  with  an  excess  (150  c.c.)  of  the 
chromate  solution ;  the  acid  is  neutralized  with  pure  powdered  chalk,  and  the 
precipitate  is  removed  by  nitration.  After  thorough  cooling,  the  filtrate  is 
acidified  with  5  c.c.  (not  more)  of  strong  HC1,  20  c.c."  of  a  10  per  cent,  solution 
of  potassium  iodide  are  added,  and  the  liquid  is  allowed  to  rest  for  five  minutes 
in  a  covered  beaker  and  in  an  atmosphere  of  carbonic  acid  (to  prevent  oxidation 
of  the  HI)  until  the  chromic  acid  is  entirely  reduced.  Finally,  it  is  diluted  to 
1  or  1^  litre,  and  titrated  quickly  with  thiosulphate  ;  three  atoms  of  iodine 
corresponding  to  1  molecule  of  S03. 

Another  variation  of  the  chromate  method  has  been  devised  by 
Mitchell  and  Smith.J  It  consists  in  using  excess  of  standard 
ammonium  dichromate,  and  titrating  the  excess  by  means  of 
standard  ferrous  ammonium  sulphate. 

METHOD  or  PROCEDURE  : — A  convenient  quantity  of  a  sulphate  is  taken  and 
dissolved  in  water  or  pure  hydrochloric  acid,  or,  if  necessary,  dilute  nitric  acid, 
and  a  slight  excess  of  standard  (2  N/s)  solution  of  barium  chloride  is  added. 
The  mixture  is  boiled  and  rendered  neutral  by  ammonium  hydroxide;  sodium 
acetate,  acetic  acid,  and  a  slight  excess  of  N/io  ammonium  dichromate  are  added. 
The  mixture  is  made  up  to  100  c.c.,  and  the  precipitate  allowed  to  settle;  25  c.c. 
of  the  clear  supernatant  liquid  are  titrated  with  N/2O  ferrous  ammonium 
sulphate,  using  potassium  ferricyanide  as  an  external  indicator,  and  taking  the 
first  appearance  of  a  green  tinge  as  the  end-point.  JTjJ 

It  should  be  noted  that  ammonium  dichromate,  which  is  N/10  in 
respect  of  oxidizing  power,  is  only  N/30  in  respect  of  precipitating 
power. 

The  method  appears  to  be  rapid,  and  the  authors'  results  are  very 
accurate. 

3.     Direct  Precipitation  with  Normal  Barium  Chloride. 

Very  good  results  may  be  obtained  by  this  method  when  carefully 
performed. 

METHOD  or  PROCEDURE  :  The  substance  in  solution  is  to  be  acidified  with 
hydrochloric  acid,  heated  to  boiling,  and  the  barium  solution  allowed  to  flow 
cautiously  in  from  the  burette  until  no  further  precipitation  occurs.  The  end 
of  the  process  can  only  be  determined  by  filtering  a  portion  of  the  liquid,  and 
testing  with  a  drop  of  the  barium  solution.  Be  ale's  filter  (shown  in  fig.  23)  is 
a  good  aid  in  this  case.  A  few  drops  of  clear  liquid  are  poured  into  a  test  tube, 
and  a  drop  of  barium  solution  added  from  the  burette  ;  if  a  cloudiness  appears, 
the  contents  of  the  tubes  must  be  emptied  back  again,  washed  out  into  the  liquid, 
and  more  barium  solution  added  until  all  the  S03"is  precipitated.  It  is  advisable 
to  use  N/io  solution  towards  the  end  of  the  process. 

*  Amer    CJtcm.  Jour.  1880,  567.  t  Chem.  Zeit.  1898,  357. 

J  J.  Chem.  Soc.  1909,  95,  2193. 


352  SULPHATES. 

Instead  of  the  test  tube  for  finding  whether  barium  or  sulphuric 
acid  is  in  excess,  a  plate  of  black  glass  may  be  used,  on  which  a  drop 
of  the  clear  solution  is  placed  and  tested  by  either  a  drop  of  barium 
chloride  or  sodium  sulphate, — these  testing  solutions  are  preferably 
kept  in  two  small  bottles  with  elongated  stoppers.  A  still  better 
plan  is  to  spot  the  liquids  on  a  small  mirror,  as  suggested  by 
Haddock  ;*  the  faintest  reaction  can  then  be  seen,  although  the 
liquid  may  be  highly  coloured. 

Wildenstein  has   arranged  another  method  for 
direct  precipitation,  especially  useful  where  a  constant 
series    of    determinations    have    to    be    made.     The 
apparatus  is  shown  in  fig.  54.     A  is  a  bottle  of  900  or 
1000  c.c.  capacity,  with  the  bottom  removed,  and 
made  of  well-annealed  glass  so  as  to  stand  heating  ; 
B  a  thistle  funnel  bent  round,  as  in  the  figure,  and 
this  siphon  filter  is  put  into  action  by  opening  the 
pinch-cock  below  the  cork.     The  mouth  of  the  funnel 
is  first  tied  over  with  a  piece  of  fine  cotton  cloth, 
then  two  thicknesses    of    Swedisli    filter-paper,  and 
again    with    a    piece    of    cotton    cloth,    the    whole 
Fig.  54.         being  securely  tied  with  waxed  thread. 
In  precipitating  SO3  by  barium  chloride,  there  occurs  a  point 
similar  to  the  so-called  neutral  point  in  silver  assay,  when  in  one  and 
the  same  solution  both  barium  and  sulphuric  acid  after  a  minute  or 
two  produce  a  cloudiness.     Owing  to  this  fact,  the  barium  solution 
must  not  be  reckoned  exactly  by  its  amount  of  BaCl2,  but  by  its 
working  effect  ;   that  is  to  say,  the  process  must  be  considered 
«nded  when  the  addition  of  a  drop  or  two  of  barium  solution  gives 
no  cloudiness  after  the  lapse  of  two  minutes. 

METHOD  OF  PROCEDURE  r'i'hc  solution  containing  the  SO3  having  been  prepared, 
and  preferably  in  HC1,  the  vessel  A  is  filled  with  warm  distilled  water  and  the 
pinch-cock  opened  so  as  to  fill  the  filter  to  the  bend  C ;  the  cock  is  then  opened 
and  shut  a  few  times  so  as  to  bring  the  water  further  down  into  the  tube,  but 
not  to  fill  it  entirely  ;  the  water  is  then  emptied  out  of  A,  and  about  400  c.c.  of 
boiled  distilled  water  poured  in  together  with  the  SO3  solution,  then,  if  necessary, 
a  small  quantity  of  HC1  added,  and  the  barium  chloride  added  in  moderate; 
quantity  from  a  burette.  After  mixing  well,  and  waiting  a  few  minutes  a  portion 
is  drawn  off  into  a  small  beaker,  and  poured  back  without  loss  into  A  ;  a  small 
quantity  is  then  drawn  off  into  a  test  tube,  and  two  drops  of  barium  chloride 
•added.  So  long  as  a  precipitate  is  produced  the  liquid  is  returned  to  A,  and 
more  barium  added  until  a  test  is  taken  which  shows  no  distinct  cloudiness  ; 
the  few  drops  added  to  produce  this  effect  are  deducted.  If  a  distinct  excess 
has  been  used,  the  analysis  must  be  corrected  with  a  solution  of  SO-  coi responding 
in  strength  to  the  barium  solution. 

A  simpler  and  even  more  serviceable  arrangement  of  apparatus 
on  the  above  plan  may  be  made  by  using  as  the  boiling  and 
precipitating  vessel  an  ordinary  beaker  standing  on  wire  gauze  or 
A  hot  plate.  The  filter  is  made  by  taking  a  small  thistle  funnel, 
tied  over  as  described,  with  about  two  inches  of  its  tube,  over  which 
is  tightly  slipped  about  four  or  five  inches  of  elastic  tubing, 


SULPHATES.  353 

terminating  with  a  short  piece  of  glass  tube  drawn  out  to  a  small 
orifice  like  a  pipette  ;  a  small  pinch-cock  is  placed  across  the  elastic 
tube  just  above  the  pipette  end,  so  that  when  hung  over  the  edge 
of  the  beaker  with  the  funnel  below  the  surface  of  the  liquid,  the 
apparatus  \vill  act  as  a  siphon.  It  may  readily  be  filled  with  warm 
distilled  water  by  gentle  suction,  then  transferred  to  the  liquid 
under  titratioii,  By  its  means  much  smaller  and  more  concentrated 
liquids  may  be  used  for  the  analysis,  and  consequently  a  more 
distinct  evidence  of  the  reaction  obtained. 

4.     Determination  by  Benzicline  Hydrochloride.* 

Benzidine  sulphate  C12H8(NH2)2.  H2S04  is  a  stable  salt  almost 
insoluble  in  water  containing  hydrochloric  acid.  Benzidine  being 
a  weak  organic  base,  neutral  to  phenolphthalein,  the  acid  in  its 
sulphate  can  be  titrated  with  standard  alkali.  For  the  determi- 
nation of  sulphuric  acid  by  benzidine  Raschigt  recommends 
treating  the  neutral  or  acid  solution  of  the  sulphate  with  benzidine 
hydrochloride  solution,  filtering  off  the  precipitated  benzidine 
sulphate,  washing  it,  and  then  suspending  it  in  water  and  titrating 
the  sulphuric  acid  with  N/10  soda. 

METHOD  OF  PROCEDURE  : — To  prepare  the  solution  of  benzidine  hydrochloride, 
6-7  gin.  of  the  free  base,  or  the  corresponding  amount  of  the  hydrochloride,  is 
rubbed  up  in  a  mortar  with  20  c.c.  of  water.  The  paste  is  rinsed  into  a  litre 
flask,  20  c.c.  of  hydrochloric  acid  (sp.  gr.  1-12)  are  added,  and  the  solution  diluted 
to  the  mark.  (1  c.c.  of  this  solution  corresponds  theoretically  to  0-00357  gm. 
H2S04.)  The  solution  has  a  brown  colour  and  may  be  filtered  if  necessary. 
After  some  time  brown  flakes  are  likely  to  separate,  but  these  do  no  harm. 

The  solution  of  the  sulphate  is  diluted  with  water  until  its  volume,  corresponds 
to  not  less  than  50  c.c.  for  each  O'l  gm.  of  sulphuric  acid  present.  An  equal 
volume  of  the  reagent  is  added  while  stirring  vigorously.  A  filter  is  prepared  by 
placing  a  perforated  porcelain  filter  plate  in  a  funnel  and  covering  it  with  two 
moistened  filter  papers,  one  of  exactly  the  same  size  as  the  plate  and  the  upper 
one  a  little  larger.  After  ten  minutes,  the  precipitate  is  filtered  off  upon  this 
filter,  using  gentle  suction.  The  last  portions  of  the  precipitate  are  transferred 
to  the  filter  with  the  aid  of  small  portions  of  the  clear  filtrate,  and  then  the 
beaker  and  precipitate  are  washed  with  20  c.c.  of  cold  water,  added  in  several 
portions.  The  precipitate  and  filter,  but  not  the  plate,  are  then  transferred  to 
an  Erlenmeyer  flask,  50  c.c.  of  water  are  added  and  the  contents  of  the 
stoppered  flask  shaken  until  a  homogeneous  paste  is  obtained.  The  rubber 
stopper  is  then  removed,  rinsed  with  water,  a  drop  of  phenolphthalein  added, 
the  water  heated  to  about  50°  C,  and  titrated  with  N/io  sodium  hydroxide. 
When  the  end  point  is  nearly  reached,  the  liquid  is  boiled  for  five  minutes,  and 
the  titration  then  finished. 

According  to  Friedheim  and  Nydegger,{  this  method  gives 
excellent  results  in  the  analysis  of  all  sulphates,  provided  no 
substances  are  present  which  attack  benzidine,  and  provided  the 
amount  of  other  salts  and  acids  present  is  not  too  great.  There 
should  not  be  more  than  10  mol.  of  HC1,  15  mol.  HNO3,  20  mol. 
HC2H3O2,  5  mol.  alkali  salt,  or  2  mol.  ferric  iron  present  to  1  mol. 
H2SO4.  A  satisfactory  determination  of  the  sulphur  in  pyrites 

»See  TreadweH's  Analytical  Chemistry,  translated  by  Hall,  2nd  edition, 
Vol.  II.,  p.  658. 

\Z.  a.  Chem.  1903,  617  and  818.  %Z.  a.  Chem.  1907,  9. 

2   A 


354  PERSULPHATES. 

may  be  made  by  dissolving  0*5  gm.  of  the  sample  according  to 
the  Lunge  method,  evaporating  off  the  nitric  acid,  taking  up  the 
residue  in  a  little  hydrochloric  acid,  diluting  to  500  c.c.  and  using 
100  c.c.  for  the  treatment  with  benzidine  hydrochloride. 


PERSULPHATES. 

The  alkali  persulphates  may  be  readily  titrated  by  adding  to 
their  solution  a  known  excess  of  ferrous  salt  and  determining 
the  amount  of  oxygen  absorbed  by  titratibn  of  the  solution  with 
permanganate.  The  salt,  say  potassium  persulphate,  decomposes 
as  follows  : — 

K2S208=K2S04+S02+02. 

The  operation  requires  a  standard  permanganate,  whose  value  is 
known  upon  a  solution  of  ammonio-ferrous  sulphate,  containing 
about  30  gm.  per  litre.  The  method  adopted  by  Le  Blanc  and 
Eckardt*  is  to  dissolve  about  2*5  gm.  of  the  persulphate  in  water 
and  dilute  to  100  c.c.  10  c.c.  of  this  solution  are  placed  in  a  flask 
with  5  c.c.  of  dilute  sulphuric  acid  of  1'16  sp.  gr.,  and  a  considerable 
excess  of  ferrous  solution,  say  100  c.c.,  then  about  100  c.c.  of 
distilled  water  at  a  temperature  of  70°  to  80°  C.  are  added,  and 
a  rapid  titration  made  with  permanganate.  The  reaction  is  the 
more  rapid  the  greater  the  excess  of  iron  solution,  within  reasonable 
limits. 

The  standard  solutions  are  best  verified  upon  a  persulphate  of 
known  purity  in  order  to  ascertain  the  comparative  composition 
of  any  given  sample. 

Another  method  consists  in  decomposing  the  persulphate  by 
means  of  potassium  iodide,  and  titrating  the  iodine  separated  with 
thiosulphate  solution.  2  to  3  gm.  of  the  sample  are  dissolved  in 
100  c.c.  of  water,  and  10  c.c.  of  the  solution  are  treated  with  an 
excess  of  potassium  iodide  (0'25  to  0'50  gm.),  and  heated  for 
10  minutes  in  a  drying  oven  at  60°  to  80°  C.  The  iodine  is  then 
titrated  with  N/10  thiosulphate,  starch  being  added  towards  the 
end  of  the  titration.  In  this  case  the  effects  of  the  process  are 
best  established  upon  a  persulphate  of  known  purity. 

B.  Griitznerf  has  discovered  that  arsenious  acid  is  completely 
oxidized  to  arsenic  acid  by  alkali  persulphates  in  alkaline  solution. 

In  applying  this  reaction,  about  0'3  gm.  of  the  alkali  persulphate 
is  heated  gradually  to  boiling  with  50  c.c.  of  N/10  As203  and  a  few 
c.c.  of  potash  or  soda-lye,  then  digested  for  a  short  time,  allowed 
to  cool,  the  liquid  made  faintly  acid  with  sulphuric  acid,  then 
strongly  alkaline  with  sodium  bicarbonate,  and  the  excess  of 
arsenious  acid  titrated  back  with  N/10  iodine  solution. 

Marie  and  BunelJ  from  careful  experiments  advocate  the 
following  method  for  alkali  persulphates  : — 

•  C.  N.  81,  38.          f  Chem.  Centr.  1900,  435.         %  Bull.  Soc.  Chim.  29,  No.  18. 


TANNIC   ACID.  355 

Dissolve  about  0'3  to  0'4  gm.  of  the  sample  in  100  c.c.  of  water ;  neutralize  the 
solution,  which  is  generally  acid,  in  the  presence  of  methyl  orange  ;  then  add 
2  c.c.  of  methylic  alcohol,  heat  for  five  minutes  to  70 — 80°,  and  then  boil  for  ten 
minutes,  cool,  and  titrate  with  methyl  orange  and  decinormal  soda. 

1  c.c.  of  decinormal  soda  corresponds  to  0*0135  gm.  K2S208. 

1  „  „  „  „  0-0119     „    Na2S208. 

1  „  „  „  „  0-0114    „    Am2S208. 

The  use  of  methyl  alcohol  is  based  on  the  fact  that  a  persulphate 
transforms  a  portion  of  the  alcohol  into  aldehyde  according  to 
a  well-known  reaction.  The  method  gave  very  satisfactory 
results. 


TANNIC    ACID. 

THE  determination  of  tannin  in  the  materials  used  for  tanning 
is  by  no  means  of  the  most  satisfactory  character.  Many  methods 
have  been  proposed,  and  given  up  as  practically  useless. 
LowenthaPs  method,  with  later  variations,  is  accepted  as  the 
best  volumetric  method  ;  but  it  is  still  deficient  in  accuracy  and 
reliability,  although  much  ingenuity  and  intelligence  have  been 
expended  on  it. 

One  difficulty  is  still  unsurmounted,  and  that  is  the  preparation 
of  a  pure  tannic  acid  to  serve  as  standard.  The  various  tannins  in 
existence  are  still  very  imperfectly  understood,*  but  so  far  as  the 
comparative  analysis  of  tanning  materials  among  themselves  is 
concerned,  the  method  in  question  is  theoretically  the  best. 

The  principle  of  the  method  depends  on  the  oxidation  of  the 
tannic  acid,  together  with  glucosides  and  other  easily  oxidizable 
substances,  by  permanganate,  regulated  by  the  presence  of  soluble 
indigo,  prepared  from  what  is  commonly  called  indigo  carmine, 
but  is  chemically  sulphindigotate  of  sodium  or  potassium,  which 
also  acts  as  an  indicator  of  the  end  of  the  reaction.  The  total 
amount  of  such  substances  having  been  found  and  expressed  by 
a  known  volume  of  permanganate,  the  actually  available  tannin 
is  then  removed  by  gelatine,  or  by  the  hide-powder  system,  and 
the  second  titration  is  made  upon  the  solution  so  obtained  in  order 
to  find  the  amount  of  oxidizable  matters  other  than  tannin. 

The  volume  of  permanganate  so  used,  deducted  from  the  volume 
used  originally,  shows  the  amount  of  tannin  actually  available  for 
tanning  purposes  expressed  in  terms  of  permanganate. 

H.  R.  Procter  in  his  Leather  Industries  Laboratory  Book  gives 
the  most  recent  methods  of  using  this  process  in  the  Yorkshire 
College  where  he  is  the  professor  of  leather  manufacture,  and 

*  Von  Schroder,  whose  suggestions  have  been  adopted  by  the  German 
Association  of  Tanners,  selects  a  commercial  pure  tannic  acid  for  use  as  a  standard 
by  dissolving  2  gm.  in  a  litre  of  water.  10  c.c.  of  this  is  titrated  with  permanganate 
as  described.  50  c.c.  are  then  digested  twenty  hours  with  3  gm.  moistened  hide- 
powder.  10  c.c.  of  the  filtrate  from  this  is  then  titrated,  and  if  the  permanganate 
consumed  amounts  to  less  than  10  per  cent,  of  the  total  consumed  by  the  tannin, 
it  is  suitable  for  a  standard.  1000  parts  being  considered  equivalent  in  reducing 
power  to  1048  parts  of  tannin  precipitable  by  hide,  according  to  Hammer's 
experiments,  therefore  Von  Schrfider,  after  titrating  as  described,  calculates 
the  dry  matter,  and  multiplies  by  the  round  number  1*05  to  obtain  the  value  in 
actual  tannin  precipitable  by  hide. 

2  A  2 


356  TANNIC   ACID. 

gives  his  opinion  of  its  value  as  a  practical  process.  "  It  is  now 
much  superseded  by  the  hide-powder  method,  but  there  are  still 
a  few  cases  in  which  it  may  be  employed  with  advantage.  Where 
only  one  or  two  analyses  are  to  be  made  at  one  time,  the  preparation 
and  adjustment  of  solutions  is  much  more  tedious  than  gravimetric 
analysis,  but  where  a  number  of  successive  titrations  are  "required 
it  is  considerably  more  rapid.  It  has  the  advantage  that  it  can  be 
applied  direct  to  solutions  however  dilute,  and  if  gelatine 
precipitation  is  used,  it  is  much  less  affected  by  the  presence  of 
gallic  acid  or  other  fixed  acids  than  the  hide-powder  method,  and 
is  therefore  well  adapted  for  the  analysis  of  weak  and  waste  liquors 
for  technical  purposes,  for  the  systematic  testing  of  spent  tans, 
and  for  the  analysis  of  sumach  and  myrabolans  which  contain 
much  gallic  acid,  and  which  in  the  gravimetric  method  is  wholly 
or  partially  estimated  as  tanning  matter." 

The  extraction  of  the  tannic  acid  from  the  raw  material  is  best 
performed  by  making  an  infusion  of  the  ground  substance  first  with 
distilled  water  to  about  500  c.c.  at  a  temperature  not  greater  than 
50°  C.  then  with  water  at  100°  C.,  and  percolating  till  free  from 
tannin,  and  diluting  when  cold  to  1  litre.  Portions  are  filtered  if 
necessary.  Concentrated  extracts  are  dissolved  before  titration 
by  adding  them  to  boiling  water,  then  cooling  and  diluting  to  the 
measure.  In  the  case  of  strong  materials  such  as  sumach  or 
valonia  10  gm.,  or  oak  bark  20  gm.,  are  used. 

The  quantity  of  these  extracts  to  be  used  for  titration  must  be 
regulated  to  some  extent  by  the  amount  of  permanganate  required  to 
oxidize  the  tannic  and  gallic  acids  present.  Practice  and  experience 
will  enable  the  operator  to  judge  of  the  proper  proportions  to  use 
in  dealing  with  the  various  materials,  bearing  in  mind  that  volu- 
metric processes  are  largely  dependent  upon  identity  of  conditions 
for  securing  concordant  results.  The  recommendation  of  the  best 
authorities  is  that  the  strength  of  the  solution  used  for  titration 
should  be  such  as  to  give  a  solid  residue  of  from  0-6  to  0'8  gm. 
from  100  c.c. 

The  working  details  according  to  Procter  adopted  at  the 
Yorkshire  College  are  as  follows.  The  solutions  required  are  : — 

(1)  Pure  potassium  permanganate,  0'5  gm.  per  litre.  As  very 
weak  solutions  do  not  keep  well,  it  is  best  to  make  up  one  of  5  gm. 
per  litre,  and  dilute  when  wanted.  The  exact  strength  of  the 
permanganate  is  not  important  so  long  as  it  is  constant  through 
a  series  of  experiments. 

.  (2)  Pure  indigo-carmine  5  gm.,  and  concentrated  H2S04,  50  gm. 
per  litre.  This  must  be  filtered,  and  should  give  a  pure  yellow  free 
from  any  trace  of  brown  where  oxidized  with  permanganate  ;  25  c.c. 
of  this  solution  should  equal  about  30  c.c.  of  the  permanganate, 
and,  if  necessary,  must  be  diluted  to  that  strength. 

(3)  Solution  of  pure  tannin,  3  gm.  to  1  litre.  Since  absolutely 
pure  tannin  cannot  be  obtained,  the  following  method  is  adopted  : — 
A  sample  of  the  purest  obtainable  tannin  (not  less  than  90-95  per 


TANNIC   ACID.  357 

cent,  pure  by  hide-powder)  is  preserved  air-dry  in  a  well-stoppered 
bottle,  and  the  moisture  carefully  determined.  The  principal 
impurity  is  gallic  acid,  which  acts  on  permanganate  like  tannin, 
but  reduces  somewhat  more  strongly,  and  1  part  of  such  tannin, 
calculated  to  dry  weight,  is  equal  on  the  average  to  1'05  parts  of 
pure  tannin.  Hence  it  is  easy  to  calculate  a  quantity  of  the  air- 
dry  tannin  equal  in  permanganate  value  to  0'3  gm.  of  pure  tannin, 
and  this  is  weighed  out  when  required  and  made  up  to  100  c.c. 
The  moisture  varies  very  little,  but  it  is  well  occasionally  to  re- 
determine  it  and  calculate  afresh. 

METHOD  OF  PROCEDURE  :  25  c.c.  of  the  indigo  solution  are  mixed  in  a  beaker 
with  about  f  litre  of  clean  tap  water,  and  the  permanganate  added  drop  by  drop 
from  a  glass-tapped  burette  till  a  pure  yellow  is  obtained,  the  liquid  being  stirred 
steadily  the  whole  time.  A  disc  stirrer  or  a  glass  rod  bent  several  times  back 
and  forward,  is  to  be  preferred  to  a  plain  rod  ;  or  some  method  of  mechanical 
stirring  may  be  adopted.  The  dropping  should  be  always  as  nearly  as  possible  at 
a  similar  rate  for  each  experiment,  and  should  be  slower  towards  the  end  of  the 
titration.  It  is  convenient  to  keep  a  second  beaker  titrated  to  a  pure  primrose 
yellow  as  a  standard  test.  Titrations  may  be  accurately  performed  by  artificial 
light,  but  usually  differ  slightly  from  those  by  daylight,  and  hence  the  light 
should  not  be  varied  in  the  course  of  an  analysis.  For  daylight  work 
Kathreiner  recommends  the  use  of  a  white  basin  instead  of  a  beaker.  The 
permanganate  solution  is  allowed  to  drop  in,  with  constant  stirring,  till  the  pure 
yellow  liquid  shows  a  faint  pinkish  rim,  most  clearly  seen  on  the  shaded  side. 
This  end-reaction  is  of  extraordinary  delicacy,  and  is  quite  different  from  the 
pink  caused  by  excess  of  permanganate,  being  an  effect  common  to  all  pure  yellow 
liquids.  The  titration  is  done  at  least  twice,  and  the  average  taken ;  f  litre  of 
water  and  25  c.c.  of  indigo  are  then  taken  as  before,  and  5  c.c.  of  the  tannin 
solution  are  added  and  similarly  titrated  repeatedly.  Deducting  amount  required 
for  the  indigo,  the  remainder  is  that  consumed  by  the  tannin,  which  should  not 
at  most  exceed  two-thirds  of  that  required  by  the  indigo.  A  similar  titration  is 
made  with  the  tannin  infusion  to  be  examined,  of  which  such  a  number  of  cubic 
centimetres  is  employed  as  will  consume  about  the  same  quantity  of  the 
permanganate  as  the  standard  tannin  solution.  The  value  of  the  total  astringent 
is  then  calculated  in  terms  of  tannin. 

Since  tanning  matters  contain  astringents  which  are  not  taken  up  by  the  hide, 
but  which  are  oxidized  by  permanganate  like  tannins,  it  is  in  most  cases  necessary 
to  remove  the  tannin  from  a  portion  of  the  infusion,  and  to  repeat  the  titration  to 
determine  the  non-tannin  ingredients. 

This  may  be  done  by  the  hide-powder  method  at  the  same  time  that  the  tannin 
substance  is  determined  gravimetrically,  but  a  much  quicker  and  even  "better 
method  is  that  of  Hunt.  The  solutions  required  are  : — 

(1)  Pure  gelatin,  2  gm.  per  100  c.c. 

(2)  Saturated  solution  of  NaCl  containing  50  c.c.  of  concentrated 
H2SO4  per  litre. 

METHOD  OF  PROCEDURE  :  To  50  c.c.  of  the  liquor  (of  about  the  strength  of 
1  to  1-5  gm.  of  tannin  per  100  c.c.)  are  added  25  c.c.  of  the  gelatin  solution  and 
25  c.c.  of  the  salt  solution,  and  about  a  teaspoonful  of  kaolin  or  barium  sulphate, 
and  the  whole  is  well  shaken  for  five  minutes  and  filtered.  This  filtrate,  which 
should  be  perfectly  bright,  is  titrated  for  non-tannin  bodies  by  the  permanganate 
method,  double  the  volume  being  taken  which  was  employed  for  determination 
of  total  astringents,  and  the  result  is  deducted  before  calculating  the  tanning 
value. 

It  is  impossible  to  give  here  the  opinions  held  by  various 
authorities  on  this  subject,  therefore  the  reader  who  desires  fuller 


358 


TANNIC   ACID. 


information  should  consult  the  various  papers  contributed  to 
various  journals,  etc.,  and  more  especially  Procter's  book  before 
mentioned. 

The  table  below  by  Hunt  is  appended,  as  the  result  of  careful 
working,  and  as  a  guide  to  the  nature  of  various  tanning 
materials  : — 

The  "  total  extract  "  in  the  table  was  determined  by  evaporating 
&  portion  of  the  tannin  solution  to  dryness  in  a  small  porcelain 
basin  and  drying  the  residue  at  110°  C.  The  "  insoluble  matter  " 
was  also  dried  at  110°  C. 

The  hide-powder  process  for  tannin  not  being  a  volumetric  one 
is  not  described  here. 


NAME  OP  MATERIAL. 

Total 
matters 
oxidized 
by  Perman- 
ganate, as 
Oxalic  Ac. 

Tannin,  as 
Oxalic  Ac. 
(Procter) 

Tannin,  as 
Oxalic  Ac. 
(Hun  t) 

Total 
Extract. 

Insoluble. 

per  cent. 

per   cent. 

per  cent. 

per   cent. 

per   cent. 

English  Oak  Bark  ,  . 

15-70 

13-54 

11-97 

18-38 

66-15 

Canadian  Hemlock  Bark 

9-03 

7-46 

7-08 

13-96 

75-25 

Larch  Bark       .  . 

8-20 

7-17 

6-15 

20-64 

60-80 

Mangrove  Bark 

31-35 

29-71 

28-48 

26-60 

49-70 

Alder  Bark       

8-27 

6-15 

5-73 

19-36 

68-00 

Blue  Gum  Bark       .  . 

10-18 

8-91 

8-91 

11-76 

74-65 

Valpnia     

37-41 

35-24 

30-50 

38-50 

46-05 

Myrabolans       .  .      .  . 

48-23 

38-43 

38-00 

42-80 

— 

Sumach     

42-53 

34-30 

31-46 

44-10 

47-77 

Betel  Nut          ..      ..      .. 

15-91 

13-87 

13-79 

17-94 

67-00 

Turkish  Blue  Galls       ,  . 

73-38 

65-83 

59-96 

48-40 

36-35 

Aleppo  Galls     .  .      .  . 

98-85 

87-82 

83-05 

68-80 

14-32 

Wild  Galls        .  .      ..... 

26-21 

18-75 

16-56 

31-70 

54-17 

Divi-*Divi 

66-98 

62-62 

61-22 

54-38 

29-90 

Balsamocarpan  (poor  and 
old  sample)  ..      ..  •  .>  = 

50-49 

37-76 

32-88 

57-14 

28-25 

Pomegranate  Rind 

27-58 

24-18 

23-12 

41-00 

49-50 

Tormentil  Root 

22-27 

20-98 

20-68 

19-70 

67-95 

Rhatany  Root          .  . 

22-27 

20-15 

19-30 

18-80 

66-00 

Pure  Indian  Tea      

23-06 

18-65 

17-40 

34-46 

53-40 

Pure  China  Tea       .  . 

18-03 

14-21 

14-09 

24-50 

62-60 

Cutch         :  .  . 

57-65 

51-95 

44-24 

61-60 

4-75 

Gum  Kino         .  .      .... 

66-39 

59-55 

51-55 

79-30 

1-00 

Hemlock  Extract 

35-16 

33-17 

30-98 

48-78 

— 

Oakwood  Extract    .  .    '.-*.. 

33-49 

26-90 

23-86 

37-78 

— 

Chestnut  Extract     .  . 

39-77 

32-63 

28-88 

50-28 

— 

Quebracho  Extract         .. 

48-22 

34-45 

40-84 

49-00 

— 

"  Pure  Tannin  "  ",Jv>i-     .•> 

135-76 

122-44 

121-93 

— 

— 

Tan  Liquor,  sp.  gr.  1-080 

4-84 

3-14 

2-10 

6.01 

— 

Spent  Tan  Liquor,  sp.  gr. 

1-0165    

1-40 

0-37 

0-25 

3-10 

— 

Absorbed 

by  Dry 

Pure  Skin. 

Gambier,  Cube        ..      .. 

70-12 



51-07 

74-40 

5-31 

„        Sarawak 

63-13 



47-09 

70-70 

3-67 

Bale         .\    .. 

56-00 

— 

43-70 

63-54 

1-40 

TANNIC   ACID.  359 

Tannin  in  Tea. — The  extract  of  this  substance  is  made  upon 
10  gm.  of  the  tea,  by  boiling  with  a  litre  of  distilled  water  for  an 
hour  in  a  flask  fitted  with  a  reflux  condenser,  filtering  and  diluting 
the  liquid  when  cool  to  a  litre. 

A.  H.  Allen^remarks  that  the  determination  of  tannin  in  tea 
affords  valuable  information  respecting  the  probable  presence  of 
previously  infused  leaves  or  extraneous  tannin  matters,  such  as 
catechu.  This  is  best  effected  in  the  aqueous  decoction  obtained  by 
exhausting  the  sample  with  boiling  water,  as  required  for  the 
determination  of  the  extract. 

The  tannin  may  be  determined  by  the  modification  of 
Loweiithal's  process,  as  previously  described.  A  volume  of  the 
above  decoction  corresponding  to  0'04  gm.  of  tea  may  be  taken 
for  the  original  titration  with  permanganate  ;  and  of  the  decoction 
deprived  of  tannin  a  volume  corresponding  to  0*080  gm.  of  tea. 
The  tannin  of  tea  is  stated  by  some  chemists  to  be  gallotannic  acid, 
and  by  others  to  be  identical  with  that  of  oak  bark.  The  reduction- 
equivalent  of  the  latter  is  almost  identical  with  that  of  crystallized 
oxalic  acid,  so  that  the  weight  of  this  substance  corresponding  to 
the  volume  of  permanganate  decolorized  gives  without  calculation 
that  of  the  tannin  present. 

The  process  of  fermentation  to  which  black  tea  has  been 
subjected  undoubtedly  causes  modification  of  the  tannin,  with 
formation  of  dark-coloured  insoluble  matter.  The  author  found 
that  a  decoction  of  green  tea  precipitated  ferric  chloride  bluish- 
black,  like  nut-galls,  while  that  of  black  tea  gave  a  green  colour 
with  iron,  just  as  catechu  does. 

A.  H.  Allen  in  his  Organic  Analysis,  vol.  iii.  part  2,  gives 
a  modification  of  the  lead  method. 

The  Lowenthal  process  distinguishes  the  tannic  acid  from  the 
small  quantity  of  gallic  acid  also  present  in  tea,  but  as  the  astringent 
character  of  the  infusion  is  due  to  both  these  substances,  a  method 
which  will  determine  the  total  amount  of  astringent  matter, 
without  distinction  of  its  nature,  is  in  some  respects  preferable  to 
a  process  that  gives  merely  the  amount  of  tannin,  while  ignoring 
the  gallic  acid.  Such  a  process  was  devised  by  F.  W.  Fletcher 
and  A.  H.  Allen*  in  1874,  and  was  based  on  the  precipitation  of 
the  tea  infusion  by  lead  acetate,  and  the  use  of  an  ammoniacal 
solution  of  potassium  ferricyanide  to  indicate  the  complete 
precipitation  of  the  astringent  matters. 

METHOD  OF  PROCEDURE  :  5  gm.  of  neutral  acetate  of  lead  should  bo  dissolved 
in  distilled  water,  and  diluted  to  1  litre,  and  the  solution  filtered  after  standing. 
The  indicator  is  made  by  dissolving  O'OSO  gm.  of  pure  potassium  ferricyanide  in 
50  c.c.  of  water,  and  adding  an  equal  bulk  of  strong  ammonia  solution.  This 
reagent  gives  a  deep  red  coloration  with  gallotanic  acid,  gallic  acid,  or  an  infusion 
of  tea.  One  drop  of  the  solution  will  detect  O'OOl  milligram  of  tannin.  In 
carrying  out  the  process,  three  separate  quantities  of  10  c.c.  each  of  the  standard 
lead  solution  should  be  placed  in  beakers,  and  each  quantity  diluted  to  about 
100  c.c.  with  boiling  water.  A  decoction  made  from  2  gm.  of  powdered  tea  in 

*  C.  N.  29,  169,  189. 


360  TANNIC   ACID. 

250  c.c.  of  water  (the  same  as  is  used  for  determining  the  extract)  is  added  from 
a  burette,  the  first  trial  quantity  receiving  an  addition  of  10,  the  second  15,  and 
the  third  18  c.c.  ;  or  if  green  tea  be  under  examination,  8,  10,  and  12  c.c.  may  be 
preferably  employed.  1  c.c.  each  of  these  trial  quantities  are  passed  through 
small  filters,  and  the  filtrates  tested  with  ammoniacal  ferricyanide  solution. 

The  approximate  volume  of  tea  decoction  required  is  thus  easily  found,  and 
after  repeating  the  test  nearly  the  requisite  measure  can  be  at  once  added.  In 
this  case  about  1  c.c.  of  the  liquid  should  be  removed  with  a  pipette,  passed 
through  a  small  filter,  and  drops  of  the  filtrate  allowed  to  fall  on  to  spots  of  the 
indicating  solution  previously  placed  on  a  porcelain  slab.  If  no  pink  coloration 
is  observed,  another  small  addition  of  the  tea  decoction  is  made,  a  few  drops  of 
the  liquid  filtered  and  tested  as  before,  and  this  process  repeated  until  a  pink 
colour  is  observed.  The  greatest  delicacy  is  obtained  when  the  drops  of  filtered 
solution  are  allowed  to  fall  directly  on  to  the  spots  of  the  indicator,  instead  of 
observing  the  point  of  junction  of  the  liquids. 

The  volume  of  tea  solution  it  is  necessary  to  add  to  100  c.c.  of  pure  water, 
in  order  that  a  drop  may  give  a  pink  reaction  with  the  indicator,  should  be 
subtracted  from  the  total  amount  run  from  the  burette. 

The  foregoing  process  is  simple,  and  gives  very  concordant 
results  ;  but  the  repeated  filtrations  requisite  for  the  observation 
of  the  end-reaction  are  apt  to  be  tedious.  It  is  difficult  to  obtain 
pure  tannin  for  setting  the  lead  solution,  and  hence  it  is  preferable 
to  abandon  the  attempt  and  make  pure  lead  acetate  the  starting- 
point.  The  author  found  that  10  c.c.  of  the  lead  solution  would 
precipitate  O'OIO  gm.  of  the  purest  gallotannic  acid  he  could 
obtain.  Hence,  if  all  the  weights  and  measures  above  mentioned 
be  adhered  to,  the  number  of  c.c.  of  tea  decoction  required,  divided 
into  125,  will  give  the  percentage  of  tannin  and  other  precipi table 
matters  in  the  sample.  The  proportion  found  in  undried  black  tea 
by  F.  W.  Fletcher  and  the  author  ranged  from  8*5  to  11*6  per 
cent.,  with  an  average  of  10  per  cent. 

Tannin  in  Wine,  Cider,  etc. — The  method  now  generally  adopted 
for  this  determination  is  that  of  treating  a  known  volume  of  the 
wine,  etc.,  with  catgut  (violin  strings  which  have  not  been  oiled, 
and  which  have  been  purified  by  washing  in  dilute  alcohol,  acid, 
and  water  until  they  have  no  reducing  action  on  permanganate  in 
the  cold).  The  digestion  is  carried  on  at  ordinary  temperature 
for  a  week,  in  a  closely  stoppered  bottle.  The  original  substance, 
and  that  from  which  the  tannin  has  been  removed,  are  then  titrated 
with  permanganate,  and  the  difference  calculated  to  tannin. 

Another  method  consists  in  mixing  equal  parts  of  an  eighth  per 
cent,  solution  of  alum  and  the  wine,  collecting  the  precipitate  on 
a  filter,  washing  slightly  with  cold  water,  transferring  the  precipitate 
by  a  stream  of  water  from  a  wash-bottle  to  a  beaker,  then  acidifying 
with  H2S04  and  titrating  with  indigo  and  permanganate  as  usual. 

Dreaper's  Copper  Process  for  Tannic  and  Gallic  Acids. — This 
is  described  in  a  paper  contributed  to  J.  C.  S.  I.  xii.  412,  from 
which  the  following  abstract  is  taken. 

The  methods  hitherto  proposed  for  the  determination  of  tannin 
may  be  divided  into  two  classes,  viz.  : — 

(1)  Those  which  act  by  precipitating  the  tannic  acid  as  an 
insoluble  compound. 


TANNIC   ACID.  361 

(2)     Those  which  act  by  oxidation. 

To  the  former  class  belongs  the  well-known  hide-powder  process, 
and  to  the  latter  Lowenthal's  permanganate  method,  which 
has  been  modified  by  Procter  and  others.  These  fairly  represent 
the  two  classes,  and  are  the  only  ones  in  general  use  at  the  present 
day. 

D  reaper,  however,  has  adopted  a  modified  form  of  Dar  ton's 
method,  the  novelty  of  which  consists  in  precipitating  the  tannic 
acid  by  means  of  an  ammonio-copper  sulphate  solution,  after 
a  preliminary  treatment  with  sulphuric  acid  to  remove  the  ellagic 
acid,  and  then  a  treatment  with  ammonia,  filtering  after  each 
treatment.  Procter  states  that  this  preliminary  treatment  is 
unnecessary  in  the  case  of  some  extracts,  but  Dreaper  has  never 
found  any  precipitation  to  take  place  in  the  case  of  the  so-called 
pure  tannic  acids,  probably  owing  to  the  removal  of  the  impurities 
during  the  process  of  purification.  The  original  solution  and  the 
filtrate  are  titrated  with  permanganate  as  in  Lowenthal's  method, 
the  difference  in  the  two  results  being  due  to  the  tannic  acid 
present.  The  copper  compound  may  be  dried  at  110°  C.  and 
weighed,  or  else  ignited  and  weighed  as  copper  oxide.  Fleck 
states  that  the  tannic  acid  can  be  calculated  from  this  by  multiplying 
by  the  factor  1'034. 

The  standard  copper  solution  used  by  the  author  contained 
30  gm.  of  pure  crystallized  copper  sulphate  in  a  litre  of  water. 
Barium  carbonate  is  also  required,  which  should  be  free  from 
calcium  salts. 

The  process  is  based  on  the  direct  precipitation  of  the  gallic  and  tannic  acids 
by  means  of  a  copper  salt,  using  as  outside  indicator  potassium  ferrocyanide.  If 
a  standard  solution  of  copper  sulphate  be  run  into  a  solution  of  the  mixed  acids, 
a  certain  amount  of  copper  tannate  and  gallate  will  be  precipitated,  depending 
on  the  dilution  of  the  solution  and  the  amount  of  acid  set  free  from  the  copper 
sulphate.  The  precipitate  is,  under  these  circumstances,  of  a  bulky  nature  and 
ill  adapted  to  any  separation  by  quick  filtration,  so  necessary  in  a  process  of  this 
description.  It  was  found  that  when  a  solution  of  copper  sulphate  was  added  to 
a  solution  of  the  mixed  acids  in  the  presence  of  barium  carbonate,  the  precipita- 
tion proceeds  with  the  utmost  regularity.  The  carbonate  immediately  forms 
insoluble  sulphate  with  the  free  acid,  and  also  helps  to  consolidate  the  precipitated 
copper  salts,  so  that  towards  the  end  of  the  reaction  they  fall  rapidly  to  the 
bottom  of  the  vessel,  leaving  the  supernatant  liquid  clear.  This  separation  is 
a  good  indication  that  the  end  of  the  titration  is  near,  and  is  supplemented  by 
the  ferrocyanide  test. 

A  molified  method  of  testing  for  the  excess  of  copper  in  the  solution  is  as 
follows : — Pieces  of  stout  Swedish  filter-paper  one  inch  square  are  folded  across 
the  middle,  and  a  drop  of  the  liquid  to  be  tested  taken  up  on  a  glass  rod  and 
gently  dropped  on  to  the  top  surface.  The  liquid  will  percolate  through  to  the 
under  fold,  leaving  the  precipitate  on  the  upper  one.  It  is  then  only  necessary 
to  unfold  the  sheet  and  apply  a  drop  of  ferrocyanide  to  the  under  surface.  If 
the  reaction  is  complete  a  faint  pink  colouration  will  take  place,  which  is  perhaps 
more  easily  recognised  by  transmitted  light. 

The  results  obtained  by  duplicate  experiments  tend  to  show  that  the  copper 
salts  are  perfectly  constant  in  composition  when  precipitated  in  this  manner,  and 
the  results  equal  in  accuracy  any  obtained  with  other  processes. 

About  1  gm.  of  barium  carbonate  was  added  in  each  case  and  the  solution 


362 


TANNIC   ACID. 


heated  up  to  90°  C.  before  titration.  The  temperature  at  the  end  of  the  titration 
should  not  be  less  thau  30°  C. 

The  precipitation  by  copper  is  done  on  say  25  c.c.  of  the  solution  of  the 
sample  and  the  results  noted.  50  c.c.  of  the  same  sample  are  then  mixed  with 
the  usual  proportions  of  gelatine,  salt,  acid,  and  barium  sulphate  ;  diluted  to 
100  c.c.,  then  filtered  through  a  dry  filter  and  50  c.c.  (  =25  c.c.  of  the  original 
liquid)  titrated  with  copper  solution  as  before,  the  difference  being  calculated  to 
available  tannin.  1 

The  experiments  show  that  the  separation  of  the  tannic  acid  by  means  of  an 
acid  solution  of  gelatine  and  salt  will  not  affect  the  general  results  obtained, 
and  this  method  for  want  of  a  better  was  used  in  the  experiments,  Procter's 
modification  being  considered  the  most  accurate,  and  therefore  adopted. 

The  following  table  was  prepared  from  experiments,  showing  the  error  due  to 
the  indicator  in  c.c.  of  standard  solution  added  to  different  quantities  of  water  : — 


c.c.  of  Water. 

c.c.  of  Standard  Solution 
required. 

20 

0-3 

30 

0-4 

60 

0-7 

100 

1-0 

150 

1-5 

The  above  correction  should  be  made  in  all  cases. 

A  sample  of  so-called  pure  tannic  acid  gave  the  following  results 


Weight  taken. 

c.c.  required. 

G-m. 

0-5 

25-0 

0-5 

25-2 

0-5 

25-2 

Slightly  lower  results  were  obtained  when  the  operation  was  conducted  in 
the  cold,  probably  owing  to  the  slower  action  of  the  carbonate  on  the  free  acid  ; 
but  the  rate  of  running  in  of  the  solution  had  no  appreciable  effect  on  the  quantity 
required. 

A  sample  of  the  purest  gallic  acid  that  could  be  obtained  gave  the  following 
figures : — 


Weight  taken. 

c.c.  required. 

Gm. 
0-5 

45-0 

0-5 

44-8 

Allowing  that  the  acid  was  of  90  per  cent,  purity,  these  results  would  give 
a  value  for  each  c.c.  of  O'Olll  gm.  This  figure  must  of  course  only  be  taken  as 
approximate.  It  will  be  seen  that  more  solution  is  required  to  precipitate  the 
gallic  than  the  tannic  acid.  This  is  also  noticed  in  Lowenthal's  method. 

The  chief  advantage  claimed  by  the  author  of  this  method  over 
Lowenthal's  are  as  follows  : — 

(1)  Both  the  tannic  and  gallic  acids  are  determined. 

(2)  Rapidity  where  a  simple  assay  is  sufficient. 


TANNIC -ACID.  363 


(3)  The  results  are  expressed  in  terms  of  the  copper  oxide 
precipitated. 

(4)  The  standard  solution  keeps  well,  and  there  is  no  correction 
necessary  for  indigo  solution  or  gelatine. 

(5J  Larger  quantities  of  the  solution  can  be  titrated,  thus 
reducing  the  working  error. 

It  seems  to  be  possible  to  use  this  method  for  substances  other 
than  tannic  or  gallic  acids,  e.g.,  Fustic. 

The  following  results  were  obtained  with  a  sample  of  pure  Fustic 
extract  51°  Tw. 

0'5  gm.  taken  required  11 '5  c.c.  of  standard  solution. 

0*5  gm.  taken  required  11*6  c.c.  of  standard  solution. 

The  end  of  the  reaction  was  sharp  when  the  titration  was  carried 
on  at  the  boiling-point  and  the  precipitate  settled  well. 

Rapid  Method  for  the  determination  of  Tannin  Materials. 

Gardner  and  Hodgson*  observed  that  tannic  and  gallic  acids 
.are  readily  attacked  by  alkaline  reducing  agents,  and  developed 
the  following  process  for  the  determination  of  tannic  acid  : — 

To  an  aqueous  solution  of  tannic  acid  standard  iodine  solution  is  added  in  excess, 
and,  after  the  addition  of  a  few  drops  of  starch  solution,  sodium  hydroxide  solution 
is  added  until  the  blue  coloration  disappears  ;  an  excess  of  NaOH  is  to  be  avoided. 
Dilute  HC1  is  then  added  in  sufficient  amount  to  liberate  the  unabsorbed  iodine, 
the  amount  of  which  is  determined  by  titration  with  standard  sodium  thiosulphate 
solution.  The  method  was  applied  to  ordinary  tannin-containing  materials, 
viz.,  gall-nuts,  sumach,  valonia,  etc.,  the  results  being  compared  with  those 
obtained  by  using  the  Lowenthal  process.  The  difference  in  the  results  ob- 
tained by  the  two  methods  was  usually  less  than  1  per  cent.  The  iodine  method 
was  applied  both  before  and  after  precipitating  the  tannic  acid  by  means  of 
gelatine,  as  in  the  Lowenthal  process. 

Other  Methods  of  Determining  Tannin. 

Direct  Precipitation  by  Gelatine. — The  difficulty  existing  with 
this  method  is  that  of  getting  the  precipitate  to  settle,  so  that  it  may 
be  clearly  seen  when  enough  gelatine  has  been  added. 

Tolerably  good  results  may  sometimes  be  obtained  by  using 
a  strong  solution  of  sal  ammoniac  or  chrome  alum  as  an  adjunct. 
The  best  aid  is  probably  barium  sulphate,  2  or  3  gm.  of  which 
should  be  added  to  each  portion  of  liquid  used  for  titration. 

Standard  solution  of  gelatine  should  contain  1-33  gm.  of  dry 
gelatine  per  litre,  together  with  a  few  drops  of  chloroform  or  a  small 
quantity  of  thymol  to  preserve  it.  45  c.c.  =0-05  gm.  tannin 
(Carles).  This  method  is  adapted  only  for  rough  technical 
purposes,  as  also  is  the  following  : — 

Direct  Precipitation  by  Antimony. — This  method  is  still  in  favour 
with  some  operators  ;  but,  like  the  gelatine  process,  is  beset  with 
the  difficulty  of  getting  the  precipitate  to  settle. 

Standard  antimony  solution  is  made  by  dissolving  2*611  gm.  of 

*  Paper  communicated  to  the  seventh  International  Congress  of  Applied 
Chemistry  (1909). 


364  TIN. 

crystals  of  tartar  emetic  dried  at  100°  C.  in  a  litre.  1  c.c.  =0*005  gm. 
tannin.  This  liquid  may  also  be  kept  from  decomposition  by  a  few 
grains  of  thymol.  50  c.c.  of  the  tannin  solution  may  be  taken  for 
titration,  to  which  is  added  1  or  2  gm.  of  sal  ammoniac,  and  the 
antimonial  solution  run  in  until  no  further  cloudiness  is  produced. 
In  both  the  above  methods  the  final  tests  must  either  be  macfe  by 
repeatedly  filtering  small  portions  to  ascertain  whether  the 
precipitation  is  complete,  or  by  bringing  drops  of  each  liquid  together 
on  black  glass  or  a  small  mirror. 

TIN. 

Sn  =  119. 

Metallic  iron  x  1  -0654  =  Tin. 

Double  iron  salt       xO'1522  =  „ 
Factor  for  N/10  iodine 

or      permanganate 

solution  0-00595 

THE  method,  originally  devised  by  Streng,  for  the  direct 
determination  of  tin  by  potassium  dichromate,  or  other  oxidizing 
agent  in  acid  solution,  has  been  found  most  unsatisfactory,  fr<5m 
the  fact  that  varying  quantities  of  water  or  acid  seriously  interfere 
with  the  accuracy  of  the  results.  The  cause  is  not  fully  understood, 
but  that  it  is  owing  partly  to  the  oxygen  mechanically  contained 
in  the  water  reacting  on  the  very  sensitive  stannous  chloride  there 
can  be  very  little  doubt,  as  the  variations  are  considerably  lessened 
by  the  use  of  water  recently  boiled  and  cooled  in  closed  vessels. 
These  difficulties  are  set  aside  by  the  processes  of  Lenssen, 
Lowenthal,  Stromeyer,  and  others,  now  to  be  described,  which 
are  found  fairly  satisfactory. 

1.     Direct  Titration  by  Iodine  in  Alkaline  Solution. 
(Lenssen). 

Metallic  tin  or  its  protosalt,  if  not  already  in  solution,  is  dissolved 
in  hydrochloric  acid,  and  a  tolerable  quantity  of  Rochelle  salt 
added,  together  with  sodium  bicarbonate  in  excess.  If  enough 
tartrate  be  present,  the  solution  will  be  clear  ;  starch  is  then  added, 
and  the  mixture  titrated  with  N/10  iodine.  Metallic  tin  is  best 
dissolved  in  HC1  by  placing  a  platinum  crucible  or  cover  in  contact 
with  it,  so  as  to  form  a  galvanic  circuit. 

Benas*  points  out  that  the  chief  error  in  the  determination 
as  above  arises  from  oxygen  dissolved  in  the  liquid,  or  absorbed 
during  the  operation.  In  order  to  obtain  constant  results,  it  is 
necessary  to  dissolve  the  tin  compound  in  HC1,  dilute  with  oxygen- 
free  water,  and  add  at  once  excess  of  standard  iodine,  which  excess 
is  found  by  residual  titration  with  standard  thiosulphate. 

S.  W.  Youngf  has  called  attention  to  the  fact  that  the  determi- 
nation of  tin  can  be  carried  out  in  acid  solution,  though  not  in  the 

«  Chem.  Centr-blatt.  51,  957.  t  J-  Am.  G.  S.  19,  809. 


TIN.  365 

same  way  as  advocated  by  Benas.  The  solution  is  best  made  in 
dilute  hydrochloric  acid,  and  must  of  course  be  free  from  other 
oxidizing  or  reducing  matters.  To  prevent  the  action  of  air  the 
stannous  compound  must  be  rapidly  prepared  and  titrated  im- 
mediately with  excess  of  standard  iodine  and  starch.  It  is  essential 
that  the  potassium  iodide  used  in  making  the  iodine  solution 
should  be  free  from  iodate.  The  determination  of  the  amount  of 
iodine  in  excess  is  best  done  with  dilute  stannous  chloride,  the 
strength  of  which  in  relation  to  the  standard  iodine  must  be  known 
either  just  before  or  after  the  tin  experiment.  The  results  obtained 
by  Young  were  a  little  higher  than  the  theoretical,  which  is 
attributed  to  the  iodine  being  standardized  by  thiosulphate  in 
a  neutral,  instead  of  an  acid,  solution,  but  as  mentioned  in  the 
beginning  of  this  section  variations  in  tin  titrations  occur  from 
several  causes  difficult  to  understand.  The  method  possesses 
some  advantage  over  the  following,  inasmuch  as  iodides,  bromides, 
and  salts  of  iron,  when  present,  cause  no  difficulty. 

2.    Indirect  Titration  by  Ferric  Chloride  and 
Permanganate  (Lowenthal,  Stromeyer,  etc.). 

This  method  owes  its  value  to  the  fact  that  when  stannous 
chloride  is  brought  into  contact  with  ferric  or  cupric  chloride  it 
acts  as  a  reducing  agent,  in  the  most  exact  manner,  upon  these 
compounds,  stannic  chloride  being  formed,  together  with  a  pro- 
portionate quantity  of  ferrous  or  cuprous  salt,  as  the  case  may  be. 
If  either  of  the  latter  be  then  titrated  with  permanganate,  the 
original  quantity  of  tin  may  be  found,  the  reaction  being,  in  the 
case  of  iron, — 

Sn012 + Fe2Cl6  =  SnCl4  +  2FeCl2. 

55-85  iron  =  59-5  tin.  If  decinormal  permanganate,  or  the  factor 
necessary  to  convert  it  to  that  strength,  be  used,  the  calculation  by 
means  of  iron  is  not  necessary. 

METHOD  OF  PROCEDURE  :  The  solution  of  stannous  chloride,  or  other  protosalt 
of  tin  in  HC1,  or  the  granulated  metal,  is  mixed  with  pure  ferric  chloride  (which, 
if  tolerably  concentrated,  dissolves  metallic  tin  readily,  and  without  evolution  of 
hydrogen)  then  diluted  with  distilled  water,  and  titrated  with  permanganate  as 
usual.  To  obtain  the  most  exact  results,  it  is  necessary  to  make  an  experiment 
with  the  same  permanganate  upon  a  like  quantity  of  water,  to  which  ferric  chloride 
is  added  ;  the  quantity  required  to  produce  the  same  rose  colour  is  deducted  from 
the  total  permanganate,  and  the  remainder  calculated  as  tin. 

Stannic  salts,  also  tin  compounds  containing  iron,  are  dissolved  in  water, 
HC1  added,  arid  a  plate  of  clean  zinc  immersed  for  ten  or  twelve  hours  ;  the  tin 
so  precipitated  is  carefully  collected  and  washed,  then  dissolved  in  HC1,  and 
titrated  as  above  ;  or  the  finely  divided  metal  may  at  once  be  mixed  with  an 
excess  of  ferric  chloride,  a  little  HC1  added,  and  when  solution  is  complete,  titrated 
with  permanganate.  4  eq.  of  iron  (  =223'4)  occurring  in  the  form  of  ferrous 
chloride  represents  1  eq.  (  =119)  of  tin. 

Tin  may  also  be  precipitated  from  slightly  acid  peroxide  solution 
as  sulphide  by  H2S,  the  sulphide  well  washed,  and  mixed  with 
ferric  chloride,  the  mixture  gently  warmed,  the  sulphur  filtered  off, 


366  TIN. 

and  the  filtrate  then  titrated  with  permanganate  as  above.  4  eq. 
of  iron  =  1  eq.  of  tin. 

Tin  Ore. — In  the  case  of  analysis  of  cassiterite,  Arnold* 
recommends  that  1  gm.  of  the  very  finely  powdered  mineral  be 
heated  to  low  redness  for  two  hours  in  a  porcelain  boat  in  a  glass 
tube  with  a  brisk  current  of  dry  and  pure  hydrogen  gas,  by  which 
means  the  metal  is  reduced  to  the  metallic  state.  It  is  then 
dissolved  in  acid  ferric  chloride,  and  titrated  with  permanganate 
or  dichromate  in  the  usual  way. 

The  Determination  of  Tin  in  White-metal  Alloys.  Ibbotson 
and  Brearley.f  Tin  may  readily  be  determined  by  reducing  the 
hot  solution  of  the  chloride,  cooling  in  an  atmosphere  of  CO2,  and 
titrating  with  iodine  and  starch.  The  reduction  can  be  conveniently 
effected  by  means  of  iron,  but  any  excess  added  must  be  dissolved 
completely.  If  antimony  is  present,  it  will  be  precipitated  as 
metal,  and  cannot  be  dissolved  again,  but  as  cold  acid  solutions 
of  stannic  chloride  are  not  reduced  by  antimony,  which  readily 
reduces  them  on  heating,  the  solution  may  be  directly  titrated  with 
iodine  as  usual,  very  good  results  being  obtained.  The  antimony 
may  be  filtered  off  after  the  titration  and  determined,  but  the 
results  obtained  are  rather  low. 

An  improvement  on  the  above  method  consists  in  reducing  the 
stannic  chloride  with  finely-powdered  metallic  antimony.  The 
reduction  of  0*15  gm.  of  tin  is  complete  after  one  minute's  boiling, 
and  the  excess  of  antimony  which  remains  undissolved  acts  as 
a  safeguard  during  the  cooling,  since  it  reduces  any  tin  which  may 
have  become  oxidized  whilst  the  solution  is  still  hot.  Cold  solutions 
of  stannous  chloride  take  up  oxygen  less  readily.  The  test  analyses 
given  show  that  the  reduction  is  complete. 

The  influence  of  various  substances  likely  to  be  present  in  an 
ordinary  analysis  on  the  above  method  was  examined,  and  it  was 
found  that  the  presence  of  iron,  chromium,  nickel,  zinc,  manganese, 
aluminium,  bismuth,  phosphorus,  and  sulphur  is  without  effect  on 
the  results.  The  quantity  of  hydrochloric  acid  present  should 
always  be  about  one-fifth  of  the  total  volume.  If  copper  is  present, 
it  will  be  reduced  to  the  cuprous  state,  but  accurate  results  may 
nevertheless  be  obtained  if  the  iodine  is  added  drop  by  drop  to 
the  vigorously  agitated  solution,  so  as  to  prevent  the  formation  of 
a  local  excess  of  iodine.  It  is  also  advisable  to  have  rather  more 
hydrochloric  acid  present,  up  to  about  one- third  of  the  total  volume. 
Cobalt  apparently  gives  very  slightly  higher  values.  Lead  is 
without  influence  if  sufficient  hydrochloric  acid  is  present  to  prevent 
the  formation  of  lead  iodide.  The  presence  of  arsenic  completely 
vitiates  the  results,  whether  the  tin  is  reduced  with  iron  or  with 
antimony.  Mercury  is  reduced  to  the  metallic  state,  but  is  not 
oxidized  in  cold  solutions.  If  molybdenum  or  tungsten  be  present,  a 
coloured  lower  oxide  is  formed,  but  this  is  not  appreciably  re-oxidized 
by  the  iodine,  and  the  starch  blue  can  be  readily  distinguished. 

•C.  N.  36,  238  f  C.  N.  84,  167. 


TITANIUM.  367 

TITANIUM. 

Ti=48'l. 

H.  L.  Wells  and  W.  L.  Mitchell*  allude  to  a  volumetric  method 
of  determining  titanic  acid  by  Pisani,|  which  does  not  appear  to 
have  been  found  satisfactory.  MarignacJ  applied  Pisani's 
method  to  the  determination  of  titanic  acid  in  the  presence  of 
niobic  acid,  special  conditions  being  adopted  to  avoid  the  reduction 
of  the  latter. 

The  authors  have  modified  Pisani's  process  as  improved  by 
Marignac,  and  employ  it  for  the  determination  of  iron  together 
with  the  titanic  acid  in  ores.  Sulphuric  acid  solutions  are  used, 
and  the  liquid  is  protected  from  the  air  during  cooling  and  titration 
by  means  of  a  current  of  carbon  dioxide. 

METHOD  OP  PROCEDURE  :  5  gm.  of  the  pulverized  ore  are  treated  with  100  c.c. 
of  concentrated  hydrochloric  acid  in  a  covered  beaker,  using  a  gradually  increasing 
heat,  and  adding  more  acid  if  necessary.  When  there  is  no  further  action,  50  c.c. 
of  a  mixture  of  equal  volumes  of  sulphuric  acid  and  water  are  added,  and  the 
liquid  evaporated  until  it  fumes  strongly.  After  cooling,  200  c.c.  of  water  are 
added,  the  whole  heated  until  the  sulphates  dissolve,  and  the  liquid  filtered  into 
a  litre  flask.  If  anything  besides  silicious  matter  is  left  on  the  filter-paper,  it 
should  be  fused  with  potassium  bisulphate,  treated  with  concentrated  sulphuric 
acid,  and  the  sulphates  dissolved  in  hot  water  and  added  to  the  main  solution. 

The  liquid  in  the  flask  is  made  up  to  the  mark  with  water,  and  4  portions  of 
200  c.c.  each  taken,  2  in  Erlenmeyer  flasks  (500  c.c.),  and  the  other  2  in 
ordinary  350  c.c.  flasks.  Each  of  these  represents  1  gm.  of  the  ore. 

To  determine  the  iron,  H2S  is  passed  into  the  solutions  in  the  ordinary  flasks  to 
saturation,  after  which  they  are  boiled  until  all  the  H2S  has  been  removed,  care 
being  taken  to  avoid  any  contact  of  the  solution  with  the  air  by  covering  the  mouths 
of  the  flasks  with  crucible  lids.  The  flasks  are  then  quickly  filled  to  the  neck 
with  cold  recently-boiled  water,  rapidly  cooled,  transferred  to  large  beakers,  and 
titrated  with  standard  potassium  permanganate. 

To  the  solutions  in  the  Erlenmeyer  flasks  25  c.c.  of  concentrated  sulphuric 
acid  are  added,  and  3  or  4  rods  of  pure  zinc,  about  50  mm.  long  and  6  or  7  mm. 
in  diameter  are  suspended  in  the  liquid  by  means  of  a  platinum  wire  attached  to 
the  loop  of  a  porcelain  crucible  lid,  which  is  inverted  over  the  mouth  of  the  flask. 
The  liquid  is  then  gently  boiled  for  30  or  40  minutes.  Then,  without  interrupting 
the  boiling,  a  rapid  current  of  CO2  is  introduced  under  the  cover.  The  flask  is 
now  rapidly  cooled,  the  zinc  washed  with  a  jet  of  water  and  removed,  and  the 
solution  titrated  with  permanganate,  while  the  current  of  C02  is  still  being  passed 
in.  The  difference  between  the  permanganate  used  in  this  case  and  that  required 
for  the  iron  alone,  represents  the  amount  corresponding  to  the  titanic  acid.  The 
factor  for  metallic  iron  divided  by  0'7  gives  the  factor  for  titanic  acid  (TiO2). 

The  most  convenient  strength  for  the  permanganate  solution  is  one  of  7 '9  gm. 
per  litre,  corresponding  to  about  0*014  gm.  of  metallic  iron. 

In  the  determination  of  iron  by  reduction  with  sulphuretted  hydrogen,  no 
effect  is  produced  on  cold  permanganate  solution  by  the  precipitated  sulphur 
present,  but  precipitated  sulphides,  such  as  copper  sulphide,  should  be  filtered  off 
before  boiling. 

The  results  of  test  analyses  of  recrystallized  potassium  titano- 
fluoride  (K2TiF6)  were  somewhat  low,  but  probably  quite  as  good  or 
better  than  any  gravimetric  method  could  furnish. 

Another  method  for  the  determination  of  titanium  and  iron  in 
iron,  ores,  etc.  is  given  by  Knecht  and  Hibbert,||  as  follows : — 

*  J.  Am,  C.  #.  1895,  878.  t  Compt.  Rend.  59,  289.  J  Z.  a.  C.  7,  112. 

II  New  Reduction  Methods  in  Volumetric  Analysis  by  Knecht  and  H  i  b  b ert,  1910. 


368 


TITANIUM. 


METHOD  OF  PROCEDURE:  About  0-5-1-0  gm.  of  the  finely  powdered  ore  is 
fused  with  about  10  times  its  weight  of  caustic  potash  in  a  nickel  dish.  The 
melt,  when  cool,  is  treated  with  water,  acidified  with  HC1,  and  the  solution  made 
up  to  250  c.c.  25  or  50  c.c.  are  then  transferred  to  a  conical  flask  (of  about  200 
c.c.  capacity)  in  which  the  titanic  salt  is  reduced.  The  apparatus  used  is  as 
shown  in  fig.  55.  The  flask  is  fitted  with  a  rubber 
stopper  having  3  holes.  The  central  one  carries  a 
glass  tube  fitted  with  a  B  u  n  s  e  n  valve  and  through 
it  passes  a  platinum  wire  carrying  a  zinc  rod  r. 
The  other  two  holes  are  temporarily  closed  by  glass 
rods.  When  the  titanic  solution  has  been  transferred 
to  the  flask,  HC1  is  added,  and  the  flask  closed  by 
the  stopper,  and  the  zinc  lowered  into  the  liquid  by 
means  of  the  platinum  wire  passing  through  the 
small  pierced  rubber  stopper,  c.  Reduction  will  be 
complete  in  15-20  minutes,  20  minutes  at  least  being 
necessary  to  ensure  complete  reduction  when  much 
iron  is  present.  One  of  the  glass  rods,  a,  is  then 
removed  and  a  stream  of  carbon  dioxide  passed 
into  the  flask  through  a  tube  inserted  in  the  same 
hole.  The  zinc  is  removed  from  the  solution  by 
raising  the  platinum  wire,  and  the  other  rod  6  is 
then  removed  also.  The  zinc  is  washed  with  a  little 
freshly  boiled  water  from  a  wash-bottle,  the  solution 
cooled,  and  titrated  with  standard  iron  alum  solution, 
using  potassium  sulphocyanate  as  indicator.  The 
standard  iron  alum  solution  consists  of  a  solution 
of  about  14  gm.  iron  alum  dissolved  in  water, 
acidified  with  sulphuric  acid  till  the  liquid  assumes 
a  pale  straw  colour,  and  made  up  to  1  litre.  By 
measuring  the  exact  volume  of  standard  titanous 
chloride  solution  (see  p.  234)  required  to  reduce  25 
c.c.  of  this  iron-alum  solution,  using  potassium 
sulpho-cyanate  as  indicator,  its  strength  is  deter- 
mined, and  as  it  will  retain  the  same  strength  for 

an   indefinite   period,  this  iron  alum   solution  may  be  used  in  all  subsequent 
cases  for  standardizing  the  titanous  chloride  solution. 

Ex.  0-4997  gm.  of  rutile  treated  as  above  and  made  up  to  250  c.c. 
25  c.c.,  after  reduction  with  zinc  and  hydrochloric  acid,  required  17-3  c.c.  of 
iron  alum  solution. 

1  c.c.  of  iron  alum  solution  contained 

0-001842  gm.  Fe^-001842  * ,80'1  =0-002642  gm.  Ti02. 


Fig.  55. 


Hence 


55-85 
•002642  x  17-3  xlOO 


0-04997 

Also,  1-960  gm.  treated  in  the  same  way,  and  titrated  with  titanous  chloride, 
required  26'1  c.c. 

1  c.c.  TiCL,  =  0-001432  gm.  Fe -0-002047  gm.  Fe203. 

„         0-002047  x26-l  x  100     0  _0  0/  _,  n 
Hence  -  —  =2-73  %  Fe03. 

URANIUM. 
U  =  238-5. 

THE  determination  of  uranium  may  be  conducted  with  great 
accuracy  by  permanganate,  in  precisely  the  same  way  as  ferrous 
salts  (p.  231).  The  metal  must  be  in  solution  either  as  acetate, 
sulphate,  or  chloride,  but  not  as  nitrate.  In  the  latter  case  it  is 
necessary  to  evaporate  to  dryness  with  excess  of  sulphuric  or 


URANIUM.  369 

hydrochloric  acid,  or  to  precipitate  with  alkali,  wash  and  redissolve 
in  acetic  acid. 

The  reduction  to  the  uranous  state  is  made  with  zinc,  but  as  the 
end  of  reduction  cannot,  like  iron,  be  known  by  the  colour,  it  is 
necessary  to  continue  the  action  for  a  certain  time  ;  in  the  case  of 
small  quantities  for  a  quarter,  of  larger  for  half  an  hour,  at 
a  temperature  of  50°  to  60°  C.,  and  in  the  presence  of  excess  of 
sulphuric  acid  ;  all  the  zinc  must  be  dissolved  before  titration. 
The  solution  is  then  freely  diluted  with  boiled  water,  sulphuric 
acid  added  if  necessary,  and  then  permanganate  until  a  faint 
permanent  rose  colour  is  obtained.  The  ending  is  distinct  if  the 
solution  be  well  diluted,  and  the  reaction  is  precisely  the  same  as 
in  the  case  of  ferrous  salts  ;  namely,  2  eq.  of  uranium  existing  in 
the  uranous  state  require  1  eq.  of  oxygen  to  convert  them  to  the 
uranic  state  ;  hence  55'85  Fe  =  119'25  IT,  consequently  the  strength 
of  any  permanganate  solution  in  relation  to  iron  being  known,  it 
is  easy  to  find  the  amount  of  uranium. 

Another  method  of  determining  uranium  has  been  published  by 
B.  Glasmann.* 

The  method  depends  on  the  reaction  of  neutral  solutions  of  uranyl  salts  on 
a  mixture  of  potassium  iodide  and  iodate  — 

3U02(N03)2  +5KI  +KI03  +3H2O  =3U02(OH)2  +6KN03  +  3I2. 
Any  excess  of  acid  in  the  solution  must  be  neutralized  with  sodium  carbonate, 
which  is  added  till  a  precipitate  begins  to  be  permanent  ;  this  precipitate  is  then 
just  redissolved  in  dilute  acid.  Place  the  solution  in  a  300  c.c.  distillation  flask 
provided  with  a  ground  stopper  carrying  a  funnel  with  stop-cock,  the  tube  of 
which  reaches  to  the  bottom  of  the  flask.  Insert  the  exit  tube  of  the  flask  into 
the  receiver  containing  potassium  iodide  solution,  add  to  the  uranyl  solution  the 
requisite  amount  of  iodide  and  iodato  mixture,  dilute  to  120  c.c.,  close  with  the 
stoppered  funnel,  and  slowly  heat  to  boiling.  When  boiling,  cool  the  receiver 
with  water,  and  lead  a  stream  of  hydrogen  through  the  boiling  liquid.  When  the 
liquid  is  reduced  to  50  c.c.,  withdraw  the  flask  from  the  receiver,  remove  the 
burner,  wash  the  delivery  tube  into  a  beaker,  rinse  the  contents  of  the  receiver 
into  the  same  beaker,  and  titrate  with  thiosulphate.  With  0*2  —  0'3  gm.  of 
uranyl  compound  the  whole  operation  requires  20  minutes.  The  results  are 
accurate,  and  are  not  affected  by  the  presence  of  alkaline-earth  chlorides. 

VANADIUM. 


VANADIUM  salts,  or  the  oxides  of  this  element,  may  be  very 
satisfactorily  titrated  by  reduction  with  a  standard  ferrous  solution  : 
thus  — 

2FeO+V205=Fe203+VA- 

1  gm.  of  Fe  represents  1-633  gm.  of  vanadic  pentoxide. 

Lindemannf  recommends  the  use  of  a  solution  of  ferrous 
ammonio-sulphate  standardized  by  N/10  potassium  dichromate. 

Of  course  it  is  necessary  that  the  vanadium  compound  should  be 
in  the  highest  state  of  oxidation,  preferably  in  pure  sulphuric  acid 
solution.  The  blue  colour  of  the  tetroxide  in  the  dilute  liquid  has 
no  misleading  effect  in  testing  with  ferricyanide. 

*  Ber.  1904,  189.  f  Z.  a.  C.  18,  99. 

2    B 


370  VANADIUM. 

With  hydrochloric  acid  great  care  must  be  taken  to  ensure 
absence  of  free  Cl  or  other  impurities.  The  end-point  in  the  case 
of  this  acid  is  different  from  that  with  sulphuric  acid,  owing  to  the 
colour  of  the  ferric  chloride,  the  mixture  becoming  clear  green. 

The  accuracy  of  the  action  is  not  interfered  with  by  ferric  or 
chromic  salts,  alumina,  fixed  alkalies,  or  salts  of  ammonia. 

Vanadic  solutions  being  exceedingly  sensitive  to  the  action  of 
reducing  agents,  great  care  must  be  exercised  to  exclude  dust  or 
other  carbonaceous  matters,  alcohol,  etc. 

The  reduction  of  vanadic  acid  by  hydriodic  or  hydrobromic  acid ; 
and  its  titration  in  alkaline  solution  with  iodine  has  been  worked 
out  by  P.  E.  Browning.*  The  solution  containing  the  vanadate 
is  boiled  in  an  Erlenmeyer  beaker  with  potassium  iodide  or 
bromide,  in  not  too  large  a  quantity,  and  a  regulated  amount  of 
sulphuric  acid,  until  no  more  iodine  or  bromine  is  liberated.  After 
cooling,  the  residual  liquid  is  nearly  neutralized  with  aqueous 
potash,  a  small  quantity  of  tartaric  acid  is  added,  and  the  solution 
made  alkaline  by  addition  of  potassium  bicarbonate.  Excess  of 
standard  iodine  is  then  added,  and  after  remaining  for  half  an  hour 
in  a  well-closed  bottle,  the  free  iodine  left  is  determined  by  means 
of  a  solution  of  arsenious  oxide  in  the  usual  way. 

One  mol.  of  iodine  represents  1  mol.  of  vanadium  pentoxide. 

Determination  of  Vanadium  (and  Chromium)  in  Steel.  The 
following  methodf  is  given  by  J.  Kent  Smith,  American  Vanadium 
Co.,  Pittsburg,  Pa.,  U.S.A. 

Dissolve  4  grams  of  the  steel  in  48  c.c.  of  water  and  12  c.c.  of  strong  sulphuric 
acid,  and  when  dissolved  oxidize  with  nitric  acid,  avoiding  a  great  excess. 
Evaporate  to  dryness  on  a  hot  plate,  take  up  with  150  c.c.  of  water  and  boil  till 
dissolved.  Add  10  c.c.  (or  excess)  of  a  2£  %  permanganate  solution,  and  boil 
for  5  minutes ;  then  add  a  little  manganese  sulphate  to  precipitate  any  undecom- 
posed  permanganate.  Cool,  dilute  to  500  c.c.,  filter  through  a  dry  filter  and  take 
375  c.c.  (  =3  grams  Steel)  of  the  clear  filtrate.  To  this  latter  add^SO  c.c.  of  dilute 
sulphuric  acid,  then  a  measured  excess  of  N/io  ferrous  sulphate  and  titrate  back 
with  N/io  permanganate  till  permanently  pink :  each  c.c.  of  permanganate 
thus  used  up  equals  0-001743  gm.  chromium.  Now  add  1  or  2  c.c.  of  ferrous 
sulphate  solution  to  dissolve  any  Mn02  that  may  have  been  formed,  and  add 
permanganate  very  gradually  until  the  solution  is  just  pink,  then  cautiously  add 
fr/ao  ferrous  sulphate  until  the  pink  is  just  discharged  (this  should  be  done 
exactly  to  one  drop).  Next  add  a  carefully  measured  5  c.c.  (or  excess)  of  N/2O 
ferrous  sulphate,  and  titrate  back  with  N/2O  dichromate,  using  potassium  ferri- 
cyanide  as  indicator.  1  c.c.  N/QQ  dichromate  =  0*00256  gram  Vanadium. 

ZINC. 

Zn  =  65-37. 

1  c.c.  N/10  solution =0-003268  gm.  Zinc. 
Metallic  iron  x  0-5852     =       Zinc. 

„  x  0-7285     =      Zinc  oxide. 

Double  iron  salt        x  0-0836     =       Zinc. 
,,  „  x  0-1041     =      Zinc  oxide. 

*  J.  Amer.  Sci.  1896,  185. 
t  See  Macfarlane's  Laboratory  Notes  on  Iron  and  Steel  Analyses,  p.  222 


ZINC.  371 

1.    Indirect  Method  (Mann). 

THIS  process  gives  exceedingly  good  results,  and  consists  in 
precipitating  the  zinc  as  hydrated  sulphide,  decomposing  the 
sulphide  with  moist  silver  chloride,  then  determining  the  zinc 
chloride  so  formed  by  Volhard's  method  (p.  145). 

The  requisite  materials  are — 

Silver  chloride. — Well  washed  and  preserved  from  the  light  under 
water. 

Standard  silver  nitrate. — 33*006  gm.  of  pure  silver  dissolved  in 
nitric  acid  and  made  up  to  1  litre,  or  51-98  gm.  silver  nitrate  per 
litre.  If  made  direct  from  silver,  the  solution  must  be  well  boiled 
to  dissipate  nitrous  acid.  1  c.c.=0'01  gm.  of  zinc. 

Ammonium  thiocyanate. — Of  such  strength  that  exactly  3  c.c, 
suffice  to  precipitate  1  c.c.  of  the  silver  solution. 

Ferric  indicator  and  pure  nitric  acid  (see  p.  146). 

METHOD  OF  PROCEDURE:  0*5  to  1  gm.  of  the  zinc  ore  is  dissolved  in  nitric 
acid.  Heavy  metals  are  removed  by  H2S,  iron  and  alumina  by  double  precipita- 
tion with  ammonia.  The  united  filtrates  are  acidified  with  acetic  acid,  and  H2S 
passed  into  the  liquid  until  all  zinc  is  precipitated  as  sulphide.  Excess  of  H2S 
is  removed  by  rapid  boiling,  so  that  a  drop  or  two  of  the  filtered  liquid  gives  no 
further  stain  on  lead  paper.  The  precipitate  is  then  allowed  to  settle,  decanted 
while  hot,  the  precipitate  brought  on  a  filter  with  a  little  hot  water,  and,  without 
further  washing,  the  filter  with  its  contents  is  transferred  to  a  small  beaker, 
30-50  c.c.  of  hot  water  added,  well  stirred,  and  so  much  moist  silver  chloride 
added  as  is  judged  necessary  to  decompose  the  sulphide,  leaving  an  excess  of 
silver.  The  mixture  is  now  boiled  till  it  shows  signs  of  settling  clear ;  5  or  6 
drops  of  dilute  sulphuric  acid  (1  :  5)  are  added  to  the  hot  mixture,  and  in  a  few 
minutes  the  whole  of  the  zinc  sulphide  will  be  converted  into  zinc  chloride.  Thei 
free  sulphur  and  excess  of  silver  chloride  are  now  filtered  off,  washed,  and  the 
chloride  in  the  mixed  filtrate  and  washings  determined  as  follows : — 

To  the  cool  liquid,  measuring  200  or  300  c.c.,  are  added  5  c.c.  of  ferric  indicator, 
and  so  much  pure  nitric  acid  as  is  necessary  to  remove  the  yellow  colour  of  the 
iron.  A  measured  excess  of  the  standard  silver  solution  is  then  delivered  in 
with  the  pipette,  and  without  filtering  off  the  silver  chloride,  or  much  agitation 
so  as  to  clot  the  precipitate,  the  thiocyanate  is  cautiously  added,  with  a  gentle 
movement  after  each  addition,  until  a  permanent  light  brown  colour  appears. 

The  volume  of  silver  solution  represented  by  the  thiocyanate 
being  deducted  from  that  originally  used,  will  give  the  volume  to 
be  calculated  to  zinc,  each  c.c.  being  equal  to  O'Ol  gm.  Zn. 

2.     Precipitation  as  Sulphide  and  subsequent  titration  with  Ferric 
Salts  and  Permanganate  (S  c  h  w  a  r  z). 

THIS  method  is  based  on  the  fact  that  when  zinc  sulphide  is 
mixed  with  ferric  chloride  and  hydrochloric  acid,  or  better  still, 
with  ferric  sulphate  and  sulphuric  acid,  ferrous  or  zinc  chloride, 
or  sulphate  respectively,  and  free  sulphur  are  produced.  If  the 
ferrous  salt  so  produced  is  determined  with  permanganate  or 
dichromate,  the  proportional  quantity  of  zinc  present  is  ascertained. 
2  eq.  Fe  represent  1  eq.  Zn. 

Preparation  of  the  Ammoniacal  Zinc  Solution.— In  the  case  of  rich  ores  1  gm., 
and  poorer  qualities  2  gm.,  of  the  finely  powdered  material  are  put  into  a  small 

2   B   2 


372  ZINC. 

wide-mouthed  flask,  and  treated  with  HC1,  to  which  a  little  nitric  acid  is  added, 
the  mixture  is  warmed  to  promote  solution,  and  when  this  has  taken  place  the 
excess  of  acid  is  evaporated  by  continued  heat.  If  lead  is  present,  a  few  drops 
of  concentrated  sulphuric  acid  are  added  previous  to  complete  dryness,  in  order 
to  render  the  lead  insoluble  ;  the  residue  is  then  extracted  with  water  and  filtered. 
Should  metals  of  the  fifth  or  sixth  group  be  present,  they  must  be  removed  by 
H2S  previous  to  the  following  treatment.  The  solution  will  contain  iron,  and  in 
some  cases  manganese.  If  the  iron  is  not  already  fully  oxidized,  the  solution  must 
be  boiled  with  nitric  acid  ;  if  only  traces  of  manganese  are  present,  a  few  drops 
of  brominated  HC1  should  be  added.  When  cold,  the  solution  may  be  further 
diluted  if  necessary,  and  then  super-saturated  with  ammonia  to  precipitate  the 
iron ;  if  the  proportion  of  this  metal  is  small,  it  will  suffice  to  filter  off  and  wash 
the  oxide  with  ammoniacal  warm  water,  till  the  washings  give  no  precipitate 
of  zinc  on  adding  ammonium  sulphide.  Owing  to  the  fact  that  this  iron 
precipitate  tenaciously  holds  about  a  fifth  of  its  weight  of  zinc,  it  will  be  necessary 
when  the  proportion  is  large  to  redissolve  the  partly  washed  precipitate  in  HC1, 
and  reprecipitate  (best  as  basic  acetate) ;  the  filtrate  from  this  second  precipitate 
is  added  to  the  oringinal  zinc  filtrate,  and  the  whole  made  up  to  a  litre. 

METHOD  OP  PROCEDURE  :  The  ammoniacal  zinc  solution  (prepared  as  described 
above)  is  heated,  and  the  zinc  precipitated  in  a  tall  beaker  with  a  slight  excess  of 
sodium  or  ammonium  sulphide,  then  covered  closely  with  a  glass  plate,  and  set 
aside  in  a  warm  place  for  a  few  hours.  The  clear  liquid  is  removed  by  a  siphon, 
and  hot  water  containing  some  ammonia  again  poured  over  the  precipitate, 
allowed  to  settle,  and  again  removed,  and  the  washing  by  decantation  repeated 
three  or  four  times  ;  finally,  the  precipitate  is  brought  on  to  a  tolerably  large  and 
porous  filter,  and  well  washed  with  warm  water  containing  ammonia,  till  the 
washings  no  longer  discolour  an  alkaline  lead  solution.  The  filter  pump  may  be 
used  here  with  great  advantage. 

The  filter  with  its  contents  is  then  pushed  through  the  funnel  into  a  large 
flask  containing  a  sufficient  quantity  of  ferric  sulphate  mixed  with  sulphuric 
acid,  immediately  well  stoppered  or  corked,  gently  shaken,  and  put  into  a  warm 
place ;  after  some  time  it  should  be  again  well  shaken,  and  set  aside  quietly  for 
about  ten  minutes.  After  the  action  is  all  over  the  mixture  should  possess  a 
yellow  colour  from  the  presence  of  undecomposed  ferric  salt ;  when  the  cork  or 
stopper  is  lifted  there  should  be  no  odour  of  H2S.  The  flask  is  then  nearly  filled 
with  cold  distilled  water,  if  necessary  some  dilute  sulphuric  acid  added,  and  the 
contents  of  the  flask  titrated  with  permanganate  or  dichromate  as  usual, 

The  free  sulphur  and  filter  will  have  no  reducing  effect  upon  the 
permanganate  if  the  solution  be  cool  and  very  dilute. 

3.  Precipitation  by  Standard  Sodium  Sulphide,  with  Alkaline 
Lead  Solution  as  Indicator  (applicable  to  most  Zinc  Ores  and 
Products). 

The  ammoniacal  solution  of  zinc  is  prepared  just  as  previously 
described  in  Schwarz's  method. 

Standard  sodium  sulphide. — A  portion  of  caustic  soda  solution 
is  saturated  with  H2S,  sufficient  soda  added  to  remove  the  odour 
of  the  free  gas,  and  the  whole  diluted  to  a  convenient  strength  for 
titrating. 

Standard  zinc  solution. — 43*993  gm.  of  pure  zinc  sulphate  are 
dissolved  to  the  litre.  1  c.c.  will  then  contain  0*01  gm.  of  metallic 
zinc,  and  upon  this  solution,  or  one  prepared  from  pure  metallic 
zinc  of  the  same  strength,  the  sulphide  solution  must  be  titrated. 

Alkaline   lead   indicator. — Is    made   by   heating   together   lead 


ZINC.  373 

acetate,  tartaric  acid,  and  caustic  soda  solution  in  excess,  until 
a  clear  solution  is  produced.  It  is  preferable  to  mix  the  tartaric 
acid  and  soda  solution  first,  so  as  to  produce  sodium  tartrate  ;  or 
if  the  latter  salt  is  at  hand,  it  may  be  used  instead  of  tartaric  acid. 
Some  operators  use  sodium  nitroprusside  instead  of  lead. 

METHOD  OF  PROCEDURE  :  50  c.c.  of  zinc  solution  (  =0'5  gm.  Zn)  are  put  into 
a  beaker,  a  mixture  of  solutions  of  ammonia  and  ammonium  carbonate  (3  of  the 
former  to  about  1  of  the  latter)  added  in  sufficient  quantity  to  redissolve  the 
precipitate  which  first  forms.  A  few  drops  of  the  lead  solution  are  then,  by 
means  of  a  glass  rod,  placed  at  some  distance  from  each  other,  on  filtering  paper, 
laid  upon  a  slab  or  plate. 

The  solution  of  sodium  sulphide  contained  in  an  ordinary  M  o  h  r '  s  burette  is 
then  allowed  to  flow  into  the  zinc  solution  until,  on  bringing  a  drop  from  the 
mixture  and  placing  it  upon  the  filtering  paper,  so  that  it  may  expand  and  run 
into  a  drop  of  lead  solution,  a  black  line  appears  at  the  point  of  contact ;  the 
reaction  is  very  delicate.  At  first  it  will  be  difficult,  probably,  to  hit  the  exact 
point,  but  a  second  trial  with  25  or  50  c.c.  of  zinc  solution  will  enable  the 
operator  to  be  certain  of  the  corresponding  strength  of  the  sulphide  solution. 
As  this  latter  is  always  undergoing  a  slight  change,  it  is  necessary  to  titrate 
occasionally. 

Direct  titration  with  pure  zinc  solution  gave  99*6  and  100'2,  instead  of  100. 

Groll  recommends  the  use  of  nickel  protochloride  as  indicator, 
instead  of  sodium  nitroprusside  or  lead.  The  drops  are  allowed  to 
flow  together  on  a  porcelain  plate  ;  while  the  point  of  contact  shows 
a  blue  or  green  colour  the  zinc  is  not  all  precipitated  by  the  sulphide, 
therefore  the  latter  must  be  added  until  a  greyish  black  colour 
appears  at  contact. 

4.     Precipitation  as  Sulphide  with  Ferric  Indicator 
(S  c  h  a  f  f  n  e  r). 

Schaffner's  modification  of  this  process,  which  is  used  constantly 
at  the  laboratory  of  the  Vieille  Montagne  and  the  Rhenish  Zinc 
Works,  is  conducted  as  follows  : — For  ores  containing  over  35  per 
cent,  zinc,  O5  gm.  is  taken  ;  for  poorer  ones,  1  gm.  to  2  gm. 
Silicates,  carbonates,  or  oxides  are  treated  with  hydrochloric  acid, 
adding  a  small  proportion  of  nitric  acid  at  boiling  heat  to  peroxidize 
the  iron.  Sulphur  ores  are  treated  with  aqua  regia,  evaporated  to 
dry  ness,  and  the  zinc  afterwards  extracted  by  hydrochloric  acid  ; 
the  final  ammoniacal  solution  is  then  prepared  as  described  on 
page  371. 

METHOD  OF  PROCEDURE  :  The  titration  is  made  with  a  solution  of  sodium 
sulphide,  1  c.c.  of  which  should  equal  about  O'Ol  gm.  Zn.  The  Vieille  Montagne 
laboratory  uses  ferric  chloride  as  an  indicator,  according  to  Schaffner's  method. 
For  this  purpose  a  single  drop  or  some  few  drops  of  this  chloride  are  let  fall  into 
the  ammoniacal  solution  of  zinc.  The  iron  which  has  been  added  is  at  once 
converted  into  red  flakes  of  hydrated  ferric  oxide,  which  sink  to  the  bottom  of 
the.  flask.  If  sodium  sulphide  be  dropped  from  a  burette  into  the  solution  of 
zinc,  a  white  precipitate  of  zinc  sulphide  is  at  once  thrown  down,  and  the  change 
in  the  colour  of  the  flakes  of  ferric  hydrate  from  red  to  black  shows  the  moment 
when  all  the  zinc  is  sulphuretted,  and  the  titration  is  ended.  It  is  advisable  to 
keep  the  solution  for  titration  at  from  40°  to  60°  C.  Titration  carried  out  under 
exactly  equal  conditions,  with  a  known  and  carefully  weighed  proportion  of  zinc, 


374  ZINC. 

gives  comparative  data  for  calculation,  and  thus  for  the  determination  of  the 
contents  of  any  zinc  solution  by  means  of  a  simple  equation.  If,  for  example, 
30'45  c.c.  of  sulphide  have  been  used  to  precipitate  0'25  gm.  of  zinc,  1  c.c.  of  it  will 
precipitate  8'21  mgm.  of  zinc  (30'45  :  0'25=1  :  x,  and  therefore  z=0'00821). 

The  following  method  is  adopted  in  the  laboratory  of  a  well- 
known  copper  works  in  Wales  : — 

Reduce  the  sample  to  fine  powder,  and  dry  at  a  temperature  of  about  105°  C. 
Dissolve  0-5  gm.  of  the  sample  thus  prepared  in  aqua  regia,  evaporate  nearly  to 
dryness,  take  up  with  hot  water,  add  20  c.c.  of  ammonia  and  10  c.c.  of  a  solution 
of  ammonium  carbonate  (1  to  10),  then  a  few  drops  of  solution  of  permanganate 
to  precipitate  lead  and  manganese.  Now  heat  nearly  to  boiling-point  and  filter 
into  a  larger  flask,  wash  the  precipitate  well  with  hot  water  containing  ammonia 
until  a  drop  of  the  washings  shows  no  reaction  with  sodium  sulphide.  The 
volume  of  the  filtrate  and  washings  should  be  about  250  c.c.,  and  the  temperature 
about  50°  C.  Now  titrate  with  a  standard  solution  of  sulphide.  The  most 
convenient  strength  is  70  c.c.  =0'5  gm.  of  pure  zinc,  heat  the  sample  liquid 
almost  to  boiling-point,  and  add  not  quite  enough  sulphide  solution  to  precipitate 
the  whole  of  the  zinc.  Now  take  a  drop  of  a  dilute  solution  of  ferric  chloride,  and 
let  it  fall  into  a  small  beaker  containing  a  few  drops  of  dilute  ammonia,  wash  the 
whole  contents  of  the  beaker  into  the  assay,  and  continue  titrating  slowly  and 
cautiously,  at  last  adding  the  sulphide  solution  by  0*1  c.c.  at  a  time,  while 
continually  agitating  the  flask  until  the  ferric  oxide  at  the  bottom  of  the  flask 
begins  to  turn  black,  when  the  assay  is  finished. 

The  number  of  c.c.  of  sulphide  solution  used  is  noted.  In  order  to  determine 
the  strength  of  the  sulphide  solution,  weigh  0*5  gm.  pure  zinc,  place  this  in  a 
flask,  dissolve  in  10  c.c.  of  HC1,  and  add  some  hot  water,  20  c.c.  of  ammonia,  and 
10  c.c.  of  ammonium  carbonate  as  above,  and  fill  up  with  hot  water  to  about 
250  c.c.  Then  titrate  with  the  sulphide  solution  as  described.  From  the  number 
of  c.c.  used  for  the  0'5  gm.  pure  zinc  (standard),  and  the  number  used  for  the 
sample,  the  zinc  contents  of  the  latter  can  easily  be  calculated. 

The  copper  present  in  blendes  and  calamines  does  not  usually  exceed  0'5  per 
cent.  It  may  be  determined  colorimetrically,  and  the  amount  deducted  from  the 
total  produced. 

If  any  considerable  amount  of  copper  or  other  impurities  be  present,  they 
must  be  separated  by  the  ordinary  well-known  methods.  In  order  to  obtain 
greater  accuracy  a  correction  is  made  by  measuring  the  volume  of  the  liquid 
after  the  assay  is  finished,  and  deducting  0'6  c.c.  from  the  sulphide  solution  used 
for  every  100  c.c.  of  the  volume  of  the  assay :  this  correction  is  equally  applied 
to  the  standard.  Experiments  have  shown  that  oxide  of  iron  prepared  as  described 
above  placed  in  100  c.c.  of  distilled  water  containing  ammonia,  requires  0'6  c.c. 
of  a  sulphide  solution  of  the  above  strength  to  turn  distinctly  black. 

The  essential  point  in  this  volumetric  process  practised  at  the 
Vieille  Montagne  is  the  perfect  uniformity  of  working  adopted  in 
the  assays  with  reference  to  the  volume  of  the  solutions  and 
reagents  used  and  the  colour  of  the  indicator.  In  titrating,  the 
same  quantities  of  ferric  chloride,  hydrochloric  acid,  and  ammonia 
are  invariably  used.  The  operation  is  carried  out  always  at  one 
temperature  and  in  the  same  time,  particularly  at  the  end  of  the 
process,  when  the  iron  begins  to  assume  that  characteristic  colour 
which  the  flakes  show  at  the  edges — points  which  should  not  be 
overlooked.  As  a  further  precaution,  the  titrating  apparatus  is 
provided  in  duplicate,  two  assays  being  always  made.  It  permits 
the  execution  of  several  titrations  without  the  necessity  of  a  too 
frequent  renewal  of  sodium  sulphide,  which  is  stored  in  a  yellow 
flask  of  large  capacity  supplying  two  Mohr's  burettes,  under 


ZINC.  375 

which  the  beakers  can  be  placed  and  warmed.  A  mirror  shows  by 
reflection  the  iron  flakes  which  settle  down  after  shaking  the 
liquid. 

Too  much  stress  cannot  be  laid  upon  the  necessity  of  standardizing 
the  sodium  sulphide  under  the  same  conditions  as  to  volume  of 
fluid,  proportions  of  NH3  and  HC1,  and  colour  of  the  indicator,  as 
will  actually  be  observed  in  the  analysis. 

The  chief  difficulty  in  this  sulphide  process  is  the  end-point. 
E.  G.  Ballard,*  has  recommended  a  good  plan  for  ascertaining 
this,  the  following  being  his  own  words  : — 

"  I  have  found  the  following  method  very  delicate  and  rapid  in  determining 
the  end  of  the  titration,  and  it  is  based  upon  the  fact  that  the  suspended  sulphide 
of  zinc  has  no  action  on  metallic  silver,  whereas  the  smallest  excess  of  sulphide  in 
the  solution  will  produce  a  stain  upon  a  bright  silver  surface.  A  small  bright 
plate  of  silver  is  procured,  and  at  intervals  during  the  titration  a  drop  of  the 
zinc  solution  containing  suspended  ZnS  is  taken  out  on  a  glass  rod  and  placed 
upon  the  silver  plate  and  allowed  to  remain  there  for  10  to  20  seconds.  No 
blackening  of  the  silver  surface  occurs  until  there  is  an  excess  of  sulphide 
present,  when  the  stain  upon  the  silver  plate  is  evident  at  once,  and  the  titration 
may  be  considered  finished. 

The  number  of  drops  of  the  standard  solution  of  sulphide  required  to  produce 
the  stain  in  the  time  mentioned,  may  be  ascertained  in  a  quantity  of  water  equal 
in  bulk  to  that  of  the  zinc  solution  operated  upon,  and  afterwards  deducted  from 
the  total  used  in  calculating  the  result.  One  part  of  Na2S  in  20,000  of  water  will 
produce  a  stain. 

Another  way  to  use  the  silver-plate  indicator  is  to  run  in  the  sulphide  to  small 
excess,  and  then  titrate  back  with  a  solution  of  zinc  of  known  strength,  watching 
the  disappearance  of  the  stain  on  the  silver  plate  in  this  instance.  Time  is  thus 
gained  when  testing  a  substance  containing  an  unknown  quantity  of  zinc  for  the 
first  time,  but  in  cases  where  the  amount  is  approximately  known,  the  former 
method  suffices,  the  greater  part  of  the  sulphide  being  run  in  at  once,  and  the 
silver-plate  indicator  applied  during  the  addition  of  the  last  portions  only. 

PRECAUTION  :  Although  it  has  been  stated  that  the  precipitated  sulphide  of 
zinc  has  no  action  upon  the  silver  plate,  yet  in  the  presence  of  a  large  excess  of 
ammonia  in  the  cold  there  is  a  slight  action ;  therefore  it  is  desirable  to  observe 
the  precaution  to  avoid,  as  far  as  possible,  adding  more  ammonia  than  is  required 
to  redissolve  the  precipitate  first  formed  in  rendering  the  solution  of  zinc  under 
examination  alkaline.  However,  if  the  temperature  of  the  solution  of  zinc  be 
raised  to  about  180°  F.,  a  much  larger  excess  of  ammonia  may  be  added  without 
interfering  with  the  accuracy  of  the  test  or  the  final  determination  of  the  zinc. 
Under  any  circumstances  I  prefer  titrating  the  solution  of  zinc  hot. 

The  first  appearance  of  the  stain  on  the  silver  plate  can  be  more  easily 
distinguished  in  a  diffused  light,  such  as  that  reflected  from  a  sheet  of  white 
paper,  or  a  white  card,  and  more  especially  if  the  drop  be  removed  at  the  end  of 
10  or  20  seconds  by  means  of  a  small  blotting  pad  or  piece  of  folded  filter-paper. 

A  porcelain  dish  is  the  most  convenient  vessel  in  which  to  perform  the 
titration.  Another  point  to  ensure  accuracy  is  to  be  careful  that  the  silver  plate 
is  clean,  and  free  from  grease.  A  little  chalk  and  ammonia  is  useful  for  this 
purpose." 

A  method  of  determining  zinc  by  the  following  means  has  been 
found  by  J.  E.  Clennellf  useful  for  ores  and  the  solutions  used 
in  gold  extraction  without  the  use  of  an  external  indicator,  as  is 
the  case  with  the  usual  sulphide  and  ferrocyanide  process. 

*  J.  S.  C  I.  16,  399.  t  C.  N.  87,  121. 


376  .         ZINC. 

The  zinc  is  precipitated  by  means  of  a  solution  of  sodium  sulphide  of  known 
strength,  added  in  slight  excess,  the  excess  of  sulphide  being  then  determined  by 
making  use  of  the  reaction — 

Na2S  +2KAgCy2  =Ag2S  +2NaCy  +2KCy. 

The  solutions  required  are — 

Sodium  Sulphide. — A  convenient  strength  being  about  0-2  per  cent.  Na2S. 

Silver  Double  Cyanide. — Prepared  by  adding  silver  nitrate  to  a  solution  of 
potassium  cyanide  (say,  2  or  3  per  cent.  KCy)  till  a  slight  permanent  precipitate 
of  AgCy  is  produced,  allowing  to  stand,  and  filtering. 

Silver  Nitrate. — Any  dilute  solution  of  known  strength.  A  convenient  standard 
is  one  containing  5 '2 15  gm.  AgN03  per  litre,  1  c.c.  being  equivalent  to  O'OOl  gm. 
zinc. 

Potassium  Iodide. — 1  per  cent,  solution. 

It  is  perhaps  advisable  also  to  have  a  standard  zinc  solution  prepared  from  pure 
metallic  zinc  or  pure  zinc  sulphate,  and  containing  0'5  or  1  per  cent.  Zn. 

METHOD  OF  PROCEDUBE  :  The  zinc  in  ores  or  similar  substances  is  brought 
into  solution  in  the  ordinary  way,  and  the  liquid  made  strongly  alkaline  with 
caustic  soda  or  ammonia,  boiled,  diluted,  and  filtered  if  necessary.  In  cyanide 
solutions,  the  sulphide  may  in  general  be  applied  direct ;  in  some  cases,  however, 
it  may  be  necessary  to  remove  the  cyanogen  by  a  preliminary  operation. 

The  liquid  to  be  examined  is  mixed  with  a  measured  volume  of  sodium  sulphide, 
slightly  in  excess  of  that  required  to  precipitate  the  whole  of  the  zinc.  The 
liquid  is  well  shaken  in  a  stoppered  flask ;  a  little  lime  may  be  added  to  promote 
settling.  The  whole,  or  an  aliquot  part,  is  then  filtered,  and  an  excess  of  the 
double  silver  cyanide  added.  The  precipitate  of  Ag2S  generally  settles  rapidly, 
and  is  easily  filtered  and  washed  (occasionally  it  may  be  necessary  to  add  a  little 
more  lime).  About  5  c.c.  of  the  1  per  cent.  KI  solution  are  added  to  the  filtrate, 
and  the  liquid  titrated  with  AgN03  till  a  slight  yellowish  turbidity  remains 
permanent. 

1  gm.  KCy  =0-3  gm.  Na2S  =0-25  gm.  Zn. 

A  table  is  given  which  illustrates  the  results  of  titrations  made  by  this  method 
with  solutions  containing  known  quantities  of  zinc. 

In  three  trials  made  with  zinc  double  cyanide,  a  separate  portion  of  the 
original  liquid  was  tested  in  the  ordinary  way  for  "  total  cyanide,"  i.e.,  by 
titration  with  silver  nitrate,  using  alkali  iodide  indicator,  and  the  cyanide  so 
found  deducted  from  the  amount  shown  by  titration  after  adding  sodium 
sulphide  and  the  double  silver  salt.  In  another  test,  however,  the  cyanide  was 
precipitated  before  making  the  determination  of  zinc,  by  adding  excess  of  AgN03, 
and  then  a  few  drops  of  HC1  to  remove  excess  of  AgN03,  boiling  and  filtering ; 
making  strongly  alkaline  with  NaOH  before  adding  Na2S. 

In  presence  of  ferrocyanide  and  thiocyanate,  it  appears  to  be  necessary  to  make 
the  solution  strongly  alkaline  to  ensure  complete  precipitation  of  the  zinc 
sulphide. 

5.     Determination  as  Ferrocyanide. 

In  Acetic  Acid  Solution  (Galetti). — When  ores  containing  zinc 
and  iron  are  dissolved  in  acid,  and  the  iron  precipitated  with 
ammonia,  the  ferric  oxide  invariably  carries  down  with  it  a  portion 
of  the  zinc,  and  it  is  only  by  repeated  precipitation  that  the  complete 
separation  can  be  made.  In  this  process  the  zinc  is  converted  into 
soluble  acetate,  and  titrated  by  a  standard  solution  of  potassium 
ferrocyanide  in  the  presence  of  insoluble  ferric  acetate. 

The  standard  solution  of  potassium  ferrocyanide,  as  used  by 
Galetti,  contains  41*250  gm.  per  litre.  1  c.c.=0;01  gm.  Zn,  but 
its  actual  working  value  must  be  fixed  by  experiment. 

Standard  zinc  solution,  10  gm.  of  pure  metallic  zinc,  dissolved 
in  hydrochloric  acid,  per  litre. 


ZINC.  377 

The  process  is  available  in  the  presence  of  moderate  quantities 
of  iron  and  lead,  but  copper,  manganese,  nickel,  and  cobalt  must 
be  absent. 

The  adjustment  of  the  ferrocyanide  solution  (which  should  be 
freshly  prepared  at  short  intervals)  must  be  made  in  precisely  the 
same  way  and  with  the  same  volume  of  liquid  as  the  actual  analysis 
of  ores,  and  is  best  done  as  follows  : — 

25  c.c.  of  zinc  solution  are  measured  into  a  beaker,  15  c.c.  of  liquid  ammonia 
of  sp.  gr.  0'900  added  to  render  the  solution  alkaline,  then  very  cautiously 
acidified  with  acetic  acid,  and  50  c.c.  of  acid  ammonium  acetate  (made  by  adding 
together  20  c.c.  of  ammonia  of  sp.  gr.  0'900,  15  c.c.  of  concentrated  acetic  acid 
and  65  c.c.  of  distilled  water),  which  is  poured  into  the  mixture,  then  diluted  to 
250  c.c.,  and  warmed  to  about  50°  C.  The  titration  is  then  made  with  the 
ferrocyanide  solution  by  adding  it  from  a  burette  until  the  whole  of  the  zinc  is 
precipitated.  Galetti  judges  the  ending  of  the  process  from  the  first  change  of 
colour  from  white  to  ash  grey,  which  occurs  when  the  ferrocyanide  is  in  excess  ; 
but  it  is  best  to  ascertain  the  ending  by  taking  drops  from  the  solution,  and 
bringing  them  in  contact  with  solution  of  uranium  acetate  on  a  white  plate  until 
a  faint  brown  colour  appears.  The  ferrocyanide  solution  should  be  of  such 
strength  that  measure  for  measure  it  agrees  with  the  standard  zinc  solution. 

In  examining  ores  of  zinc,  such  as  calamine  and  blende,  Galetti  takesO'Sgm. 
for  the  analysis,  and  makes  the  solution  up  to  500  c.c.  Calamine  is  at  once 
treated  with  HC1  in  sufficient  quantity  to  bring  it  into  solution.  Blende  is 
treated  with  aqua  regia,  and  evaporated  with  excess  of  HC1  to  remove  nitric 
acid.  The  solutions  of  zinc  so  obtained  invariably  contain  iron,  which  together 
with  the  zinc  is  kept  in  solution  by  the  HC1,  but  to  ensure  the  peroxidation  of 
the  iron,  it  is  always  advisable  to  add  a  little  potassium  chlorate  at  a  boiling  heat 
during  the  extraction  of  the  ore.  The  hydrochloric  solution  is  then  diluted  to 
about  100  c.c.,  30  c.c.  of  ammonia  added,  heated  to  boiling,  exactly  neutralized 
with  acetic  acid,  100  c.c.  of  the  acid  ammonium  acetate  poured  in,  and  diluted  to 
about  500  c.c.  The  mixture  so  prepared  will  contain  all  the  zinc  in  solution,  and 
the  iron  will  be  precipitated  as  acetate.  The  titration  may  at  once  be  proceeded 
with  at  a  temperature  of  about  50°  to  60°  C.  by  adding  the  ferrocyanide  until 
the  necessary  reaction  with  uranium  is  obtained.  As  before  mentioned,  Galetti 
takes  the  change  of  colour  as  the  ending  of  the  process,  and  when  iron  is  present 
this  is  quite  distinguishable,  but  it  requires  considerable  practice  to  rely  upon, 
and  it  is  therefore  safer  to  use  the  uranium  indicator.  When  using  the  uranium, 
however,  it  is  better  to  dilute  the  zinc  solution  less,  both  in  the  adjustment  of 
the  standard  ferrocyanide  and  the  analysis  of  ores.  The  dilution  is  necessary 
with  G  a  1  e  1 1  i '  s  method  of  ending  the  process,  but  half  the  volume  of  liquid, 
or  even  less,  is  better  with  the  external  indicator. 

In  Hydrochloric  Acid  Solution  (Fahlberg). — This  method  is 
not  available  in  the  presence  of  iron,  copper,  nickel,  cobalt,  cadmium, 
lead,  or  manganese. 

Both  these  processes  have  been  thoroughly  investigated  by 
L.  de  Koninck  and  E.  Prost*  with  a  view  to  ascertain  the  exact 
reactions  which  take  place  in  adding  potassium  ferrocyanide  to 
a  solution  of  zinc.  The  reaction  takes  place  somewhat  slowly  ; 
therefore  there  may,  at  first,  be  an  excess  of  ferrocyanide  as  proved 
by  the  uranium  reaction.  Soon,  however,  this  excess  disappears, 
as  an  insoluble  double  compound  of  zinc  and  potassium  ferrocyanide 
is  formed.  The  direct  titration  of  zinc  by  means  of  potassium 
ferrocyanide  is,  therefore,  not  to  be  recommended.  The  following 
is  found  by  the  authors  to  give  trustworthy  results. 

*  Z.  a.  C.  1896,  460,  also  C.  N.  76,  6 


378  ZINC. 

METHOD  OP  PROCEDURE  :  10  gm.  of  pure  zinc  are  dissolved  in  hydrochloric 
acid,  nearly  neutralized  with  soda,  and  made  up  to  1  litre.  27  gm.  of  potassium 
ferrocyanide  are  dissolved  in  a  litre  of  water.  These  solutions  are  then  checked 
by  mixing  20  c.c.  of  the  zinc  solution  with  50  c.c.  of  a  20  per  cent,  solution  of 
ammonium  chloride,  two  drops  of  a  10  per  cent,  solution  of  sodium  sulphite,  and 
10  c.c.  of  hydrochloric  acid  (sp.  gr.  T075) ;  the  zinc  solution  must  be  measured 
from  an  accurate  pipette,  but  the  others  are  only  roughly  measured.  40  c.c. 
exactly  of  the  ferrocyanide  solution  are  now  added,  and,  after  being  left  for  at 
least  ten  minutes,  the  excess  is  titrated  with  the  zinc  solution  until  the  uranium 
reaction  is  no  longer  obtained.  The  relation  between  the  zinc  and  the  ferro- 
cyanide is  thus  determined. 

The  determination  of  zinc  in  any  of  its  ores  is  carried  out  as  follows : — 2*5  gm. 
of  the  sample  are  dissolved  in  nitrohydrochloric  acid  and  evaporated  to  dryness 
to  render  any  silica  insoluble,  the  residue  being  taken  up  with  5  c.c.  of  hydrochloric 
acid  and  a  little  water.  The  nitrate  from  this  is  freed  from  lead,  cadmium,  etc., 
by  a  current  of  hydrogen  sulphide,  boiled  to  expel  the  gas,  and,  after  cooling, 
mixed  with  25  c.c.  of  saturated  bromine  water.  After  pouring  the  liquid  into 
a  500  c.c.  flask,  containing  100  c.c.  of  ammonia  (sp.  gr.  0'9),  and  10  c.c.  of  a  25  per 
cent,  solution  of  ammonium  bicarbonate,  it  is,  when  cold,  made  up  to  the  mark. 

When  the  precipitate  has  quite  settled,  the  liquid  is  passed  through  a  dry 
filter.  100  c.c.  are  then  pipetted  off,  acidified  with  hydrochloric  acid,  and  titrated 
with  the  ferrocyanide  in  the  way  described. 

Maxwell  Lyte,*  who  used  the  original  method  of  Fahlberg, 
gives  the  following  method  of  treating  a  blende  containing  lead, 
copper,  and  iron  in  small  quantities  : — 

2  gm.  of  finely  powdered  ore  were  boiled  with  strong  HC1  and  a  little  KC103, 
the  insoluble  matter  again  treated  in  like  manner,  the  solutions  mixed  and 
evaporated  somewhat,  washed  into  a  beaker,  cooled,  and  moist  barium  carbonate 
added  to  precipitate  iron,  copper,  etc.,  allowed  to  stand  a  few  hours,  then  filtered 
into  a  200  c.c.  flask  containing  10  c.c.  of  strong  HC1,  and  washed  until  the  exact 
measure  was  obtained.  20  c.c.  (  =0'2  gm.)  of  blende  were  measured  into  a  small 
beaker,  diluted  with  the  same  quantity  of  water,  3  drops  of  uranic  solution  added 
as  indicator,  and  the  ferrocyanide  delivered  in  from  a  burette.  When  70  c.c. 
were  added  the  brown  tinge  disappeared  slowly ;  the  testing  on  a  white  plate  was 
then  resorted  to,  and  the  ferrocyanide  added  drop  by  drop,  until  the  proper  effect 
was  observed  at  73  c.c.  As  a  slight  excess  of  ferrocyanide  was  necessary  to  produce 
the  brown  colour,  0-2  c.c.  was  deducted,  leaving  72'8  c.c.  as  the  quantity  necessary 
to  precipitate  all  the  zinc.  The  0*2  gm.  of  blende  therefore  contained  0'0728  gm. 
of  Zn  or  3  6 '4  per  cent. 

The  sample  in  question  contained  about  2*7  per  cent,  of  copper, 
but  this  was  precipitated  with  the  iron  by  the  barium  carbonate  ; 
had  it  contained  a  larger  quantity,  the  process  would  not  have  been 
available  unless  the  copper  was  removed  by  other  means. 

Mahonf  uses  the  ferrocyanide  method  much  in  the  same  way 
as  above  described,  but  finds  that  Mn  must  be  absent  to  ensure 
good  results.  In  the  presence  of  Mn  he  separates  the  Zn  from 
a  strong  acetic  solution  with  H2S.  The  sulphide  is  then  dissolved 
in  HC1  and  titrated  as  before. 

A  modification  of  the  ferrocyanide  method  so  as  to  be  available 
for  the  determination  of  both  zinc  and  manganese  in  the  presence 
of  each  other  has  been  devised  by  G.  C.  Stone. f 

The  standard  solutions  required  are  : — 

Potassium  ferrocyanide,  about  30  gm.  per  litre.  Its  actual 
working  strength  is  found  by  titrating  it  upon  a  known  weight  of 

«  C.  N.  21,  222.  f  Amer.  Chem.  Journ.  4,  53,  J  J.  Am.  C.  S.  17,  437. 


ZINC.  379 


either  zinc  or  manganese  in  slightly  acid  solution,  using  a  very 
dilute  solution  of  cobalt  nitrate  as  outside  indicator.  A  correction 
is  made  in  all  cases  for  the  amount  of  ferrocyanide  required  to  give 
the  reaction  with  the  indicator,  and  may  be  taken  as  0'5  c.c.  for 
every  100  c.c.  of  the  solution  titrated. 

Potassium  permanganate,  1*99  gm.  of  the  pure  salt  per  litre. 
1  c.c.  =  l  mgm.  of  Mn. 

The  end-point  of  reaction  with  the  indicator  is  found  by  placing 
drops  of  the  cobalt  solution  on  a  white  tile,  and  bringing  a  drop  of 
the  liquid  under  titration  in  contact  with  it,  but  not  actually 
mixing.  The  immediate  production  of  a  faint  green  line  at  the 
junction  of  the  drops  is  accepted  as  the  correct  reading. 

METHOD  or  PROCEDURE  :  The  ore  is  dissolved  in  HC1  with  the  addition  of 
KC103  as  an  oxidizer,  and  care  must  be  taken  to  have  sufficient  acid  to  keep  all 
the  manganese  in  solution. 

Lead  alone  need  not  be  separated  ;  copper  can  be  precipitated  by  lead  ;  or 
lead  and  copper  can  both  be  precipitated  by  aluminium.  Cadmium  should 
be  precipitated  by  H2S,  and  the  filtrate  oxidized.  Iron  and  aluminium  are 
best  separated  by  barium  carbonate,  but  the  latter  must  be  free  from  alkali 
carbonates  and  hydroxides,  barium  hydroxide,  and  ammonium  salts.  A  salt 
sufficiently  pure  for  the  purpose  may  be  obtained  by  suspending  the  ordinary 
pure  carbonate  (first  proved  free  from  ammonium  salts)  in  warm  water  for  several 
hours  with  2  or  3  per  cent,  of  its  weight  of  barium  chloride. 

The  well-oxidized  solution  of  the  ore  is  put  into  a  500  c.c.  flask,  and  barium 
carbonate  suspended  in  water  added  until  the  precipitate  coagulates.  The  whole 
is  then  poured  into  a  beaker,  well  mixed,  allowed  to  settle,  and  the  clear  liquid 
decanted  through  a  dry  filter,  and  diluted  to  500  c.c.  Portions  of  50,  100  or 
200  c.c.  of  the  filtrate  are  used  for  each  titration.  One  portion,  which  should 
contain  between  0*01  and  0*04  gm.  of  manganese,  is  diluted  to  200  c.c.,  heated 
nearly  to  boiling  in  a  porcelain  dish,  and  titrated  rapidly  with  permanganate 
with  vigorous  stirring. 

A  second  portion  is  made  slightly  acid  with  hydrochloric  acid,  the  zinc  and 
manganese  are  titrated  together  in  the  cold  with  ferrocyanide  ;  the  dark  colour 
of  the  precipitate  suddenly  changes  to  light  yellowish  green  shortly  before  the 
end  of  the  reaction.  It  is  not  necessary  to  test  with  the  cobalt  solution  until 
1  or  2  c.c.  of  the  ferrocyanide  have  been  added  after  the  lightening  of  the 
precipitate. 

EXAMPLE  :  1  c.c.  of  the  ferrocyanide  solution  equalled  0 '00606  gm.  of  zinc,  or 
0*00384  of  manganese ;  1  c.c.  of  the  permanganate  equalled  0*001  gm.  of 
manganese.  2£  gm.  of  the  ore  were  dissolved,  and  the  iron  precipitated  and 
filtered  off.  50  c.c.  of  the  solution  were  diluted,  heated,  and  titrated  with 
permanganate,  requiring  18*45  c.c.  =7*38  per  cent,  of  manganese.  100  c.c. 
titrated  with  ferrocyanide  required  27*85  c.c.  of  which  9*61  c.c.  would  be  used  by 
the  manganese  present.  Deducting  this,  18*24  c.c.  was  left  for  the  zinc,  equal  to 
0*1 1053  gm.,  or  22*11  per  cent.  The  amounts  of  zinc  and  manganese  as  determined 
gravimetrically  were  22*05  and  7*58  per  cent,  respectively. 

Von  Schulz  and  Low's  method*  as  modified  by  Pattinson 
and  Redpathf  {applicable  to  blende,  flue-dust,  etc.). 

Treat  1  gram  of  the  ore  with  hydrochloric  acid,  heat  gently,  and  after  some  time 
add  nitric  acid.  Evaporate  to  dryness,  and  extract  the  residue  with  1  gram  of 
ammonium  chloride  and  3 — 5  c.c.  of  ammonia ;  redissolve  the  residue  in  hydro- 
chloric acid,  evaporate  to  dryness,  and  extract  a  second  time,  using  the  same 
quantities  of  ammonium  chloride  and  ammonia  as  before.  Filter,  and  wash 

*  J    S.  C.  I.  1892,  846.        t  •/•  S.  C.  I.  1905,  228. 


380  ZINC. 

several  times  with  hot  chloride  of  ammonium  solution  of  5  %  strength.  If 
manganese  is  present,  precipitate  it  by  adding  bromine-water  to  the  ammoniacal 
solution,  filter,  redissolve  the  precipitate  in  hydrochloric  acid,  add  ammonium 
acetate,  precipitate  by  H2S  any  zinc  carried  down  with  the  manganese,  dissolve 
the  ZnS  in  hydrochloric  acid,  and  add  it  to  the  main  solution.  Add  hydrochloric 
acid  till  the  solution  is  just  acid,  then  a  further  10  c.c.  and  titrate  the  solution 
hot,  reserving  a  portion  at  first  in  case  two  much  ferrocyanide  should  be  run  in. 
Use  uranium  acetate  spotted  on  a  white  porcelain  plate  as  indicator.  (When 
copper  is  present,  it  is  removed  by  Seaman's  method,  see  below). 

W.  H.  Seaman*  recommends  the  use  of  aluminium  (in  place 
of  granulated  lead  previously  recommended,  which  gave  rise  to 
high  results)  for  the  removal  of  copper,  the  process  being  carried 
out  as  follows  : — 

0*5  gm.  of  the  ore  is  covered  with  7  c.c.  of  concentrated  nitric  acid,  and  an  equal 
volume  of  hydrochloric  acid  is  then  added.  The  mixture  is  allowed  to  act  for 
15  minutes  at  a  temperature  not  exceeding  60°  C.  7  grams  of  ammonium  chloride 
are  now  introduced,  and  the  whole  evaporated  to  dryness.  After  making  alkaline 
with  5  c.c.  of  ammonia,  15  c.c.  of  bromine  water  are  added,  and  the  mixture  is 
boiled  for  three  minutes,  filtered  hot,  and  the  precipitate  washed  with  a  mixture 
of  ammonium  chloride  and  ammonia.  The  filtrate  faintly  acidified  with  hydro- 
chloric acid,  is  boiled  for  three  minutes  with  aluminium  foil,  which  precipitates 
all  the  copper,  lead  and  cadmium.  The  foil  is  then  removed,  and  the  liquid 
titrated  hot,  the  precipitated  metals  in  no  way  interfering.  In  standardizing 
the  ferrocyanide  solution,  the  amount  of  zinc  used  should  correspond  as  closely 
as  possible  to  that  in  the  ore,  and  a  correction  should  always  be  made  for  the 
amount  of  ferrocyanide  required  to  produce  the  coloration  of  the  indicator  in 
a  blank  experiment. 

6.     Determination  as  Zinc-Ammonium  Phosphate. 

The  method  has  been  devised  by  P.  H.  Walker.f  This  process 
is  a  modification  of  that  devised  by  Stolba  for  the  determination 
of  magnesium,  and  is  carried  out  as  follows  :— 

To  the  zinc  solution,  which  should  contain  ammonium  chloride,  a  large  excess 
of  ammonia  is  added,  then  a  large  excess  of  sodium  phosphate  solution. 
Hydrochloric  acid  is  now  gradually  added  until,  after  stirring,  the  solution 
remains  milky,  when  the  liquid  is  heated  to  about  75°  C.,  and  the  gradual  addition 
of  acid  continued,  with  constant  stirring,  until  nearly  complete  neutralization  is 
attained.  By  this  means  the  precipitate  becomes  crystalline,  and  after  five 
minutes'  standing  it  should  be  filtered  off  and  washed  with  cold  water  until  the 
washings  show  only  a  faint  trace  of  chloride.  The  filter  paper  and  precipitate 
are  now  returned  to  the  beaker  in  which  the  precipitation  was  carried  out,  an 
excess  of  standard  acid  added,  and  the  exact  point  of  neutrality  determined  by 
means  of  standard  alkali,  using  methyl  orange  as  indicator.  From  the  equation — 

ZnNH4P04  +  H2S04  =ZnS04  +NH4H2P04 

it  is  seen  that  1  c.c.  of  normal  acid  corresponds  with  32-7  mgm.  of  zinc.  The 
method  gives  good  results.  Since  the  zinc  ammonium  phosphate  is  not 
precipitated  in  presence  of  a  large  excess  of  ammonia,  any  magnesium  present, 
which  will  be  precipitated,  may  be  removed  by  filtration,  and  the  filtrate 
neutralized  to  throw  down  the  zinc.  Fairly  good  results  are  obtained  by  this 
method  also  in  the  presence  of  iron,  calcium,  and  magnesium,  but  any  manganese 
must  be  previously  separated,  best  by  means  of  nitric  acid  and  potassium 
chlorate. 

*  J.  Amer.  Chfm.  Soc.  1907,  205-211  and  J.  S.  C.  I.  1907,  258. 
t  J.  Am.  C.  S.  1901,  23,  [7],  468—470. 


ZINC.  381 

7.     Zinc  and  Lead. 

Rupp*  has  recently  devised  the  following  method  for  the 
determination  of  zinc  (and  lead).  It  depends  on  the  fact  that  when 
a  neutral  solution  of  zinc  is  added  to  a  solution  of  potassium  cyanide 
the  soluble  double  cyanide  K2Zn  (CN)4  is  formed  at  first,  but  the 
least  excess  of  zinc  causes  the  production  of  insoluble  zinc  cyanide. 
The  method  is  inapplicable  in  the  presence  of  acetates. 

METHOD  OF  PROCEDURE  :  The  solution  of  zinc  as  sulphate  or  chloride  is 
neutralized  by  means  of  caustic  soda,  using  methyl  orange  as  indicator,  and  made 
up  to  a  known  volume.  Into  a  flask  containing  about  0*5  gm.  of  ammonium 
chloride  dissolved  in  a  little  water  20  c.c.  of  N/g  KCN  is  accurately  measured, 
and  the  neutral  solution  of  zinc  is  run  in  from  a  burette  into  this  mixture,  with 
constant  agitation,  until  a  permanent  turbidity  appears.  The  cyanide  solution 
is  standardized  in  the  same  way  with  a  solution  of  pure  zinc  sulphate.  The 
presence  of  a  salt  of  ammonia  is  essential,  as  it  causes  the  re-solution  of  precipitate 
locally  formed  in  the  cyanide  solution.  Too  much  ammonium  chloride  causes 
high  results. 

1  c.c.  N/2  KCN  =0-008171  gm.  Zn. 

Lead, — Lead  cyanide  is  not  soluble  in  excess  of  alkali  cyanide,  but  is  readily 
so  in  acids.  On  adding  excess  of  KCN  to  a  lead  solution  and  filtering,  the  excess 
of  KCN  can  be  titrated  with  HC1.  The  solution  of  lead  as  nitrate  is  neutralized 
by  means  of  NaOH,  using  methyl  orange  as  indicator,  and  added  to  25  c.c.  of 
N/2  KCN  solution  contained  in  a  100  c.c.  measuring  flask,  the  amount  of  lead 
present  being  such  that  an  excess  of  cyanide  remains  after  it  has  all  been 
precipitated  as  Pb  (CN)2.  The  flask  is  made  up  to  the  mark,  well  shaken,  allowed 
to  stand  for  10  minutes,  and  filtered.  50  or  75  c.c.  of  the  filtrate  are  then  titrated 
with  N/g  or  N/4  HC1,  using  methyl  orange  as  indicator. 

1  c.c.  N/2  HC1=1  c.c.  N/2  KCN  =0-0518  gm.  Pb. 

IPb  =2KCN  =2HC1. 
The  cyanide  is  standardized  by  a  pure  lead  salt. 

Greenwood  and  Brisleef  consider  that  of  all  volumetric 
processes  for  the  determination  of  zinc  Schaffner's  (see  p.  373) 
is  of  the  widest  application  and  affords  the  highest  degree  of 
accuracy.  The  method  was  found  to  give  the  best  results  with 
the  assay  solution  at  a  temperature  between  60°  and  80°  C.,  the 
volume  being  150-170  c.c.  and  containing  excess  of  ammonia. 
The  indicator  employed  was  ferric  hydroxide  (formed  in  the  assay 
solution  by  the  addition  of  1  drop  of  a  strong  solution  of  ferric 
chloride),  and  corrections  for  the  excess  volume  of  liquid  and  for 
the  excess  of  sulphide  required  to  blacken  the  indicator  were 
applied.  The  ferrocyanide  method  is  recognised  as  being  very 
rapid  and  fairly  accurate,  but  in  the  authors'  opinion  its  value  is 
greatly  impaired  by  the  unsatisfactory  end-point  of  the  titration 
when  a  uranium  indicator  is  employed.  Ammonium  tetramolybdate 
is  stated  to  be  nearly  twice  as  delicate  an  indicator  as  the  uranium 
salt,  and  its  use  is  recommended  for  this  purpose.  Walker'  s 
method  (see  6)  is  considered  of  limited  application  only,  the  results 
being  seriously  affected  in  the  presence  of  lime  and  magnesia. 

*  Chem.  Zetf.11910,  34,  121. 

tThe  Technical  Assay  of  Zinc,  Inst.  of  Metals,  October,  1909,  also  in  J.  S.  C.  /.. 
1909,  1138. 


382  ZINC. 

8.     Zinc  Dust. 

The  value  of  this  substance  depends  upon  the  amount  of  metallic 
zinc  contained  in  it ;  but  as  it  generally  contains  a  large  proportion 
of  zinc  oxide,  the  foregoing  methods  are  not  available  for  its 
valuation.  The  volume  of  hydrogen  yielded  by  it  on  treatment 
with  acids  appears  to  be  the  most  accurate,  as  suggested  by 
Fresenius  or  by  Barnes.*  This  may  very  well  be  done  in  the 
nitrometer  with  decomposing  flask,  comparing  the  volume  of  gas 
yielded  by  pure  zinc  and  the  sample  of  dust  under  examination. 

L.  deKoninckf  has  shown  that  it  is  necessary  to  avoid  india- 
rubber  connections  in  the  apparatus  used,  as  otherwise  there  is 
a  loss  of  hydrogen,  due  to  diffusion. 

Many  other  methods  have  been  proposed  for  the  valuation  of 
this  substance.  The  best  is  that  of  Klemp,}  which  consists  in 
treating  the  dust  with  an  excess  of  caustic  potash  and  potassium 
iodate  ;  the  latter  is  reduced  in  definite  proportion  by  the  metallic 
zinc  to  potassium  iodide,  which  is  determined  by  distillation  in  the 
iodimetric  apparatus,  fig.  38  or  39. 

The  solutions  of  potash  and  iodate  must  be  somewhat  concentrated,  and  the 
mixture  with  the  zinc  dust  must  be  intimate,  which  may  be  best  secured  by 
shaking  the  whole  together  in  a  well -stoppered  200  c.c.  flask  with  glass  beads. 
A  5  per  cent,  solution  of  iodate  should  be  used,  and  the  potash  solution  should 
be  about  40  per  cent.  For  1  gm.  of  the  dust,  30  c.c.  of  the  iodate  and  so  much  of 
the  potash  solution  should  be  used  as  to  measure  130  c.c.  The  weighed  substance, 
together  with  the  beads,  being  already  in  the  flask,  the  solutions  are  added,  the 
stopper  greased  with  vaseline,  tied  down  and  shaken  for  five  minutes,  then  heated 
on  the  water-bath,  with  occasional  shaking  for  one  hour.  (Digestion  without  heat 
gives  practically  the  same  results.)  The  flask  is  then  cooled  and  the  contents 
diluted  to  250  or  500  c.c.,  and  50  or  100  c.c.  placed  in  the  distilling  flask,  acidified 
with  sulphuric  acid,  and  the  iodine  so  set  free  distilled  into  solution  of  potassium 
iodide,  and  titrated  with  thiosulphate  in  the  usual  way.  Each  gram  of  iodine 
equals  1-5451  gm.  Zinc. 

A  simpler  method  has  been  devised  by  A.  R.  Wahl,|]  but  like 
many  others  it  gives  no  protection  against  metallic  iron,  but  this 
of  course  can  be  ascertained  by  other  means. 

It  was  found  that  when  solid  ferric  sulphate  is  added  to  zinc  dust 
suspended  in  a  little  cold  water  with  exclusion  of  free  acid,  a  reaction 
occurs  with  evolution  of  heat,  and  the  zinc  quickly  and  totally 
dissolves  with  formation  of  a  clear  greenish  solution.  A  small 
residue  remains  consisting  of  lead  and  other  impurities.  The 
solution  of  the  zinc  takes  place  without  any  evolution  of  hydrogen, 
and  the  reaction  is  therefore  represented  by  the  equation — 

Fe2(SO4)3 + Zn = ZnSO4  +  2FeSO4. 

When  all  has  dissolved,  it  is  only  necessary  to  acidify  the  solution 
with  sulphuric  acid  and  titrate  with  permanganate  to  find  the 
quantity  of  ferrous  salt  formed,  and  hence  the  quantity  of  metallic 
zinc  in  the  sample  under  examination. 

«  J.  8.  C.  I    5,  145.  f  J-  S-  O.  /.  1908,  450. 

t  Z.  a.  C.  29,  253.  ||  J.  S.  C.  /.  16,  15. 


zmc.  383 

PREPARATION  OF  PURE  FERRIC  SULPHATE. — 500  gm.  of  pure  ferrous  sulphate 
are  dissolved  in  as  little  water  as  possible,  and  to  it  are  added  100  gm.  of  sulphuric 
acid  and  gradually  210  gm.  of  nitric  acid  (60  per  cent.).  On  adding  the  nitric 
acid,  torrents  of  nitrous  gas  are  evolved,  the  solution  acquiring  a  nearly  black 
colour,  which  disappears  again  when  the  whole  of  the  acid  is  added.  The  solution 
is  evaporated  on  the  water-bath  until  it  becomes  solid,  when  it  is  ground  with 
alcohol  in  a  mortar,  put  on  a  filter,  and  washed  with  alcohol  until  the  filtrate  is 
no  longer  acid.  The  product  is  then  dried  thoroughly  on  the  water-bath  to  re- 
move all  alcohol,  and  the  salt,  which  is  a  perfectly  white  powder,  is  kept  in  stoppered 
bottles  for  use. 

METHOD  OF  PROCEDURE  :  About  ^  gm.  of  zinc  dust  is  put  into  a  stoppered 
250  c.c.  flask  and  to  it  are  added  25  c.c.  of  cold  water.  The  mixture  is  agitated, 
and  when  the  zinc  is  thoroughly  suspended,  7  gm.  of  ferric  sulphate  are  added. 
There  is  a  gentle  evolution  of  heat,  and  after  shaking  for  a  quarter  of  an  hour 
the  zinc  will  have  completely  dissolved,  with  the  exception  of  a  slight  residue  of 
impurities.  25  c.c.  of  strong  sulphuric  acid  are  then  added,  and  the  flask  is 
made  up  to  the  mark  with  water.  50  c.c.  of  this  solution,  after  dilution  with 
50  c.c.  of  water,  are  titrated  with  standard  permanganate. 

From  the  quantity  of  the  latter  employed,  the  percentage  of  metallic  zinc  is  at 
once  found. 

Another  method  is  as  follows  : — * 

One  gm.  of  the  sample  is  weighed  into  a  dry  stoppered  200  c.c.  flask,  mixed 
with  100  c.c.  of  potassium  dichromate  solution  (30  gm.  per  litre)  and  10  c.c.  of 
1  :  3  sulphuric  acid,  and  agitated  for  five  minutes.  Another  10  c.c.  of  acid  are 
then  added,  and  the  shaking  continued  for  ten  or  fifteen  minutes,  when  every 
thing,  except  a  small  earthly  residue,  should  be  dissolved.  The  liquid  is  diluted 
to  500  c.c.,  and  in  50  c.c.  thereof  the  excess  of  dichromate  is  determined  by 
introducing  10  c.c.  of  10  per  cent,  potassium  iodide  and  5  c.c.  of  sulphuric  acid, 
titrating  the  liberated  iodine  with  decinormal  thiosulphate. 

9.     Zinc  Oxide  and  Carbonate. 

Benedikt  and  Can  to  if  show  that  zinc  oxide  and  carbonate 
can  be  accurately  titrated  with  standard  acid  and  alkali,  using 
methyl  orange  as  indicator  ;  and  other  zinc  salts,  using  phenol- 
phthalein.  The  oxide  or  carbonate  is  dissolved  in  excess  of  acid, 
and  the  excess  titrated  back  by  soda  solution.  Zinc  salts  are 
dissolved  in  water  (50  c.c.  ;to  CO'l  gm.  ZnO),  phenolphthalein  is 
added,  and  then  standard  soda  solution  to  intense  red  colour. 
A  few  more  c.c.  of  soda  are  then  added,  the  mixture  is  boiled  for 
some  minutes,  and  the  excess  of  soda  titrated.  If  either  free  acid 
or  zinc  oxide  is  present  in  the  zinc  salt,  it  is  neutralized  in  presence 
of  methyl  orange  by  alkali  or  acid,  as  the  case  may  be. 


ACETONE. 

Dimethyl-Ketone  CO(CH3)2  = 

ACETONE  is  an  important  constituent  of  wood-spirit  and  is 
largely  used  as  a  solvent  in  the  manufacture  of  cordite,  the 
insoluble  cellulose  hexanitrate  being  by  its  aid  worked  into  a  homo- 
geneous mixture  with  the  nitroglycerine.  For  such  purposes  the 
acetone  must  be  free  from  high  boiling  constituents  and  especially 

*  Analyst  25,  279.  f  Zett.  angew.  Chem.  1888,  236,  237. 


384  ACETONE. 

from  acid  or  acid-forming  substances,  which  are  injurious.     The 
British  Government  specification*  for  apetone  is  as  follows  : — 

"  The  liquid  is  to  be  genuine  Acetone  and  must  contain  no  other 
ingredients  except  small  quantities  of  substances  which  are  normal 
by-products  of  the  manufacture  of  Acetone.  It  must  be  colour- 
less and  absolutely  transparent,  and  when  mixed  with  distilled 
water  in  any  proportions  it  must  show  no  turbidity.  It  must 
leave  no  residue  when  evaporated  upon  a  boiling  water-bath. 

The  sp.  gr.  must  not  be  greater  than  0*800  at  15*5°  C.  compared 
with  water  at  the  same  temperature. 

One  cubic  centimetre  of  a  0*10  per  cent,  solution  in  distilled 
water  of  pure  permanganate  of  potash  added  to  100  cubic  centi- 
metres of  the  Acetone  must  retain  its  distinctive  colour  for  not  less 
than  30  minutes.  This  test  is  to  be  conducted  at  a  temperature 
of  15-5°  C. 

The  Acetone  is  not  to  contain  more  than  0'002  per  cent,  of 
carbon  dioxide,  and  is  otherwise  to  be  quite  neutral." 

The  above  stringent  requirements  ensure  that  only  the  middle 
.fractions  from  the  distillation  of  acetone  from  acetate  of  lime  are 
supplied,  and  that  the  acetone  is  not  contaminated  with  foreign 
matter,  and  contains  no  more  than  traces  of  any  substances  except 
ethyl  methyl  ketone,  and  only  a  small  quantity  of  that.f 

Commercial  acetones  may  contain  both  basic  bodies  and  acids. 
The  basic  bodies  and  strong  acids  may  be  determined  by  diluting 
a  measured  volume  of  the  sample  with  an  equal  volume  of  boiled 
distilled  water,  adding  2-4  drops  of  a  saturated  solution  of  para- 
nitrophenol  in  water,  and  titrating  with  standard  acid  or  alkali 
solution.  Weak  acids  may  be  detected  and  determined  by  mixing 
with  water  as  before,  boiling  for  5  or  10  minutes,  adding  phenol- 
phthalein  and  titrating  with  standard  caustic  alkali.  Carbon 
dioxide  is  readily  determined  by  adding  water  and  phenolphthalein 
and  titrating  at  once  without  boiling. 

ConroyJ  points  out  the  importance  of  protecting  from  light 
all  samples  of  acetone  intended  for  testing. 

Acetone  may  be  determined  by  Messinger's  method, ||  which 
depends  upon  the  fact  that  when  acetone  is  treated  with  iodine 
and  potash  iodoform  and  potassium  acetate  are  formed.  It  is 
carried  out  as  follows  : — 

25  c.c.  of  N/!-KOH  and  1 — 2  c.c.  of  the  sample  (e.g.,  methyl  alcohol)  are 
measured  into  a  stoppered  250  c.c.  flask,  the  mixture  well  shaken,  and  allowed  to 
stand  for  5  to  10  minutes.  N/5  iodine  solution  is  then  run  in  from  a  burette, 
drop  by  drop,  vigorously  shaking  all  the  time,  until  the  upper  portion  of  the 
solution,  after  standing  a  minute,  becomes  quite  clear.  A  few  more  c.c.  of  N/5 
iodine  are  added,  as  an  excess  is  essential.  After  shaking,  the  mixture  is  allowed 
to  stand  for  10 — 15  minutes  and  [then  25  c.c.  N/i  sulphuric  acid  are  added. 

*  This  was  kindly  supplied  to  me  by  the  Director  of  Artillery,  Royal  Gunpowder 
Factory,  Waltham  Abbey,  in  June,  1910. — A.E.J. 

t  Marshall  "Acetone:  its  manufacture  and  purification,"  J.  S.  C.  I.  1904,  23,  645. 

J  J.  S.  C.  I.  1900,  19,  209. 
II  Ber.  ^888,  21,  36G6 ;  also  J.  S.  C.  I.  1889,  8,  138. 


ANILINE.  385 

The  excess  of  iodine  thus  liberated  is  then  titrated  with  N/io  sodium  thiosulphate 
solution  and  starch. 

Let  n  be  the  c.c.  of  N/5  iodine  required  by  the  acetone  (  =  volume  added  less 
half  the  volume  of  thiosulphate  required),  m  be  the  c.c.  of  sample  taken  and 
then 

n  x  0-19334 

=gm.  of  acetone  per  100  c.c. 

m 

This  includes  as  acetone  any  aldehydes,  etc.,  capable  of  yielding  iodoform  by 
this  reaction. 

ANILINE. 

C6H5NH2  =  93-07. 

A  PROCESS  for  determining  aniline  or  its  salts  has  been  devised 
by  M.  Fran9ois.*  The  method  depends  on  the  fact  that  if 
bromine  water  is  added  to  an  aniline  solution,  which  contains 
a  little  soluble  indigo  as  indicator,  the  bromine  does  not  act  on  the 
indigo  until  all  the  aniline  has  been  converted  into  tribromaniline. 

METHOD  or  PROCEDURE  :  The  bromine  water  (5  gm.  bromine  in  1000  c.c.  water) 
is  standardized  by  means  of  an  aqueous  solution  of  aniline  hydrochloride,  which 
contains  1-392  gm.  of  the  pure  salt  in  1000  c.c.  (1  c.c.  =  OO01  gm.  aniline).  The 
bromine  water,  if  exposed  to  the  air,  is  continually  losing  bromine  ;  it  is  therefore 
essential  to  use  a  burette  of  such  capacity  that  it  contains  enough  bromine  water 
for  both  the  standardization  and  determination  without  refilling  ;  to  close  the 
end  of  the  burette  with  a  plug  of  cotton  wool ;  to  find  approximately  the  number 
of  c.c.  of  bromine  water  required,  and  then,  in  the  final  titration  to  add  nearly 
the  whole  at  once  in  order  to  avoid  the  slight  loss  of  bromine  which  occurs  when 
drops  of  the  solution  fall  through  the  air.  The  method  may  be  applied  to  solutions 
containing  aniline  or  its  hydrochloride,  the  presence  of  ammonium  chloride  does 
not  vitiate  the  result,  and  finally,  if  the  solution  to  be  titrated  contains  mineral 
substances  which  would  react  with  the  bromine,  the  aniline  may  be  liberated  by 
potash  and  distilled  in  steam.  The  degree  of  dilution  of  the  aniline  solution 
does  not  influence  the  result. 

Another  method  devised  by  Reinhardt  is  described  by 
Liebmann  and  Studer,f  who  use  a  slight  modification  of  it  for 
determining  aniline  or  mixtures  of  aniline  and  o-  and  p-toluidines 
which  are  sometimes  present  in  technical  oils.  Reinhardt 
accomplishes  this  by  titration  of  the  oil  in  hydrobromic  acid  solution 
by  potassium  bromate  and  bromide. 

Aniline  requires  three  molecules  of  bromine  to  form  tribromo- 
aniline,  whilst  o-  and  p-  toluidine  only  absorb  two  molecules. 

METHOD  OF  PROCEDURE:  Reinhardt  prepares  his  standard  solution  by  boiling 
480  gm.  of  Br  with  336  gm.  of  KOH  (100  per  cent.)  and  1  litre  of  water  for  2-3 
hours,  then  dilutes  to  9  litres. 

Hypobromites  should  not  be  present. 

To  carry  out  the  analysis,  he  dissolves  l'5-2  gm.  of  oil  in  1000  c.c.  of  water  and 
100  c.c.  of  hydrobromic  acid  of  1-4-1-5  sp.  gr.  He  adds  his  bromate  solution 
until  iodized  starch-paper  indicates  the  presence  of  free  bromine. 

The  following  equation  gives  him  the  result — 

X+(VT-X)  ^  |=A  or  X=^  TO_|  A> 

in  which  X  means  aniline,  V  the  volume  of  bromate  used,  T  its  titer,  and  A  th§ 
«  J.  Pharm.  1899,  521.  t  J-  8.  C.  7..1899,  110. 

2    C 


386  ANILINE. 

weight  of  oil  used  for  analysis.  Toluidine  is  found  by  difference.  To  determine 
the  relative  quantities  of  o-  and  p-toluidine,  use  is  made  of  the  property  of 
p-toluidine  and  aniline  to  be  precipitated  from  their  hydrochloric  acid  solution  by 
oxalic  acid,  whilst  o-toluidine  remains  in  solution. 

160  gm.  of  the  oil  are  dissolved  in  106  gm.  of  HC1  of  20°  B,  and  the  mixture 
is  then  added  to  a  hot  solution  of  oxalic  acid  in  10  times- its  quantity  of  water. 

The  solution  will  at  first  be  clear.  It  has  to  stand  for  48  hours.  The  oxalates, 
which  will  then  have  separated  out,  are  filtered  and  washed  three  times  with 
25  c.c.  of  distilled  water. 

After  decantation  with  hot  dilute  KOH  (100  c.c.  KOH  45°  B.,  200  c.c.  H20), 
the  oil  is  separated,  weighed,  and  finally  titrated  by  the  bromine  solution  to  find 
the  amount  of  p-toluidine  present. 

Liebmann  and  Studerhave  adopted  this  method,  with  slight  modifications, 
for  determining  the  aniline  and  toluidine  oils,  and  also  for  analyzing  the  aniline 
salts.  To  prepare  the  standard  solution,  16'7  gm.  of  pure  potassium  bromate  and 
59 '5  gm.  of  potassium  bromide  are  dissolved  in  1  litre  of  water,  and  standardized 
by  titration  with  sodium  thiosulphate,  using  potassium  iodide  and  starch  as 
indicator. 

For  aniline  they  have  found  that  concentrated  hydrochloric  acid  can  be  used 
as  solvent  instead  of  hydrobromic  acid,  but  that  the  latter  is  essential  when 
toluidines  are  present.  Instead,  however,  of  using  ordinary  hydrobromic  acid, 
they  found  that  by  dissolving  100  gm.  of  potassium  bromide  in  100  c.c.  of  hot 
water,  and  100  c.c.  of  hydrochloric  acid,  sp.  gr.  1*18,  an  acid  is  obtained  which 
gives  accurate  results. 

For  pure  aniline  0*5  gm.  of  the  oil,  or  about  0'6  gm.  of  salt  is  dissolved  in 
about  500  c.c.  of  water  and  30  c.c.  of  pure  hydrochloric  acid  of  1'18  sp.  gr.,  and 
add  the  standard  solution  until  a  distinct  excess  of  bromine  is  observable.  The 
reaction  grows  slower  at  the  end  of  the  operation,  but  it  is  found  that  to  wait  for 
two  minutes  is  quite  sufficient  to  determine  whether  free  bromine  is  present  in 
solution  or  not.  The  excess  of  bromine  is  determined  by  titration  with  N/io 
thiosulphate  solution,  using  potassium  iodide  and  starch  as  indicator,  6  c.c.  of  the 
thiosulphate  corresponding  to  1  c.c.  of  the  bromate  solution. 

For  aniline  containing  toluidine  0*5  gm.  is  dissolved  in  32  c.c.  of  hydrobromic 
acid,  prepared  as  above,  and  500  c.c.  of  water,  and  the  titration  is  carried  out 
in  the  same  way  as  with  pure  aniline.  A  number  of  analyses  of  aniline  oil  and 
salt  and  of  mixtures  of  aniline  oil  with  toluidine  of  known  composition  gave 
excellent  results. 

Another  method  has  been  adopted  of  carrying  out  Reinhardt's 
process  for  determining  the  aniline  and  toluidine  in  aniline  oil, 
which  depends  on  the  bromination  of  these  two  amines  by 
potassium  bromate  in  hydrobromic  acid  solution. 

An  8  per  cent,  solution  of  pure  potassium  bromate  is  prepared,  the  strength 
being  determined  by  mixing  25  c.c.  with  5  gm.  of  potassium  iodide  and  3  c.c.  of 
25  per  cent,  hydrobromic  acid  solution,  and  determining,  by  titration  with 
standard  thiosulphate,  the  iodine  set  free  according  to  the  equation : 
KBr03+6HBr+6KI=3I2+7KBr+3H20.  One  gm.  of  iodine  corresponds 
with  0-21933  gm.  of  potassium  bromate,  that  is,  with  G'12225  gm.  of  aniline,  or 
0'1393  gm.  of  toluidine.  About  1  gm.  of  the  aniline  oil  is  dissolved  in  about 
60  gm.  of  25  per  cent,  hydrobromic  acid  solution,  and  the  bromate  solution  run 
in  until  the  clear  liquid  above  the  bromide  precipitate  assumes  a  yellow 
coloration.  Then,  if  a  is  the  weight  of  oil  taken,  n  the  number  of  c.c.  of  bromate 
solution  employed,  ta  and  tf  the  amounts  of  aniline  and  toluidine  respectively 
corresponding  with  1  c.c.  of  the  bromate  solution,  the  percentage  of  aniline  in 
the  oil  is  given  by:  lOOt  (ntt-a)/a(t(-t  ),  and  that  of  the  toluidine  by 
lMtt(a-nta)/a(tt-ta). 

Aniline  hydrochloride  may  be  titrated  direct  by  standard  caustic 
alkali,  using  phenolphthalein  or  litmus  (but  not  methyl  orange)  as 
indicator,  as  it  acts  exactly  like  an  equivalent  quantity  of  free 


AZO-DYES,    ETC.  387 

hydrochloric  acid.     The  presence  of  neutral  ammonium  salts  has 
no  detrimental  effect. 

AZO-DYES,    NITRO-    AND    NITROSO-COMPOUNDS,  Etc. 
Dr.  E.  Knecht's  process. 

THE  powerful  reducing  properties  of  titanous  chloride  (TiCl3) 
render  this  reagent  capable  of  being  put  to  a  variety  of  uses  in 
volumetric  analysis.  Thus,  in  addition  to  its  uses  in  the  volumetric 
determination  of  iron  (and  arsenic),  it  has  been  found  available  for 
the  exact  determination  of  several  series  of  aromatic  compounds, 
including  the  azo-compounds,  the  nitro-  and  the  nitroso-compounds. 
In  all  the  cases  hitherto  observed,  the  reduction  takes  place  in  such 
a  manner  that  each  azo-group  present  requires  (in  accordance  with 
theory)  four  equivalents,  each  nitro  group  (NO2)  six  equivalents, 
and  each  nitroso-group  (NO)  four  equivalents  of  TiCl3.  Thus  with 
benzeneazo-betanaphthol,  the  reaction  takes  place  according  to 
the  following  equation  :  — 

C6H5N  :  N.C10H 


+  C10H6(OH)(NH2)  +  4TiCl4 
with  picric  acid, 

C6H2(OH)(NO2)3  +  18TiCl3  +  18HC1  = 
C6Ha(OH)(NHa)8  +  18TiCl4  +  12H2O 
and  with  nitrosodimethylaniline, 

C6H4.N(CH3)2.NO  +  4TiCl3  +  4HC1  = 
C8H4.N(CH3)2.NH2  +  4TiCl,  +  H2O. 

In  using  titanous  chloride  for  these  determinations,  it  is  essential 
that  the  reagent  should  be  kept  out  of  contact  with  the  air,  both  in 
the  storage  vessel  and  in  the  burette,  and  for  this  purpose  the 
arrangement  shown  in  fig.  45  should  be  used. 

The  most  suitable  strength  of  titanous  chloride  for  titrating  is 
a  one  per  cent,  solution,  obtained  by  letting  down  the  commercial 
product  with  water.  For  this  purpose  50  c.c.  of  the  commercial 
20  per  cent,  solutions  are  first  mixed  with  an  equal  volume  of  strong 
hydrochloric  acid  and  boiled  for  a  few  minutes  in  a  flask.  The 
solution  is  then  made  up  to  a  litre  with  distilled  water  which  has 
been  previously  boiled.  The  method  of  ascertaining  its  strength 
is  shown  on  page  235.  When  two  or  more  titrations  agree, 
the  iron  value  per  c.c.  of  the  titanous  chloride  solution  is  easily 
calculated. 

In  case  the  azo-compound  under  examination  is  not  precipitated 
by  dilute  hydrochloric  acid,  it  may  be  titrated  directly,  its  own 
intense  colour  serving  as  indicator. 

For  the  titrations  a  solution  of  0'5  gm.  of  the  dyestuff  made  up  to 
500  c.c.  with  distilled  water,  and  of  this  100  c.c.  are  taken.  The 
following  example  may  serve  as  an  illustration  :  — 

CRYSTAL  SCARLET  6R.—  C2oH12N2S202Na2  +7H20  (colouring  matter  from  alpha 
naphthylamine  and  G.  salt.) 

2   C   2 


388  AZO-DYES,    ETC. 

0*5  gm.  of  the  dyestuff  was  dissolved  in  distilled  water  and  the  solution  made 
up  to  500  c.c.     Of  this,  100  c.c.  were  measured  out  into  a  conical  flask,  and  after 
adding  about  10  c.c.  concentrated  hydrochloric  acid,  boiled  for  about  a  minute. 
This  amount  required  22 '6  c.c.  of  titanous  chloride  solution. 
The  calculation  is  as  follows  : — 

1  c.c.  TiCl3  =0-00158  gm.  Fe 

and  502  gm.  colour  require  by  theory  224  gm.  Fe. 
...  0-00158  x  22-6  x  502  _0.nsnn9  gm   colom. 

224  calc. 

and  1  gm.  contains  0-8002  or       80-02  %  79*96 

Water  of  cryst.  at  140°  C.  =         19 '96  20-04 

TotaJ         99-98  100 -00 

In  the  case  of  dyestuff s  which,  like  the  majority  of  the  benzidine 
derivatives,  are  thrown  out  of  solution  by  hydrochloric  acid,  the 
reaction  is  too  slow  and  the  end  of  the  reaction  often  not  sufficiently 
sharp  to  admit  of  exact  determinations.  In  such  cases  it  is  best  to 
run  in  an  excess  of  titanous  chloride  solution  into  the  boiling 
solution  of  the  dyestuff,  taking  the  precaution  to  keep  a  gentle 
current  of  carbonic  acid  passing  into  the  flask  by  a  tube  which 
almost  touches  the  surface  of  the  liquid.  The  reduction  will 
usually  be  completed  in  less  than  two  minutes,  when  the  flask  is 
cooled  under  the  tap  without,  however,  interrupting  the  current 
of  carbonic  acid.  When  cold,  the  excess  of  titanous  chloride  is 
determined  as  already  described  by  running  in  iron  alum  solution 
of  known  strength  (but  preferably  nearly  equivalent  to  that  of  the 
titanous  chloride  solution)  until  a  drop  taken  out  and  spotted  on 
potassium  sulphocyanide  solution  just  shows  a  red  colour.  By 
subtracting  the  number  of  c.c.'s  of  the  iron  alum  solution  (or  tKeir 
equivalent  in  titanous  chloride,  should  the  two  solutions  not  be  of 
equal  strength)  from  the  total  number  of  c.c.'s  of  titanous  chloride 
run  in,  the  exact  amount  of  the  latter  used  up  in  the  reduction  of 
the  dyestuff  is  arrived  at. 

The  following  example  will  serve  to  illustrate  the  application  of 
the  indirect  method  : — 

BENZOPURPTJRIN  4  B,  C34H26N6S206K?  +4£  H20  ( potassium  salt  of  the 
colouring  matter  from  tolidine  and  naphthionic  acid). 

0'5  gm.  of  the  dyestuff  is  dissolved  in  distilled  water,  and  the  solution  made 
up  to  500  c.c.  Of  this  100  c.c.  were  measured  into  a  conical  flask  and  heated  to  the 
boil.  10  c.c.  concentrated  hydrochloric  acid  and  50  c.c.  titanous  chloride  solution 
were  then  added,  carbonic  acid  being  passed  into  the  flask.  The  contents  of  the 
flask  were  now  boiled  for  about  a  minute,  when  complete  reduction  took  place, 
and,  after  cooling,  the  solution  required  22-9  c.c.  iron  alum  (equivalent  to  21-0  c.c. 
titanous  chloride).  The  excess  of  titanous  chloride  added  was  therefore  21-0  c.c., 
and  this,  subtracted  from  50,  gives  29 '0  c.c.  titanous  chloride  as  having  been 
used  for  the  reduction.  The  calculation  is  as  follows  : — 
1  c.c.  TiCl3  =0-001845  gm.  Fe 
and  756  gm.  colour  require  by  theory  448  gm.  Fe. 

'••  000184Sx29x7J6=0-09026  gm.  colour. 

448         .  Examd.  Calc. 

And  1  gm.  contains  0-9026  or      90-26  %        90-33  % 
Water  of  cryst 9'63J  9'67 

99-89  100-00 


CARBON    BISULPHIDE.  389 

For  the  determination  of  picric  acid  and  other  nitro-compounds, 
it  is  also  necessary  to  employ  the  indirect  method,  the  end  of  the 
reaction  not  being  perceptible  by  the  direct  method. 

CARBON    BISULPHIDE    AND    THIOGARBONATES. 

CS2  =  76-14. 

FOR  the  purpose  of  determining  carbon  disulphide  in  the  air  of 
soils,  gases,  or  in  thiocarbonates,  Gas  tine*  has  devised  the 
following  process  : — 

The  gas  or  vapour  to  be  tested  is  carefully  dried,  and  then  passed  through  a 
concentrated  solution  of  recently  fused  potassium  hydroxide  in  absolute  alcohol. 
The  presence  of  even  traces  of  water  seriously  diminishes  the  delicacy  of  the 
reaction.  The  alcoholic  solution  is  afterwards  neutralized  with  acetic  acid, 
diluted  with  water,  and  tested  for  xanthic  acid  by  adding  copper  sulphate. 

In  order  to  determine  the  distribution  of  carbon  disulphide  introduced  into 
the  soil,  250  c.c.  of  the  air  in  the  soil  is  drawn  by  means  of  an  aspirator  through 
sulphuric  acid,  and  then  through  bulbs  containing  the  alcoholic  potash.  For 
quantitative  determinations,  a  larger  quantity  of  air  must  be  used,  and  the 
xanthic  acid  formed  is  determined  by  means  of  the  reaction  2C3H6OS2  +I2 
=2C3H5OS2  +2HI.  The  alkaline  solution  is  slightly  acidified  with  acetic  acid, 
mixed  with  excess  of  sodium  bicarbonate,  and  titrated  in  the  usual  way  with 
a  solution  of  iodine,  containing  l'68gm.  per  litre,  1  c.c.  of  which  is  equivalent  to 
1  mgm.  of  carbon  disulphide. 

To  apply  this  method  to  thiocarbonates,  about  1  gm.  of  the  substance,  together 
with  about  10  c.c.  of  water,  is  introduced  into  a  small  flask  and  decomposed  by 
a  solution  of  zinc  or  copper  sulphate,  the  flask  being  heated  on  a  water-bath,  and 
the  evolved  carbon  disulphide  passed,  first  through  sulphuric  acid  and  then  into 
alcoholic  potash.  In  the  case  of  gaseous  mixtures  of  carbon  disulphide,  nitrogen, 
hydrogen  sulphide,  carbonic  anhydride,  carbonic  oxide  and  water  vapour  the  gas 
is  passed  through  a  strong  aqueous  solution  of  potash,  then  into  sulphuric  acid, 
and  finally  into  alcoholic  potash.  The  thiocarbonate  formed  in  the  first  flask  is 
decomposed  by  treatment  with  copper  or  zinc  sulphate  as  above,  and  the  xanthic 
acid  obtained  is  added  to  that  formed  in  the  third  flask,  and  the  whole  titrated 
with  iodine. 

Harding  and  Doran's  Method,  f  For  CS2  in  Benzene.  A  known  volume  of  the 
benzene  is  shaken  in  a  separating  funnel  with  a  solution  of  potassium  hydroxide 
in  absolute  alcohol.  After  standing  half  an  hour,  the  mixture  is  extracted 
several  successive  times  with  water  containing  a  little  alkali,  about  40  c.c.  of 
water  and  1  c.c.  of  alcoholic  potassium  hydroxide  solution  being  used  each  time. 
The  aqueous  extractions  are  diluted  to  a  known*  volume,  and  a  portion  is 
acidified  with  acetic  acid  and  precipitated  by  the  addition  of  a  distinct  excess 
of  standard  cupric  acetate  solution.  The  precipitate  is  stirred  for  about  ten 
minutes,  collected  on  a  filter,  and  washed  with  four  quantities  of  water,  using 
15  c.c.  each  time.  About  3  grams  of  potassium  iodide  are  added  to  the  filtrate, 
and  the  liberated  iodine  is  titrated  with  a  sodium  thiosulphate  solution  whose 
strength  in  terms  of  cupric  acetate  is  known.  The  authors  find  that  the  ratio 
of  CuO  to  CS2  is  1  to  1-927.  This  ratio  approaches  nearest  to  that  between 
cupric  oxide  and  carbon  disulphide  in  cupric  xanthate,  which  has  the  formula 
(CS.OC2H5S)2Cu,  and  in  which  the  ratio  is  1  to  1-9126;  this  tends  to  strengthen 
the  belief  that  the  compound  formed  is  cupric  xanthate. 

For  CS2  in  Illuminating  Gas. — The  method  consists  in  passing  the  gas  through 
a  meter,  absorbing  the  carbon  dioxide  with  potassium  hydroxide,  drying  the  gas 
by  means  of  concentrated  sulphuric  acid,  and  absorbing  the  carbon  disulphide  in 
an  alcoholic  solution  of  potassium  hydroxide.  The  water  in  the  meter  must  be 
saturated  previously  with  gas,  and  about  2  cubic  feet  of  the  latter  are  employed 
for  the  determination,  the  gas  being  allowed  to  flow  at  the  rate  of  0-5  cubic  foot 
*  Compt.  Rend.  98,  1588.  f  Journ.  Amer.  Chem.  Soc.  1907,  29,  1476. 


390  FORMALDEHYDE. 

per  hour.  The  xanthate  solution  obtained  is  boiled  to  expel  absorbed  gases, 
cooled,  acidified  with  acetic  acid,  and  precipitated  by  the  addition  of  standard 
cupric  acetate  solution.  The  determination  is  then  carried  out  as  described 
above. 

FORMALDEHYDE. 

H.CHO  =  30-02. 

THIS  substance  is  met  with  in  commerce  in  the  form  of  a  40  % 
(by  volume)  aqueous  solution  ("  Formalin ")  containing  usually 
from  36-38  per  cent,  by  weight  of  formaldehyde.  Solutions  contain- 
ing more  than  40  %  of  formaldehyde  polymerise  spontaneously 
to  paraformaldehyde. 

For  the  purpose  of  determining  the  amounts  of  formaldehyde  in 
various  solutions  the  following  processes  are  employed  : — 

LEGLEB'S  METHOD:  10  c.c.  of  the  solution  to  be  tested  are  neutralized, 
if  necessary,  with  N/ioo  soda  and  placed  in  a  flask,  diluted  with  water,  and 
treated  with  an  excess  of  standard  ammonia  solution.  The  excess  is  removed  by 
a  current  of  steam  and  received  hi  standard  acid,  the  result  being  calculated  from 
the  following  equation:— 6CH20  +  4NH3  =  (CH2)(.N4  +  6H20,  which  represents  the 
reaction  which  occurs.  A  small  quantity  of  hexamethylene-tetramine  is,  however, 
carried  over  by  the  steam.  68*12  parts  of  ammonia  react  with  180-12  parts  of 
formaldehyde,  or  1  part  of  ammonia  equals  2-6442  parts  of  formaldehyde. 
Hexamethylene-tetramine,  N4(CH2),.,  is  formed  as  a  condensation  product.  The 
action  is  slow,  and  according  to  the  experiments  of  L.  F.  Kebler*  a  digestion  of 
6  hours  is  necessary  hi  order  to  get  the  full  proportion  of  CH20.  Equally  good 
results  were  obtained  when  the  mixed  solutions  were  left  overnight. 

Another  method  which  gives  good  results  is  that  of  Blank  and 
Finkenbeiner.f  It  is  based  on  the  oxidation  of  formic  aldehyde 
into  formic  acid  by  peroxide  of  hydrogen  in  alkaline  solution,  and 
titration  of  the  excess  of  alkali. 

METHOD  OF  PROCEDURE  :  3  gm.  of  the  solution  of  formic  aldehyde  under 
examination  (or  1  gm.  in  the  case  of  a  solid)  are  weighed  out  carefully  and  placed 
in  a  tall  conical  flask  containing  25  c.c.  of  double  normal  soda  (30  c.c.  when  the 
concentration  of  the  formic  aldehyde  is  greater  than  45  per  cent.).  The  mixture 
is  then  immediately  treated  with  50  c.c.  of  peroxide  of  hydrogen  at  from  2-5  to 
3  per  cent,  strength  ;  in  the  case  of  the  peroxide  having  an  acid  reaction,  the 
acidity  should  be  determined  and  deducted  from  the  final  result.  The  peroxide 
of  hydrogen  must  be  added  gradually  (taking  about  three  minutes)  by  means  of 
a  funnel ;  after  two  or  three  minutes  the  funnel  is  rinsed  with  water,  and  the 
excess  of  alkali  is  titrated  with  a  double  normal  solution  of  sulphuric  acid  ;  in 
very  exact  analyses  the  water  used  for  rinsing  should  be  boiled  first  to  drive  off 
any  carbonic  acid.  Litmus  is  used  as  an  indicator. 

With  solutions  containing  less  than  30  per  cent,  of  formic  aldehyde  the 
mixture  should  be  allowed  to  stand  for  about  ten  minutes  after  the  addition  of 
the  peroxide  of  hydrogen,  for  the  reaction  to  be  complete. 

The  volume  of  standard  alkali  used,  multiplied  by  6,  gives  the  formaldehyde 
in  1  gm.  of  the  solid  or,  multiplied  by  2,  in  3  gm.  of  the  solution. 

The  reaction  takes  place  with  the  disengagement  of  a  considerable  amount  of 
heat  and  production  of  froth. 

Experiments  on  other  aldehydes  did  not  give  satisfactory  results. 

A  further  method,  especially  applicable  to  dilute  solutions,  is 
furnished  by  R.  Orchard.  J  It  is  based  on  the  reaction  of 
formaldehyde  with  ammoniacal  silver  solution,  and  in  this  process 

*  Amer.  Journ.  Pharm.  1898,  432. 
t  Berichte  1898,  2979,  also  Analyst,  1899,  92.  J  Analyst,  22,  4. 


FORMALDEHYDE.  391 

it  is  arranged  quantitatively,  and  can  be  carried  out  either  by 
weight  or  the  residual  silver  found  volume trically. 

METHOD  OF  PROCEDURE  :  In  the  actual  experiments  10  c.c.  of  on  approxi- 
mately 0-1  per  cent,  solution  of  formaldehyde  were  added  to  25  c.c.  N/io  silver 
nitrate,  10  c.c.  of^dilute  ammonia  (1  of  0'88  solution  to  50  of  water)  added,  and 
the  whole  boiled  in  a  conical  flask  attached  to  a  reflux  condenser.  The  precipitate, 
after  filtration  and  washing,  was  ignited  and  weighed  as  metallic  silver,  and  as 
a  check  the  excess  of  silver  was  determined  in  the  filtrate  as  silver  chloride.  As 
the  first  experiments  showed  that  the  reduction  was  incomplete  after  boiling  for 
half  an  hour,  the  boiling  was  continued  for  four  hours.  In  order  to  ascertain  if 
any  loss  took  place  during  boiling,  a  duplicate  determination  was  made,  in  which 
a  bottle  with  a  tied-down  stopper,  heated  in  a  water-bath,  was  employed.  The 
actual  results  obtained  were  in  the  first  case  0-01038  gm.  formaldehyde,  and  in 
the  second  0-0104  gm.,  consequently  there  was  practically  no  loss. 

In  the  calculation,  as  one  molecule  of  CH2O  reduces  two  molecules  of  Ag20, 
the  weight  of  the  precipitated  silver  multiplied  by  the  factor  0*0696  gives  the 
weight  of  the  formaldehyde,  and  1  c.c.  N/io  silver  nitrate  corresponds  to  0 '00075 
gm.  formaldehyde  ;  it  is,  therefore,  possible  to  determine  extremely  small  quantities 
by  this  process. 

A  number  of  experiments  were  carried  out  on  the  determination 
of  formaldehyde  by  G.  Romijn.*  The  methods  of  Legler, 
Brochet,  and  Cambier  were  studied,  and  the  results  obtained 
with  them  compared  with  those  given  by  two  new  methods  de- 
scribed below.  For  this  purpose  four  aqueous  solutions  were 
prepared  containing  in  500  c.c.  :  (1)  2*075  gm.  of  formalin  ;  (2) 
2-075  gm.  of  formalin +  1-3  gm.  acetaldehyde  ;  (3)  2-075  gm.  of 
formalin +0-355  gm.  of  acetone  ;  (4)  2-075kgm.  of  formalin+1  gm. 
of  benzaldehyde. 

IODIMETRIC  METHOD. — 10  c.c.  of  the  aldehyde  solution  are  mixed  with  25  c.c. 
of  decinormal  iodine  solution,  and  sodium  hydrate  added  drop  by  drop  until  the 
liquid  becomes  clear  yellow.  After  ten  minutes  hydrochloric  acid  is  added  to 
liberate  the  uncombined  iodine,  which  is  then  titrated  back  with  standard 
thiosulphate.  Two  atoms  of  iodine  are  equivalent  to  1  molecule  of  formaldehyde. 
The  amo'unt  of  iodine  taken  up  multiplied  by  0-1183  gives  the  amount  of  formal- 
dehyde. The  results  obtained  with  the  first  solution  showed  that  the  formalin 
used  contained  (1)  37 -38  and  (2)  37 '40  per  cent,  of  formaldehyde. 

With  the  second  solution  a  certain  amount  of  iodoform  was  produced,  and  the 
results  were  too  low.  With  the  third  solution  the  acetone  was  entirely  converted 
to  iodoform,  and  in  the  fourth  solution  the  benzaldehyde  was  partially  oxidized. 
Hence  this  method,  though  suitable  for  the  valuation  of  pure  formaldehyde,  does 
not  give  correct  results  in  the  presence  of  other  aldehydes. 

POTASSIUM  CYANIDE  METHOD. — This  is  based  on  the  fact  that  formaldehyde 
combines  with  potassium  cyanide.  The  addition  product  reduces  silver  nitrate 
in  the  cold.  But  if  the  silver  nitrate  be  acidified  with  nitric  acid  before  the 
addition  of  the  aldehyde  cyanide  mixture,  no  precipitate  results  if  the  aldehyde 
in  the  latter  be  in  excess.  If,  on  the  other  hand,  the  potassium  cyanide  is  in 
excess,  1  molecule  of  potassium  cyanide  is  left  in  combination  with  1  molecule  of 
the  formaldehyde,  while  the  excess  precipitates  silver  cyanide  from  the  silver  . 
nitrate  solution. 

10  c.c.  of  decinormal  silver  nitrate,  acidified  with  nitric  acid,  are  mixed  with 
10  c.c.  of  potassium  cyanide  solution  (prepared  by  dissolving  3-1  gm.  of  the  96  per 
cent,  salt  in  500  c.c.),  the  whole  diluted  to  500  c.c.,  filtered,  and  25  c.c.  of  the 
filtrate  titrated  by  Volhard's  method  (p.  145).  The  difference  between  this 
blank  result  and  that  obtained  by  titrating  the  filtrate  after  the  addition  of  the 
aldehyde  solution  gives  the  amount  of  standard  sulphocyanide  corresponding  to 
the  silver  not  precipitated  by  the  excess  of  potassium  cyanide.  From  this  the 
amount  of  formaldehyde  can  be  calculated.  With  solution  1  the  results  showed 
*  Z.  a.  C.  1897,  18-24. 


392  ALDEHYDES. 

37-39  and  37 '67  per  cent,  of  formaldehyde  in  the  formalin.  With  solution  2,  if 
the  titration  was  made  immediately  after  shaking,  only  the  formaldehyde  had 
combined,  but  if  left  for  some  time  the  acetaldehyde  also  began  to  combine,  and 
erroneous  results  were  obtained.  Solutions  3  and  4  gave  correct  results,  even 
after  standing  for  30  minutes. 

HYDROXYLAMISTE  METHOD  (B  r  o  c  h  e  t  and  C  a  m  b  i  e  r)*. — This  gave  satisfactory 
results  with  pure  formaldehyde,  but  quite  irregular  figures  with  the  other  three 
solutions. 

LE GLEE'S  METHOD.f — The  four  solutions  were  made  more  concentrated  in 
order  to  lessen  the  difficulty  of  observing  the  end-reaction.  In  each  case  the  correct 
amount  of  formaldehyde  was  found,  but  the  author  does  not  consider  the  method 
so  accurate  as  the  others. 

Acetaldehyde. — A  method  originally  proposed  by  ReiterJ  has 
been  modified  by  Roques  with  good  results. 

METHOD  OP  PROCEDURE  :  A  sodium  sulphite  solution  is  made  by  dissolving 
12-6  gm.  of  anhydrous  sodium  sulphite  in  400  c.c.  of  water,  adding  100  c.c.  of 
normal  sulphuric  acid,  diluting  to  1000  c.c.  with  alcohol  of  96  %,  and  filtering 
after  24  hours.  A  convenient  quantity  of  the  alcoholic  solution  of  aldehyde  to 
be  examined  is  placed  in  a  100  c.c.  stoppered  flask,  mixed  with  50  c.c.  of  the 
^sulphite  solution  and  made  up  to  100  c.c.  with  alcohol  of  50  %.  A  second  quantity 
of  50  c.c.  of  the  sulphite  solution  is  placed  in  a  similar  flask,  and  made  up  to  100 
c.c.  with  the  same  alcohol.  After  heating  to  50°  C.  at  least  4  hours,  50  c.c.  are 
withdrawn  from  each  flask,  and  the  sulphurous  acid  determined  by  means  of 
N/1O  iodine  solution';  the  difference  is  the  quantity  of  sulphurous  acid  that  is  in 
combination  with  the  aldehyde ;  1  c.c.  of  N/io  iodine  =0*0022  gm.  of  aldehyde. 

If  the  liquid  to  be  examined  contains  less  than  1  per  cent,  of  aldehyde  the 
sulphite  solution  must  be  diluted ;  for  0'5  per  cent.,  it  should  be  diluted  with  an 
equal  volume  of  alcohol  of  50  %,  and  N/go  iodine  should  be  used ;  for  O'l  per  cent., 
the  sulphite  should  be  diluted  with  alcohol  of  50  %  to  10  times  its  ordinary  volume, 
and  centinormal  iodine  solution  should  be  used. 

A  Volumetric  Method  for  the  Determination  of  various  Aldehydes 
has  been  devised  by  M.  Ripper.|| — The  method  is  based  on  the 
combination  of  alkali  bisulphites  with  aldehydes.  25  c.c.  of 
the  solution  to  be  examined,  which  should  not  contain  more  than 
J  per  cent,  of  the  aldehyde,  are  run  into  50  c.c.  of  a  solution  of 
potassium  bisulphite  containing  12  gm.  KHSO3  per  litre,  placed  in 
a  150  c.c.  flask,  which  is  then  securely  corked,  and  allowed  to  stand 
for  a  quarter  of  an  hour.  During  this  time  another  50  c.c.  of  the 
potassium  bisulphite  solution  are  titrated  with  N/10  iodine.  The 
excess  of  bisulphite  added  to  the  aldehyde  solution  is  then 
determined  with  the  same  iodine  solution,  and  from  the  difference 
the  amount  of  aldehyde  present  is  calculated. 

The  amount  of  the  aldehyde  is  obtained  by  the  formula — 

T      M 

-I2-  =  r_^ 
~J  26-92"  2531*4 

in  which  A  represents  the  amount  of  aldehyde,  I  the  iodine  corresponding  to  the 
combined  sulphurous  acid,  and  M  the  molecular  weight  of  the  aldehyde  in 
question. 

From  this  the  following  factors  are  obtained  : — 
Formaldehyde  =1  xO'1179. 
Acetaldehyde    =1x0-1729 
Benzaldehyde   =  I  x  0  -4 1 66 
Vanillin  =1x0-5974. 

*  Comp.  Rend.  120,  449.  f  Ber.  16,  1335.  J  J.  S.  C.  I.  abslr.  1897,  GOG. 

li  Monatfthe/te  /.  Chem.21,  1079. 


GLYCEROL.  393 

The  method  is  said  to  yield  reliable  results  in  all  cases  in  which  the  aldehyde 
is  soluble  in  water,  or  can  be  brought  into  solution  by  the  addition  of  a  little 
alcohol. 

In  the  case  of  the  four  aldehydes  mentioned  above,  test  analyses  are  described 
in  detail  to  show  that  the  results  are  in  close  agreement  with  those  obtained  by 
recognised  reliable  methods. 

Solutions  of  potassium  bisulphite  stronger  than  the  above  should  not  be 
employed,  as  the  larger  quantities  of  hydriodic  acid  formed  would  exert  a 
reducing  action  on  the  sulphuric  acid  formed.  The  use  of  alcohol  to  dissolve  the 
aldehyde  should  also  be  avoided  as  far  as  possible,  as  even  relatively  small 
quantities  of  alcohol  (upwards  of  5  per  cent.)  interfere  with  the  iodide  of  starch 
reaction.  With  the  very  dilute  solutions  of  aldehyde  used,  however,  a  very 
small  addition  of  alcohol  will  be  sufficient  in  most  cases. 


GLYCEROL. 
GLYCERIN. 

C3H5(OH)3  =  92-06. 

UP  to  a  recent  time  no  satisfactory  method  of  determining 
glycerin  had  been  devised,  but  the  problem  has  now  been  solved 
in  a  tolerably  satisfactory  manner.  The  permanganate  method  of 
oxidation  appears  to  have  been  originally  suggested  by  Wanklyn, 
improved  by  him  and  Fox,  and  further  elaborated  by  Benedikt 
and  Zsigmondy.*  With  fatty  matters  it  depends  on  the 
saponification  of  the  fat,  and  oxidation  of  the  resultant  glycerol 
by  permanganate  in  alkaline  solution,  with  formation  of  oxalic 
acid,  carbon  dioxide,  and  water,  thus — 

C3H8O3  +  3O2 = C2H204 + CO2 + 3H2O. 

Aqueous  solutions  of  glycerin  may  of  course  be  submitted  to  the 
method  very  easily. 

The  excess  of  permanganate  is  destroyed  by  a  sulphite,  the  liquid 
filtered  from  the  manganese  precipitate,  the  oxalic  acid  then  pre- 
cipitated by  a  soluble  calcium  salt  in  acetic  solution,  and  the 
precipitated  calcium  oxalate,  after  ignition  to  convert  it  into 
carbonate,  titrated  with  standard  acid  in  the  usual  way,  or  the 
oxalic  precipitate  titrated  with  permanganate.  The  oxalic  solution 
may  be  titrated  direct  after  addition  of  H2S04  with  permanganate  ; 
but  Allen  and  Belcher  have  found  this  method  faulty,  probably 
from  the  formation  of  a  dithionate,  due  to  the  sulphite.  On  the 
other  hand,  they  have  obtained  very  satisfactory  results  by  the 
alkalimetric  or  the  permanganate  titration  on  known  weights  of 
pure  oxalic  acid  and  glycerin. 

These  operators  have  also  shown  that,  in  the  case  of  dealing  with 
fats,  where  it  has  been  recommended  by  Wanklyn  and  Fox  to 
use  ordinary  alcohol  as  the  solvent,  and  by  Benedikt  methyl 
alcohol,  both  these  media,  especially  ethylic  alcohol,  themselves 
produce  a  variable  quantity  of  oxalic  acid  when  treated  with 
alkaline  permanganate,  and  hence  vitiate  the  process.  Again,  if 

*  Chem.  Zett.  9,  975,  and  J.  S.  C.  I.  1885   610. 


394  GLYCEROL. 

it  be  attempted  to  avoid  this  by  boiling  off  the  alcohols,  there  is 
a  danger  of  losing  glycerin.* 

Allen's  methodf  with  oils  and  fats  is  as  follows  : — 

10  gm.  of  the  fat  or  oil  are  placed  in  a  strong  small  bottle,  together  with  4  gra. 
of  pure  KHO  dissolved  in  25  c.c.  of  water.  A  solid  rubber  stopper  is  then  used 
to  close  the  bottle,  and  tied  down  firmly  with  wire.  It  is  then  placed  in  boiling 
water,  or  in  a  water  oven,  and  heated,  with  occasional  shaking,  from  6  to  10  hours, 
or  until  the  contents  are  homogeneous,  and  all  oily  globules  have  disappeared. 
When  saponification  is  complete,  the  bottle  is  emptied  into  a  beaker  and  diluted 
with  hot  water  which  should  give  a  clear  solution,  the  fatty  acids  are  then  separated 
by  dilute  acid,  filtered,  and  the  filtrate  made  up  to  a  given  volume. 

This  solution,  which  will  usually  contain  from  0'2  to  0'5  gm.  of  glycerol, 
according  to  its  origin,  is  transferred  to  a  porcelain  basin  and  diluted  with  cold 
water  to  about  400  c.c.  From  10  to  12  gm.  of  caustic  potash  should  next  be 
added,  and  then  a  saturated  aqueous  solution  of  potassium  permanganate  until 
the  liquid  is  no  longer  green  but  blue  or  blackish.  An  excess  does  no  harm. 
The  liquid  is  then  heated  and  boiled  for  about  an  hour,  when  a  strong  solution  of 
sodium  sulphite  should  be  added,  drop  by  drop,  to  the  boiling  liquid  until  it  just 
becomes  colourless.  The  liquid  containing  the  precipitated  oxide  of  manganese 
is  then  poured  into  a  500  c.c.  flask,  and  hot  water  added  to  15  c.c.  above  the  mark, 
the  excess  being  an  allowance  for  the  volume  of  the  precipitate  and  for  the 
increased  measure  of  the  hot  liquid.  The  solution  is  then  passed  through  a  dry 
filter,  and,  when  cool,  400  c.c.  of  the  filtrate  should  be  measured  off,  acidified 
with  acetic  acid,  and  precipitated  with  calcium  chloride.  The  solution  is  kept 
warm  for  three  hours,  or  until  the  deposition  of  the  calcium  oxalate  is  complete, 
and  is  then  filtered,  the  precipitate  being  washed  with  hot  water.  The  precipitate 
consists  mainly  of  calcium  oxalate,  but  is  liable  to  be  contaminated  more  or  less 
with  calcium  sulphate,  silicate,  and  other  impurities,  and  hence  should  not  be 
directly  weighed.  It  may  be  ignited,  and  the  amount  of  oxalate  previously 
present  deduced  from  the  volume  of  normal  acid  neutralized  by  the  residual 
calcium  carbonate,  but  a  preferable  plan  is  to  titrate  the  oxalate  by  standard 
permanganate.  For  this  purpose,  the  filter  should  be  pierced  and  the  precipitate 
rinsed  into  a  porcelain  basin.  The  neck  of  the  funnel  is  then  plugged,  and  the 
filter  filled  with  dilute  sulphuric  acid.  After  standing  for  five  or  ten  minutes 
this  is  allowed  to  run  into  the  basin  and  the  filter  washed  with  water.  Acid  is 
added  to  the  contents  of  the  basin  in  quantity  sufficient  to  bring  the  total 
amount  used  to  10  c.c.  of  concentrated  acid,  the  liquid  diluted  to  about  200  c.c., 
brought  to  a  temperature  of  about  60°  C.,  and  decinormal  permanganate  added 
gradually  till  a  distinct  pink  colouration  remains  after  stirring.  Each  c.c.  of 
permanganate  used  corresponds  to  0'0045  gm.  of  anhydrous  oxalic  acid,  or  to 
0'0046  gm.  of  glycerol.  Operating  in  the  way  described,  the  volume  of 
permanganate  solution  required  will  generally  range  between  70  and  100  c.c. 

C.  MangoldJ  advocates  the  reduction  of  the  excess  of 
permanganate  by  hydrogen  peroxide  in  preference  to  sodium 
sulphite  as  used  by  Allen.  The  author  simplifies  the  method  by 
carrying  out  the  oxidation  in  the  cold. 

METHOD  OF  PROCEDURE  :  2-4  gm.  of  fat  are  saponified  ;  the  filtrate  from  the 
liberated  fatty  acids  is  put  into  a  litre  flask,  diluted  to  300  c.c.,  10  gm.  potassium 
hydroxide  added,  and  as  much  of  a  5  per  cent,  permanganate  solution  as  will 
correspond  to  1£  times  the  theoretical  quantity  required  for  the  oxidation  of 
the  glycerol  (1  gm.  C3H803  theoretically  requires  6 '87  gm.  KMnO4).  The 
operation  is  conducted  in  the  cold  and  with  constant  shaking.  The  mixture  is 
allowed  to  stand  at  the  ordinary  temperature  for  half  an  hour.  Hydrogen  peroxide 

*  In  dealing  with  waxes  or  similar  bodies  including  sperm  oil,  potash  dissolved  in 
methyl  alcohol  must  be  used  for  the  saponiflcation,  as  it  is  almost  impossible  to 
saponify  with  aqueous  potash. 

t  Commercial  Org.  Analysis,  2,  290.  J  Zeit.  /.  angew.  Chem.  1891,  400. 


GLYCEROL.  395 

is  then  added  (avoiding,  however,  a  large  excess)  until  the  liquid  becomes  colourless. 
Then  make  up  to  1000  c.c.,  shake  well,  and  filter  off  500  c.c.  through  a  dry  filter. 
Boil  the  filtrate  for  half  an  hour  to  decompose  all  hydrogen  peroxide,  allow  to 
cool  to  60°  C.,  acidify  with  sulphuric  acid,  and  titrate  with  standard  permanganate 
solution. 

Otto  Hehner*  has  experimented  largely  on  the  determination  of 
glycerol  in  soap  lyes  and  crude  glycerins.  The  volumetric  methods 
recommended  in  preference  to  the  permanganate  are  oxidation 
with  potassium  dichromate,  or  conversion  of  the  glycerol  into 
triacetin. 

The  Dichromate  Method. — One  part  of  glycerol  is  quantitatively 
oxidized  to  carbonic  acid  by  7 -486  parts  of  dichromate  in  the  presence 
of  sulphuric  acid.  The  solutions  required  are  : — 

Standard  potassium  dichromate. — 74g86  gm.  of  pure  potassium 
dichromate  are  dissolved  in  water.  150  c.c.  of  concentrated 
sulphuric  acid  added,  and  when  cold  diluted  to  a  litre.  1  c.c. 
=0-01  gm.  glycerol. 

A  weaker  solution  is  also  made  by  diluting  100  c.c.  of  the  strong 
solution  to  a  litre. 

These  solutions  should  be  controlled  by  a  ferrous  solution  of 
known  strength,  if  there  is  any  doubt  about  the  purity  of  the 
dichromate. 

Solution  of  double  iron  salt. — 240  gm.  of  ferrous  ammonium 
sulphate  are  dissolved  with  50  c.c.  of  concentrated  sulphuric  acid 
to  a  litre,  and  its  relation  to  the  standard  dichromate  must  be 
accurately  found  from  time  to  time  by  titration  with  the  latter, 
using  the  ferricyanide  indicator  (p.  126). 

METHOD  OF  PROCEDURE  :  With  concentrated  or  tolerably  pure  samples  of 
glycerin  it  is  only  necessary  to  take  a  small  weighed  portion,  say  0'2  gm.  or  so, 
dilute  moderately,  add  10  or  15  c.c.  of  concentrated  sulphuric  acid  and  30  or  40  c.c. 
of  the  stronger  dichromate,  place  the  beaker  covered  with  a  watch  glass  in  a 
water-bath  and  digest  for  two  hours  ;  the  excess  of  dichromate  is  then  found  by 
titration  with  the  standard  iron  solution.  The  weaker  dichromate  is  useful  in 
completing  the  titration  where  accuracy  is  required.  As  the  stronger  dichromate 
and  the  iron  solution  are  both  concentrated,  they  mus«  be  used  at  a  temperature 
as  near  15°  C.  as  possible.  If  the  operation  be  carried  out  on  a  water-bath  and 
kept  at  normal  temperature  during  the  operation  no  correction  will  be  necessary. 
In  the  case  of  crude  glycerin  it  must  be  purified  from  chlorine  or  aldehyde 
compounds  as  follows : — About  1  -5  gm.  of  the  diluted  sample  is  placed  in  a 
100  c.c.  flask,  some  moist  silver  oxide  added,  and  allowed  to  stand  10  minutes. 
Basic  lead  acetate  is  then  added  in  slight  excess,  the  measure  made  up  to  100  c.c., 
filtered  through  a  dry  filter,  and  25  c.c.  or  so  digested  with  excess  of  dichromate, 
and  titrated  as  before  described. 

Richardson  and  Jaffef  have  published  a  modification  of  this 
method  for  the  treatment  of  crude  glycerins. 

METHOD  or  PROCEDURE  :  25  gm.  of  the  samples  are  made  up  with  "water  to 
50  c.c.  of  solution,  and  of  this  25  c.c.  are  taken,  and  precipitated  with  7  c.c.  of  the 
official  solution  of  basic  acetate  of  lead  (Liquor  Plumbi  Subacetatis  B.P.).  The 
mixture  is  filtered  through  a  Swedish  filter  into  a  250  c.c.  flask.  Repeated 
washings  are  made  with  about  150  c.c.  of  cold  water.  The  excess  of  lead  (which 

*  J.  S.  C.  I.  8,  4.  t  J .8.  C.  I.  1898,  330. 


396  GLYCEROL. 

should  be  small)  is  precipitated  by  an  excess  of  dilute  sulphuric  acid.  After 
making  to  the  mark  and  shaking,  the  liquid  is  poured  on  to  a  dry  Swedish  filter, 
20  c.c.  of  the  filtrate  (representing  2  gm.  of  the  original  sample  of  crude  glycerin) 
are  pipetted  into  a  beaker,  the  mouth  of  which  is  closed  by  a  funnel  with  short 
stem  ;  then  25  c.c.  of  Hehner'  s  strong  standard  dichromate  solution  are  added  ; 
finally  25  c.c.  of  pure  sulphuric  acid  are  cautiously  mixed  with  the  other  fluids. 
After  20  minutes'  heating  in  a  water-bath,  the  oxidation  is  complete.  After 
cooling,  the  liquid  is  made  to  250  c.c.  with  water,  and  this  solution  is  then  titrated 
upon  20  c.c.  of  a  solution  containing  2  '982  per  cent,  of  the  double  sulphate  of 
iron  and  ammonia,  using  ferricyanide  of  potassium  to  determine  the  end-reaction 
in  the  usual  manner. 

The  portion  of  the  iron  solution  taken  represents  O'Ol  gm.  of  glycerin  ;  there- 
fore, if  A  is  the  number  of  c.c.  of  the  dichromate  mixture  required,  and  x  the 
percentage  of  glycerin  sought,  we  have  the  simple  formula  — 


(0*25  gm.  is  the  equivalent  of  glycerin  represented  by  that  25  c.c.  of  dichromate 
used,  containing  74-86  gm.  per  litre). 

In  the  case  of  a  spent  lye  we  take  2'5  gm.  and  dilute  to  50  c.c.  ;  the  precipitation 
of  chlorides  and  organic  impurities  is  effected  by  the  addition  of  a  slight  excess 
of  the  solution  of  basic  lead  acetate,  and  the  operation  proceeds  as  in  the  case  of 
the  crude  glycerin,  with  the  exception  that  the  lead  sulphate  is  filtered  off  and 
the  filtrate  is  concentrated  to  about  25  c.c.  before  the  addition  of  the  dichromate 
and  the  sulphuric  acid  is  made* 

The  Acetin  Method.  —  This  process  is  based  on  the  quantitative 
conversion  of  glycerol  into  triacetin  by  heating  concentrated 
glycerol  with  acetic  anhydride,  the  reaction  being 


Glycerol.  Acetic  Triacetin.  Acetic 

anhydride.  acid. 

If  the  product  of  this  reaction  is  then  dissolved  in  water,  and  the 
free  acetic  acid  neutralized,  the  dissolved  triacetin  can  easily  be 
determined  by  saponifying  with  a  known  volume  of  standard 
alkali  and  titrating  back. 

It  is  due  to  Benedikt  and  Cantor,*  and  recommends  itself  by 
its  simplicity  and  rapidity  as  compared  with  other  methods. 
Hehner  has  pointed  out  the  precautions  necessary  to  ensure 
accuracy  as  follows  :  — 

METHOD  or  PROCEDURE  :  About  1  -5  gm.  of  the  crude  glycerin,  accurately 
weighed,  is  placed  in  a  round-bottomed  flask  holding  about  100  c.c.,  together  with 
7  gm.  of  acetic  anhydride  and  3  gm.  of  perfectly  anhydrous  sodium  acetate  ;  an 
upright  condenser  is  attached  to  the  flask,  and  the  contents  are  heated  to  gentle 
boiling  for  one  hour  and  a  half.  After  cooling  a  little,  50  c.c.  of  warm  water  are 
added  through  the  tube  of  the  condenser,  and  the  mixture  heated,  but  not  boiled, 
until  all  triacetin  has,  by  shaking,  dissolved.  As  triacetin  is  volatile  with  water 
vapour,  these  operations  should  be  conducted  whilst  the  flask  is  still  connected 
with  the  condenser.  The  solution  is  then  filtered  into  a  large  flask  (500  —  600  c.c.), 
the  residue  or  filter  well  washed,  the  liquid  allowed  to  cool  down  to  the  ordinary 
temperature,  some  phenolphthalein  added,  and  the  acidity  exactly  neutralized  by 
a  dilute  solution  of  caustic  soda  ;  whilst  running  in  the  soda  the  liquid  must  be 
shaken  continually  to  prevent  local  excess  of  the  alkali.  The  neutral  point  is 
reached  when  the  slightly  yellowish  colour  is  just  changed  to  reddish-yellow. 
It  must  not  become  pink  or  the  test  is  spoiled,  as  the  excess  of  soda  cannot  be 
titrated  back  owing  to  any  excess  of  alkali  saponifying  a  portion  of  the  acetin. 

*  MvnatsheftQ,  521. 


GLYCEROL.  397 

The  triacetin  is  then  saponified  by  adding  25  c.c.  of  an  approximately  10  per  cent, 
solution  of  pure  caustic  soda  standardized  with  normal  sulphuric  or  hydrochloric 
acid,  and  boiling  for  10  minutes,  taking  care  to  attach  a  reflux  condenser  to  the 
flask.  The  excess  of  alkali  is  then  titrated  back  with  normal  acid,  each  c.c.  of 
which  represents  0 -03067  gm.  of  glycerol. 

It  is  essential  that  the  processes  of  analysis  should  be  rapid  and  continuous, 
and  especially  that  the  free  acetic  acid  in  the  first  process  be  neutralized  very 
cautiously,  and  with  constant  agitation  to  avoid  the  local  action  of  alkali. 

EXAMPLE. — 1*324  gm.  of  a  sample  were  treated  as  above.  25  c.c.  of  the  strong 
soda  solution  required  60'5  c.c.  of  normal  hydrochloric  acid,  and  21-5  c.c.  were 
required  for  titrating  back. 

Hence  60'5  -21'5  =39*0  c.c.  had  been  used,  and  the  sample  contained  39  x 
0-03067  =1-196  gm.  or  90'3  per  cent,  of  glycerol. 

Weak  soap  lyes  should  be  concentrated  to  50  per  cent,  of  glycerin 
if  determined  by  the  acetin  method  ;  if  not  the  dichromate  method 
must  be  used. 

For  fats  and  soaps  about  3  gm.  should  be  saponified  with  alcoholic 
potash,  diluted  with  200  c.c..  of  water,  the  fatty  acids  separated 
and  filtered  off.  The  filtrate  and  washings  are  then  rapidly  boiled 
to  one  half  and  titrated  with  dichromate. 

In  the  case  of  crude  glycerins  the  permanganate  method  is  not 
so  reliable  as  the  acetin  or  dichromate  method,  owing  probably  to 
the  oxidation  of  foreign  matters  into  oxalates.  Hehner  has 
shown  by  comparative  experiments  with  both  acetin  and  dichromate 
methods  that  the  results  agree  well,  and  the  same  has  been  verified 
by  Lewkowitsch. 

The  latter  authority  has  called  attention  to  the  difficulties  which 
occur  in  examining  crude  lyes  for  glycerol  at  the  present  time  owing 
to  the  very  impure  fats  used  in  soap  making,  etc.*  The  dichromate 
method  is  liable  to  produce  very  high  results.  The  acetin  process 
is  only  applicable  to  strong  lyes  containing  not  less  than  about 
60  per  cent,  of  glycerol  and  cannot  therefore  be  used  with  weak 
lyes.  The  best  method  is  therefore  to  take,  say,  1000  gm.  or  c.c., 
purify  the  lye  and  concentrate  down  so  as  to  prepare  a  crude 
glycerin  in  which  the  glycerol  may  be  determined  by  the  acetin 
method. 

INDIGO. 

(Indigotin  C16H10N202.) 

THE  valuation  of  indigo  for  its  real  dyeing  property  has  created 
for  many  years  past  a  large  number  of  chemical  processes,  but 
those  which  give  anything  like  reliable  results  seem  to  necessitate 
an  enormous  amount  of  time  and  care,  together  with  very  compli- 
cated forms  of  apparatus,  the  use  of  which,  successfully,  requires 
the  purification  of  the  commercial  material  from  various 
accompanying  substances  in  order  to  get  satisfactory  results. 

One  of  the  earliest  methods  used  was  the  permanganate  test,  but 
owing  to  the  presence  of  other  substances  in  the  natural  product 

*  AnalystZS,  104. 


398  INDIGO. 

which  affected  the  test  as  though  they  were  true  indigotin  it  ceased 
to  command  much  confidence. 

Longer  experience  and  the  discovery  of  methods  for  purifying 
the  raw  material  have,  however,  overcome  the  former  difficulty  to 
a  great  extent,  and  C.  Raws  on*  has  contributed  to  various 
journals  improved  permanganate  methods.  In  the  first  com- 
munication the  oxidation  of  sulphindigotic  acid  by  permanganate 
is  described  as  follows  : — 

METHOD  OF  PROCEDURE  :  To  obtain  a  solution  of  sulphindigotic  acid,  1  gm. 
of  finely-powdered  indigo  is  intimately  mixed  in  a  small  mortar  with  its  own 
weight  of  ground  glass.  The  mixture  is  gradually  and  carefully  added  during 
constant  stirring  with  a  glass  rod  to  20  c.c.  of  concentrated  H2S04  (sp.  gr.  1-845) 
contained  in  a  cylindrical  porcelain  crucible  (cap.  50  c.c.)  ;  the  mortar  is  rinsed 
out  with  a  little  powdered  glass,  which  is  added  to  the  contents  of  the  crucible, 
and  the  whole  is  exposed  in  a  steam-oven  for  a  period  of  one  hour  to  a  temperature 
of  90°  C.  The  sulphindigotic  acid  thus  formed  is  diluted  with  water  and  made 
up  to  a  litre.  The  solution  must  be  filtered,  in  order  to  separate  certain  insoluble 
impurities,  which  otherwise  would  interfere  with  the  subsequent  operations. 
50  c.c.  of  the  clear  solution  are  measured  into  a  porcelain  dish,  to  which  are  added 
250  c.c.  of  distilled  water.  To  this  diluted  liquid  a  solution  of  potassium  per- 
manganate (0*5  gm.  per  litre)  is  gradually  run  in  from  a  burette  until  the  liquid, 
which  at  first  has  a  greenish  tint,  changes  to  a  light  yellow,  the  sulphindigotic 
acid  being  converted  by  oxidation  into  a  yellow  body  named  sulphiatic  acid. 
It  would  appear  that  indigo-red  acts  upon  permanganate  in  the  same  way  as 
indigotin,  whereas  indigo-brown  is  precipitated  from  its  solution  in  strong  H2S04 
on  diluting,  and  does  not  affect  the  result ;  but  indigo-gluten  and  the  mineral 
portion  strongly  decolourize  permanganate.  As  indigo-red  cannot  be  regarded 
as  an  impurity,  the  inaccuracy  in  the  analysis  may  be  chiefly  ascribed  to  the 
gluten  and  mineral  impurities.  To  eliminate  this  source  of  error,  the  author 
makes  use  of  the  property  of  sodium  sulphindigotate  of  being  almost  insoluble 
in  solutions  of  common  salt.  The  50  c.c.  of  the  filtered  solution  of  indigo  instead 
of  being  directly  titrated  with  permanganate,  are  mixed  in  a  small  flask  with 
50  c.c.  of  water  and  32  gm.  of  common  salt.  The  liquid,  which  is  almost  saturated 
with  the  salt,  is  allowed  to  stand  for  two  hours,  when  it  is  filtered,  and  the  pre- 
cipitate washed  with  about  50  c.c.  of  a  solution  of  salt  (sp.  gr.  1  '2).  The  precipitated 
sulphindigotate  of  soda  is  dissolved  in  hot  water,  the  solution  is  cooled,  mixed  with 
1  c.c.  sulphuric  acid  and  diluted  to  300  c.c.  The  liquid  is  then  titrated  with 
potassium  permanganate  as  before.  A  small  correction  is  necessary  owing  to 
the  slight  solubility  of  the  sodium  sulphindigotate  in  the  salt  solution.  For 
OO5  gm.  of  the  indigo  sample  O'OOOS  gm.  must  be  added  to  the  amount  of  indigotin 
found. 

In  the  later  contribution,  C.  Raws  on  gives  a  new  method  for 
removal  of  impurities  from  indigo  solutions  previous  to  testing, 
which  answers^well  for  technical  purposes  and  is  described  as 
follows  : — 

When  commercial  indigo  is  dissolved  in  concentrated  sulphuric  acid  and  the 
liquid  diluted  with  water,  the  colouring  matter  remains  in  solution  as  a 
disulphonic  acid,  and  various  impurities  are  held  in  suspension.  Before  pro- 
ceeding further  with  the  testing  it  is  necessary  to  remove  the  suspended  matter, 
and  this  is  usually  done  by  filtration.  Filter-paper  abstracts  some  of  the  colouring 
matter,  and  on  this  account  the  first  portions  coming  through  are  rejected  in  the 
same  way  as  in  testing  tannins.  Some  qualities  of  filter-paper  abstract  more 
colouring  matter  than  others,  and  the  rate  of  filtration  also  causes  a  difference. 

Moreover,  some  of  the  suspended  impurities  are  in  an  exceedingly  fine  state  of 
division,  and  are  liable  to  pass  through  many  kinds  of  filter-paper,  and  thus  lead 

*  Journ.  Soc.  Dyers  and  Colourists,  1885,  74  ;  and  J.  S.  C- 1.  1899,  251. 


INDIGO.  399 

to  inaccurate  results.  In  order  to  avoid  these  sources  of  error,  a  number  of  tests 
were  made  with  solutions  where  the  impurities  were  allowed  to  subside,  but  it 
was  found  that  with  some  classes  of  indigo,  subsidence  was  not  complete  'after 
many  hours.  Various  precipitants  were  then  tried  and  barium  chloride  was 
found  to  give  most  satisfactory  results.  The  proportions  recommended  are 
as  follows : — 

METHOD  OF  PROCEDURE  :  O5  gm.  of  powdered  indigo  mixed  with  glass  powder 
is  digested  with  25  c.c.  pure  concentrated  sulphuric  acid  at  a  temperature  of 
70°  C.  for  an  hour.  When  cold,  the  liquid  is  diluted  with  water,  mixed  with 
10  c.c.  of  a  20  per  cent,  solution  of  barium  chloride  and  made  up  to  500  c.c.  In 
15  to  20  minutes  the  barium  sulphate  formed,  carrying  down  with  it  all  suspended 
impurities,  will  have  settled,  and  the  requisite  amount  of  perfectly  clear  solution 
may  be  withdrawn  by  a  pipette  for  titration.  By  this  means  not  only  are  the 
results  more  concordant,  but  the  solution  is  clearer,  than  when  filter-paper  is 
used.  In  fact,  the  results  thus  obtained  are  practically  the  same  as  those  given 
by  "  salting  out."  Tests  made  with  pure  indigotin  show  that  no  colouring  matter 
is  precipitated  by  the  barium  chloride. 

Raws  on  lays  special  stress  on  the  importance  of  using  pure 
sulphuric  acid  for  dissolving  the  indigo.  It  should  contain  not  less 
than  97  per  cent,  of  H2SO4,  and  should  be  quite  free  from  nitrogen 
acids  and  sulphurous  acid. 

With  indigo  containing  more  than  1  or  2  per  cent,  of  indirubin 
(or  red  indigo),  the  ordinary  methods  of  analysis  suitable  for 
determining  indigotin  are  not  applicable.  Very  good  results  may 
be  obtained  by  a  colorimetric  method.  For  this  purpose  the 
following  is  recommended  : — 

From  O'l  to  0-25  gm.  of  the  finely  powdered  sample  is  boiled  with  about 
150  c.c.  of  ether  for  half  an  hour  in  a  flask  attached  to  an  inverted  condenser. 
When  cold,  the  solution  is  made  up  to  200  c.c.  with  ether  and  mixed  with  10  c.c. 
of  water  in  a  bottle.  Shaking  up  with  a  little  water  causes  the  suspended 
particles  of  indigo  to  settle  immediately,  and  a  clear  solution  of  indirubin  is  at 
once  obtained  without  filtering.  A  measured  quantity  of  the  solution  is  with- 
drawn and  compared  in  a  colorimeter  with  a  standard  solution  of  indirubin. 
The  proportion  of  ether  recommended  may  seem  large,  but  although  pure  indirubin 
is  freely  soluble  in  ether,  it  is  by  no  means  readily  extracted  from  indigo. 

For  the  determination  of  indigotin  in  indigo  rich  in  indirubin,  it 
is  advisable  to  boil  up  repeatedly  with  alcohol,  and  collect  on  an 
asbestos  filter. 

Indirubin  may  also  be  conveniently  removed  by  boiling  with 
glacial  acetic  acid,  as  recommended  byW.  F.  Kopperschaar. 

In  view  of  the  difficulties  attending  the  separation  of  pure 
indigotin  and  indirubin  from  the  other  constituents  of  indigo,  and 
the  possible  presence  of  substances  similar  to  the  yellow  body 
described,*  perhaps  the  best  general  commercial  method  of 
examination  will  be  found  to  be  one  based  on  colorimetry.  For 

*  In  this  second  paper  R  a  w  s  o  n  describes  the  existence  of  a  yellow  compound 
foxind  in  Java  indigo,  amounting  in  some  cases  to  20  per  cent.,  and  the  existence 
of  which  interferes  with  any  of  the  ordinary  technical  processes  of  analysis  used  for 
indigo.  It  may  be  discovered  by  adding  a  solution  of  caustic  soda  or  ammonia  to 
the  powdered  indigo  in  a  white  basin  or  on  filter-paper.  If  present  the  alkali  produces 
a  deep  yellow  colour.  In  cases  where  this  occurs  it  must  be  removed  by  boiling 
the  weighed  sample  of  indigo  in  alcohol  and  the  indigo  collected  on  an  asbestos 
niter,  washed  with  alcohol  and  dried  before  being  converted  into  sulphindigotic 
acid.  It  must  be  borne  in  mind,  however,  that  the  boiling  with  alcohol  also  removes 
indirubin. 


400  INDIGO. 

this  purpose,  in  order  that  indigotin  and  indirubin  may  be 
determined  at  the  same  time,  a  good  and  delicate  colorimeter  in 
conjunction  with  Lovibond's  tintometer  is  a  desideratum. 
The  relation  between  the  standard  permanganate  used  and  indigotin 
is  best  established  upon  the  purest  indigotin  obtainable. 

A  much  more  troublesome  method,  but  one  which  is  believed  to 
give  the  most  accurate  results,  is  one  originated  by  Miiller  and 
further  improved  by  Bernthsen.*  The  apparatus  used  is 
complicated,  and  is  practically  on  the  same  principle  as  that 
described  for  determining  oxygen  in  waters  and  figured  here  on 
page  293.  A  somewhat  simpler  arrangement  for  indigo  is  described 
by  B.  W.  Gerland,f  it  is,  in  fact,  the  same  apparatus  as  was  used 
byTiemann  and  PreussJ  for  determination  of  oxygen  in  waters, 
but  even  with  this  method  commercial  indigo  cannot  be  successfully 
tested  without  previous  troublesome  purification,  and  is  therefore 
hardly  a  suitable  substance  for  technical  examinations. 


OILS,    FATS,    AND    WAXES. 

UNDER  the  terms  oils,  fats,  and  waxes  (liquid  and  solid)  are 
comprised  all  those  naturally-formed  substances,  derived  from 
both  the  vegetable  and  the  animal  kingdoms,  which  consist  mostly 
of  glyceryl  or  other  esters  of  the  higher  members  of  the  several 
series  of  fatty  or  aliphatic  acids,  with  which  in  some  cases  notable 
amounts  of  the  free  fatty  acids  and  of  the  free  alcohols  themselves 
are  admixed.  The  term  wax  is  also  used  to  designate  certain 
solid  hydrocarbons,  the  mineral  waxes  (e.g.,  paraffin  wax,  ozokerite). 

In  the  examination  of  the  above  as  to  their  identity  and  for  the 
detection  of  adulteration,  valuable  aid  is  lent  by  physical  methods 
of  examination,  such  as  specific  gravity,  refractive  index,  rotatory 
power,  etc.,  for  an  account  of  which  other  works  must  be  consulted. 
The  chemical  methods  of  examination  are  based  on  the  determination 
of  "  values,"  which  furnish  a  measure  of  the  quantity  of  the  acids, 
alcohols,  etc.,  present  in  the  sample  examined,  without,  however, 
fixing  their  absolute  quantity.  In  order  to  secure  comparable 
results,  it  is  absolutely  essential  to  adhere  strictly  to  the  minutest 
details  as  to  the  preparation  of  reagents  and  manipulation  of 
experiment  prescribed  for  each  determination.  The  most  important 
"  values  "  are  the  following  : — 

(1)  Acid  value. 

(2)  Saponification  orKottstorfer  value. 

(3)  Reichert    (Reichert-Meissl    or     Reichert-Wollny) 
value. 

(4)  Acetyl  value. 

(5)  (Bromine  or)  Iodine  value. 

In  addition  to  the  above,  the  percentage  of  insoluble  fatty  acids 

*  BericMe  18.  2277.  f  J.  S.  C.  I.  189G,  15.  {  Berichte  12, 1768. 


OILS,    FATS,    AND    WAXES.  401 

is  sometimes  referred  to  as  the  "Hehner  value."  It  should  be 
noted,  however,  that  the  insoluble  fatty  acids  obtained  from  oils 
and  fats  after  saponification  contain  varying  amounts  of  un- 
saponifiable  matter.  Hence  the  term  "Hehner  value"  comprises 
insoluble  fatty  acids  +  unsaponifiable  matter.  The  "Polenske 
value  "  will  be  referred  to  in  connection  with  the  Reichert  value. 

The  Acid  Value.  —  This  is  determined  by  the  number  of  milligrams 
of  potassium  hydrate  (KOH)  required  to  saturate  the  free  fatty 
acids  in  one  gram  of  oil,  fat,  or  wax. 

The  standard  alkali  used  in  this  process  may  be  of  N/2,  N/5,  or 
N/10  strength,  according  to  the  nature  and  amount  of  fat,  and  may 
be  either  in  aqueous  or  alcoholic  solution,  and  the  indicator  is 
preferably  phenolphthalein.  The  sample  may  be  dissolved  in  pure 
alcohol,  methyl  alcohol,  purified  methylated  spirit,  or  a  mixture  of 
alcohol  and  ether  ;  but  whatever  solvent  is  used  it  should  be  tested 
for  acidity,  and  if  any  is  present  it  is  best  neutralized  exactly  with 
N/10  alkali. 

Liquid  fats,  say  about  10  gm.,  are  weighed  into  a  flask,  and 
about  50  c.c.  of  the  neutral  solvent,  with  a  few  drops  of  indicator, 
-  added.     The  titration  is  then  made  with  constant  shaking,   the 
alkali  solution  being  run  in  slowly. 

The  first  appearance  of  a  pink  colour  is  accepted  as  the  end  ; 
otherwise  by  standing  a  little  time  the  colour  may  disappear, 
owing  to  saponification  of  neutral  esters.  Solid  fats  or  waxes 
should  be  heated  on  a  water-bath  until  the  solvent  boils,  then  at 
once  titrated. 

In  some  substances  alcohol  alone  will  not  give  a  clear  solution 
(which  does  not  really  matter),  but  if  a  clear  solution  is  desired 
a  mixture  of  ether  and  alcohol  may  be  used  and  the  titration  made 
with  alcoholic  alkali.  The  number  of  c.c.  of  standard  potash  used 
taken  in  milligrams  of  KOH  will  give  the  calculation  for  acid  value. 

Lewkowitsch  mentions  that  the  free  acid  is  sometimes  calcu- 
lated into  percentage  of  oleic  acid  (mol.  wt.  282-27),  in  which  case 
the  value  will  be  obtained  by  multiplying  the  number  of  c.c.  of  N/10 
alkali  used  by  0-0282,  dividing  by  the  weight  of  sample  and 
multiplying  by  100.  In  other  cases,  such  as  lubricating  oils,  the 
free  fatty  acids  are  sometimes  calculated  as  SO3,  in  which  case  the 
factor  will  of  course  be  0-004. 

Kottstorferon  the  other  hand  records  the  "degrees  of  acidity" 
by  the  number  of  c.c.  of  N/j  KOH  required  by  100  gm.  of  the  fat. 

EXAMPLE.—  3-254  gm.  of  tallow  treated  as  above  required  3-5  c.c.  of  N/iO  KOH 

or  3-5  x5-61  mgm.  KOH.     Hence  acid  value  =3  5  *  ^'61  =6-03. 

' 


Saponification  or  Kottstorfer  value.  —  This  indicates  the 
number  of  milligrams  of  potassium  hydroxide  required  for  the 
complete  saponification  of  one  gram  of  a  fat  or  wax.  It  expresses 
the  amount  of  potassium  hydroxide,  in  tenths  per  cent.,  required 
to  neutralize  the  total  fatty  acids  in  1  gram  of  a  fat  or  wax. 

2  D 


402  SAPONIFICATION   VALUE. 

The  solutions  required  are  : — 

Standard  hydrochloric  acid. — Semi-normal  strength,  i.e.,  18-23 
grams  per  litre. 

Standard  solution  of  caustic  potash  in  alcohol. — This  should 
contain  about  30  gm.  of  KOH  per  litre.  Methylated  spirit,* 
previously  digested  with  permanganate,  a  little  dry  calcium 
chloride  afterwards  added,  then  distilled,  rejecting  the  first 
portions,  may  be  used  in  place  of  pure  alcohol.  In  any  case  the 
strength  should  not  be  less  than  90  per  cent.,  and  the  solution  should 
be  made  from  alcohol  which  will  not  give  a  yellow  colour  after 
being  boiled  with  very  strong  solution  of  caustic  potash  and  left 
standing  for  half  an  hour.  As  the  solution  changes  in  strength, 
it  is  not  possible  to  rely  upon  its  being  semi-normal,  but  it  should 
be  roughly  adjusted  at  about  that  strength  with  absolutely  accurate 
hydrochloric  acid,  and  a  blank  experiment  made  side  by  side  with 
each  titration  of  fat.  It  is  best  kept  in  the  dark.  The  excess  of 
potash  used  in  the  fat  titration  is  thus  expressed  in  terms  of  N/2 
acid,  and  to  arrive  at  the  percentage  of  potash,  each  c.c.  is 
multiplied  by  0-02805.  The  "  saponification  equivalent "  of  the 
fat  or  oil  is  found  by  dividing  the  weight  in  milligrams  of  the  sample 
by  the  number  of  c.c.  of  normal  (not  N/2)  acid  corresponding  to 
the  alkali  neutralized  by  the  oil.  If  the  percentage  of  potash  is 
known,  the  saponification  equivalent  may  be  found  by  dividing 
this  percentage  into  5611,  or  if  NaHO  is  the  alkali  used,  into  4001. 

METHOD  OF  PROCEDURE  :  From  1  '5  to  2  gm.  of  the  fat,  previously  purified 
by  melting  and  filtration,  are  carefully  weighed  into  a  Jena  or  other  good  glass 
flask  fitted  with  a  long  cooling  tube  or  an  inverted  condenser.  25  c.c.  of  standard 
alcoholic  potash  are  then  added,  the  mixture  heated  on  the  water-bath  to  gentle 
boiling,  with  occasional  agitation,  until  a  perfectly  clear  solution  is  obtained. 
Kottstorfer  recommends  heating  for  fifteen  minutes;  but  in  the  case  of  butters 
this  is  generally  more  than  sufficient ;  with  other  fats  twenty  minutes  to  half  an 
hour  may  be  required.  At  the  end  of  the  saponification  the  flasks  are  removed  from 
the  bath,  a  definite  (not  too  small)  quantity  of  phenolphthalein  added,  and 
the  excess  of  potash  titrated  back  with  N/2  hydrochloric  acid  with  as  little  exposure 
to  the  air  as  is  possible. 

EXAMPLE. — 1-532  gm.  of  olive  oil  were  saponified  with  25  c.c.  of  alcoholic  potash, 
and  12-0  c.c.  of  N/2  hydrochloric  acid  were  required  to  titrate  back.  Another 
25  o.c.  of  alcoholic  potash  measured  out  at  the  same  time  required  for  the  blank 
test  22-5  c.c.  of  the  standard  acid. 

Hence,  the  amount  of  potash  used  for  saponification  was 
(22-5-12-0)  xO-02805  gm.  =294-5  mgm.  KOH  for 

1  -532  gm.  fat. 
or  for  1  gm.  of  fat 

^1=192-2  mgm.  KOH. 

The  method  of  calculation  adopted  by  Kottstorfer  is  to  ascertain 
the  number  of  milligrams  of  KHQ  required  to  saturate  the  acids 
contained  in  1  gm.  of  fat,  or,  in  other  words,  parts  per  1000.  He 
found  that,  operating  in  this  way,  pure  butters  required  from  221-5 
to  232-4  mgm.  of  KHO  for  1  gm.,  whereas  the  fats  usually  mixed 

*  "  Industrial "  methylated  spirit  is  the  most  suitable  for  this  purpose,  as  it 
remains  clear  on  dilution  with  distilled  water. 


OILS,    FATS,    AND    WAXES. 


403 


Linseed 
Cotton  Seed 
Whale 
Seal      . 
Rape  (Colza) 
Cod  Liver  Oil 
Castor 
Sperm  . 
Shark  Liver  . 


192—195 
193—195 
188—194 
189—196 
170—179 
171—189 
183—186 
123—147 
161 


with  butter,  such  as  beef,  mutton,  and  pork  fat,  required  a  maximum 
of  197  mgm.  for  1  gm.,  and  other  oils  and  fats  much  less. 

Practically  this  means  that  the  amount  of  KHO  required  for 
genuine  butters  ranges  from  23-24  to  22-15  per  cent.,  the  latter  being 
the  inferior  limit.  If  caustic  soda  is  used  instead  of  potash,  other 
numbers  must  of  course  be  used. 

The  following  list  shows  the  parts  of  KHO  required  per  1000  of 
fat  ;  the  first  four  being  calculated  from  their  known  equivalents, 
the  rest  obtained  experimentally  by  Kottstorfer,  Allen, 
Stoddart,  orArchbutt : — 

Tripalmitin  208-8 

Tristearin  189-1 

Triolein  190-4 

Tributyrin  557 '3 

Cocoamit  Oil  246—260 

Dripping  197'0 

Lard     .  195-4 

Horse  Fat  195—197 

Lard  Oil  191—196 

Olive  Oil  185—196 

Niger  Oil  190-2 

A  further  application  of  this  method  may  be  made  in  determining 
separately  the  amounts  of  alkali  required  for  saturating  the  free 
fatty  acids  and  saponifying  the  neutral  glycerides  or  other  esters 
of  any  given  sample  of  fat,  oil,  or  wax  (see  Allen,  Organic  Analysis 
ii.  45,  76,  also  Lewkowitsch,  4th  edit.,  vol.  I.,  p.  301). 

The  Ester  Value. — This  indicates  the  number  of  milligrams  of 
KOH  required  for  the  saponification  of  the  neutral  esters  in  one 
gram  of  a  fat  or  wax. 

Where  the  fat  contains  no  free  fatty  acids  the  ester  value  is  the 
same  as  the  previously  mentioned  saponification  value,  but  as  many 
fats  or  waxes  do  contain  small  quantities  of  free  fatty  acids  the 
saponification  value  includes  both,  and  therefore  the  ester  value  is 
the  difference  between  the  saponification  and  the  acid  value. 

The  Reichert  (Reicher  t-M  eissl,  Reichert- 
W  o  1 1  n  y)  value. — This  indicates  the  number  of  cubic  centimetres 
of  N/10  KOH  required  for  the  neutralization  of  that  portion  of  the 
soluble  volatile  fatty  acids  which  is  obtained  from  2*5  (or  5)  grams 
of  a  fat  or  wax  by  the  Reichert  distillation  process. 

Reichert  originally  used  2-5  gm.  of  substance,  but  M eissl 
suggested  that  5  gm.  would  be  a  more  convenient  quantity,  and 
this  is  the  amount  now  generally  used.  Wollny,  in  his  modifi- 
cation of  the  Reichert  process,  also  uses  5  gm.  of  substance.  It  is 
important  to  note  that  this  process  and  its  modifications  do  not  yield 
absolute  values  ;  they  merely  give  a  measure  of  the  total  volatile 
acids  present  in  an  oil,  fat  or  wax.  For  purposes  of  comparison, 
especially  in  the  examination  of  butter  fat,  the  relative  numbers 
thus  obtained  are  of  great  value.  The  numbers  given  by  the 
M  eissl  and  Wollny  modifications  are  not  necessarily  twice  the 

2  D  2 


404 


BUTTER. 


Rei chert  value.  In  the  case  of  butter  fat,  however,  it  is  quite 
admissible  to  work  with  2*5  gm.  and  to  multiply  the  result 
by  2-2  in  order  to  obtain  numbers  comparable  with  those  found 
by  the  Reichert-Meissl  or  the  Reichert-Wollny  process. 
On  the  other  hand  the  quantity  of  5  gm.  must  be  rigorously  adhered 
to  in  the  case  of  cocoanut  oil  and  palm  kernel  oil. 

The  description  of  the  process  as  used  for  butter  will  practically 
apply  to  other  fatty  matters. 


BUTTER. 

The  Reichert  or  Reicher t-M e i s s  1  Method. — This  process 
consists  in  saponifying  the  fat  to  be  examined  by  an  alkali,  separating 
the  fatty  acids  by  neutralizing  the  alkali,  and  distilling  off  the 
volatile  acids  (chiefly  butyric  and  caproic)  for  titration  with  standard 
acid.  In  this  and  Kottstorfer's  method,  where  also  alcoholic 
solution  of  caustic  alkali  is  used,  it  is  essential  to  avoid  absorption 
of  C02  by  long  exposure. 

The  necessary  solutions  are  : — 

1.  Standard  barium  or  potassium  hydrate.  N/10  strength  is  most 
convenient,  but  any  solution  approximating  to  that  strength  may 
be  used,  and  a  factor  found  to  convert  it  to  that  strength  in 
calculating  the  results  of  titration.     It  must  be  carefully  preserved 
from  C02  by  any  of  the  usual  arrangements,  and  where  a  constant 
series  of  titrations  are  carried  on,  it  is  best  to  have  a  store  bottle 
and  burette  fitted,  as  shown  p.  12,  fig.  11. 

2.  Phenolphthalein  indicator,  see  p.  39. 

3.  Alcohol  of  about  90  per  cent,  strength,  and  free  from  acid  or 
aldehyde. 


Fig.  56. 

4.     Solution  of  caustic  soda.     Made  by  dissolving  100  gm.  of 
good  sodium  hydrate  in  100  c.c.  of  distilled  water  which  has  been 


REICHERT-MEISSL   METHOD.  405 

recently  well  boiled  and  cooled  ;  this  solution  will  not  be  con- 
taminated with  C02  to  any  extent,  since  any  Na2CO3  which  might  be 
formed  is  quite  insoluble  in  the  strong  solution  ;  it  must  be  allowed 
to  stand  until  quite  clear,  then  poured  off  and  well  preserved.  Or 
better  than  this,  about  2  gm.  of  solid  stick  potash  or  soda  may  be 
added  with  50  c.c.  of  70  per  cent,  alcohol  to  5  gm.  of  butter  when 
commencing  saponification. 

5.  Sulphuric   acid  for  separating  the  fatty  acids  is  made  by 
diluting  25  c.c.  of  strongest  H2SO4  to  a  litre  with  water. 

6.  The  apparatus  for  digestion  and  distillation  are  shown  in 
fig.  56,  the  same  Erlenmeyer  flask  being  used  for  the  digestion 
and  for  the  distillation.     The  distilled  liquid  drops  into  a  small 
funnel  containing  a  small  porous  filter  for  separating  any  scum 
which  may  pass  over  with  the  distillate  ;  the  receiver  holding  the 
funnel  is  marked  at  50  c.c.  and  100  c.c.,  so  as  to  be  available  for 
either  2 -5  gm.  or  5  gm.  of  butter  fat. 

The  following  method  of  manipulation  as  drawn  up  by  the 
Association  of  Official  Agricultural  Chemists,  U.S.A.,  is  recom- 
mended as  being  all  that  is  required  to  ensure  accuracy,  and  applies 
to  the  treatment  of  approximately  5  gm.  of  fat  for  each  operation. 
Many  operators  prefer  to  take  about  half  that  quantity,  which  saves 
time,  and  need  not  be  any  the  less  accurate. 

PROCESS  :  WEIGHING  THE  FAT  :  The  butter  or  fat  to  be  examined  should  be 
melted  and  kept  in  a  dry,  warm  place  at  about  60°  C.  for  two  or  three  hours  until 
the  moisture  and  curd  have  entirely  settled  out.  The  clean  supernatant  fat  is 
poured  off  and  filtered  through  a  dry  filter-paper  in  a  jacketed  filter  containing 
boiling  water,  to  remove  all  foreign  matter  and  any  traces  of  moisture.  Should 
the  filtered  fat  in  a  fused  state  not  be  perfectly  clear  the  treatment  above  mentioned 
must  be  repeated. 

The  saponification  flasks  are  prepared  by  having  them  thoroughly  washed  with 
water,  alcohol,  and  ether,  wiped  perfectly  dry  on  the  outside,  and  heated  for  one 
hour  to  100°  C.  The  flasks  should  then  be  placed  in  a  tray  by  the  side  of  the 
balance  and  covered  with  a  silk  handkerchief  until  they  are  perfectly  cool.  They 
must  not  be  wiped  with  a  silk  handkerchief  within  fifteen  or  twenty  minutes  of 
the  time  they  are  weighed.  The  weight  of  each  flask  is  determined  accurately, 
using  a  flask  for  a  counterbalance  or  not,  as  may  be  convenient.  The  weight  of 
the  flasks  having  been  accurately  determined  they  are  charged  with  the  melted 
fat  in  the  following  way  : — 

A  pipette  with  a  long  stem  marked  to  deliver  5'75  c.c.  is  warmed  to 
a  temperature  of  about  50°  C.  The  fat  having  been  poured  back  and  forth  once 
or  twice  into  a  dry  beaker  in  order  to  thoroughly  mix  it,  it  is  taken  up  in  the 
pipette,  the  nozzle  of  the  pipette  carried  to  near  the  bottom  of  the  flask,  it  having 
been  previously  wiped  to  remove  any  adhering  fat.  The  5'75  c.c.  of  fat  are 
allowed  to  flow  into  the  flask  and  the  pipette  is  removed.  After  the  flasks  have 
been  charged  in  this  way  they  should  be  re-covered  with  the  silk  handkerchief 
and  allowed  to  stand  fifteen  or  twenty  minutes,  when  they  are  again  weighed  to 
ascertain  the  exact  amount  of  fat. 

THE  SAPONIFICATION  :  10  c.c.  of  90  per  cent,  alcohol  are  added  to  the  fat  in 
the  flask,  2  c.c.  of  the  concentrated  soda  solution  or  2  gm.  of  solid  alkali  are  added, 
a  soft  cork  stopper  inserted  in  the  flask  and  tied  down  with  a  piece  of  twine.  The 
saponification  is  then  completed  by  placing  the  flasks  upon  the  water  or  steam 
bath.  The  flasks  during  the  saponification,  which  should  last  for  one  hour,  should 
be  gently  rotated  from  time  to  time,  being  careful  not  to  project  the  soap  for 
any  distance  up  the  sides  of  the  flask.  At  the  end  of  an  hour  the  flasks,  after  having 
been  cooled  to  near  the  room  temperature,  are  opened.  If  solid  alkali  is  used 


406  BUTTER. 

instead  of  aqueous  solution,  alcohol  of  75  or  80  per  cent,  in  larger  quantity  may 
be  used. 

REMOVAL  OF  THE  ALCOHOL  :  The  stoppers  having  been  laid  loosely  in  the  mouth 
of  the  flasks,  the  alcohol  is  removed  by  dipping  the  flasks  into  a  steam  bath.  The 
steam  should  cover  the  whole  of  the  flask  except  the  neck.  After  the  alcohol  is 
nearly  removed,  frothing  may  be  noticed  in  the  soap,  and  to  avoid  any  loss  from 
this  cause,  or  any  creeping  of  the  soap  up  the  sides  of  the  flask,  it  should  be  taken 
from  the  bath  and  shaken  to  and  fro  until  the  frothing  disappears.  The  last 
traces  of  alcohol  vapour  may  be  removed  from  the  flask  by  waving  it  briskly, 
mouth  down,  to  and  fro.  Complete  removal  of  the  alcohol  with  the  precautions 
above  noted  'should  take  about  forty-five  minutes. 

DISSOLVING  THE  SOAP  :  After  the  removal  of  the  alcohol  the  soap  should  be 
dissolved  by  adding  100  c.c.  of  recently  boiled  distilled  water,  and  warmed  on  the 
steam  bath  with  occasional  shaking  until  the  soap  is  completely  dissolved. 

SETTING  FREE  THE  FATTY  ACIDS  :  When  the  soap  solution  has  cooled  to  about 
60°  or  70°  C.,  the  fatty  acids  are  separated  by  adding  40  c.c.  of  the  dilute  sulphuric 
acid  mentioned  above. 

MELTING  THE  FATTY  ACIDS  :  The  flasks  should  now  be  re-stoppered  as  in  the 
first  instance,  and  the  fatty  acids  melted  by  replacing  the  flasks  on  the  steam 
bath.  According  to  the  nature  of  the  fat  examined  the  time  required  for  the 
fusion  of  the  fatty  acids  may  vary  from  a  few  minutes  to  hours. 

THE  DISTILLATION  :  After  the  fatty  acids  are  completely  melted,  which  can 
be  determined  by  their  forming  a  transparent  oily  layer  on  the  surface  of  the 
water,  the  flasks  are  cooled  to  room  temperature  and  a  few  pieces  of  pumice  stone 
added.  The  pumice  stone  is  prepared  by  throwing  it,  at  white  heat,  into  distilled 
water,  and  keeping  it  under  water  until  used.  The  flask  is  now  connected  with 
the  condenser,  slowly  heated  with  a  naked  flame  until  ebullition  begins,  and  then 
the  distillation  continued  by  regulating  the  flame  in  such  a  way  as  to  collect 
100  e.c.  of  the  distillate  in  as  nearly  as  possible  thirty  minutes. 

Some  operators  distil  110  c.c.  from  5  gm.  of  butter  into  an  ordinary 
measuring  flask,  then  filter  and  use  100  c.c.  for  titration,  the  number 
of  c.c.  of  alkali  used  is  multiplied  by  1*1  which  gives  the  Reichert- 
Meissl  value. 

The  above  methods  of  preparation  are  somewhat  tedious,  but 
experienced  operators  will  find  methods  of  working  so*  as  to  occupy 
less  time  without  loss  of  accuracy. 

TITRATION  OF  THE  VOLATILE  ACIDS  :  The  100  c.c.  of  the  filtered  distillate 
are  poured  into  a  beaker  holding  from  200-250  c.c.,  0-5  c.c.  of  phenolphthalein 
solution  added,  and  decinormai  barium  or  potassium  hydrate  run  in  until  a  red 
colour  is  produced.  The  contents  of  the  beaker  are  then  returned  to  the 
measuring  flask  to  remove  any  acid  remaining  therein,  poured  again  into  the 
beaker,  and  the  titration  continued  until  the  red  colour  produced  remains 
apparently  unchanged  for  two  or  three  minutes. 

Where  the  greatest  accuracy  is  required  it  is  best  to  carry  out 
side  by  side  a  blank  experiment  with  the  same  amounts  of  alcohol, 
alkali,  etc. 

It  must  be  borne  in  mind  that  this  method  yields  only  a  portion 
of  the  volatile  fatty  acids,  but  the  experience  of  the  author  and 
a  host  of  other  very  competent  operators  clearly  shows  that  the 
distillate  from  5  gm.  of  genuine  normal  butter  fat  produced  in 
districts  of  medium  temperature,  when  carried  out  as  described, 
should  require  not  less  than  24  c.c.  of  N/10  alkali  to  neutralize  the 
volatile  acids  present.  It  is  true  that  butters  known  to  be  genuine 
have  occasionally  been  found  to  give  lower  figures  from  some 


REICHERT-WOLLNY   METHOD. 


407 


unexplained  causes,  one  of  which  seems  to  be  due  to  milk  taken 
from  cows  towards  the  end  of  their  period  of  lactation.  The  figure 
may  also  rise  to  32  or  33  c.c.  of  alkali.  This  is  often  the  case  with 
butters  produced  in  warmer  climates  than  Great  Britain.  The 
general  average  for  butters  taken  from  the  mixed  milk  of  a  number 
of  cows  will  be  between  27  and  28  c.c.,  whereas  margarine  (except 
when  consisting  largely  of  cocoanut  oil)  will  rarely  require  more 
than  0-5  c.c.,  beef  fat  and  lard  about  the  same,  while  cocoanut 
oil,  which  gives  the  highest  figures,  requires  about  7  c.c. 

It  may,  therefore,  be  concluded  that  any  sample  of  butter  fat 
which  requires  less  than  24  c.c.  of  N/10  alkali  must  be  looked  upon 
with  suspicion. 

The  minimum  value  recommended  in  Great  Britain,  France,  and 
Germany,  is  24  ;  Sweden,  23  ;  and  Italy,  20. 

A  Joint  Committee  of  the  Principal  of  the  Government  Laboratory 
and  the  Society  of  Public  Analysts,  adopted  in  1900*  the  Reichert- 
Wollny  method  as  the  official  method  for  determining  the  per- 
centage of  butter-fat  in  margarine,  the  object  in  view  being  to  avoid 
discrepancies  which  might  arise  through  the  employment  of  different 
methods  by  individual  analysts.  The  following  is  the  official 
description  of  the  method  : — 


Fig.  57. 

The  Reichert-Wollny  method  for  determination  of  volatile 
fatty  acids  in  Margarine  and  Butter. — "  Five  gm.  of  the  liquid  fat 
are  introduced  into  a  300  c.c.  flask,  of  the  form  seen  in  the  figure 
(length  of  neck  7  to  8  centimetres,  width  of  neck  2  centimetres). 
Two  c.c.  of  a  solution  of  caustic  soda  (98  per  cent.)  in  an  equal 

*  See  Analyst,  1900,  309. 


408  BUTTER. 

weight  of  water — preserved  from  the  action  of  atmospheric  carbonic 
acid — and  10  c.c.  of  alcohol  (about  92  per  cent.)  are  added,  and  the 
mixture  is  heated  under  a  reflux  condenser,  connected  with  the  flask 
by  a  T-piece,  for  fifteen  minutes  in  a  bath  containing  boiling  water. 
The  alcohol  is  distilled  off  by  heating  the  flask  on  the  water-bath 
for  about  half  an  hour,  or  until  the  soap  is  dry.  One  hundred  c.c. 
of  hot  water,  which  have  been  kept  boiling  for  at  least  ten  minutes, 
are  added,  and  the  flask  heated  until  the  soap  is  dissolved.  Forty 
c.c.  of  normal  sulphuric  acid  and  three  or  four  fragments  of  pumice 
or  broken  pipe-stems  are  added,  and  the  flask  is  at  once  connected 
with  a  condenser  by  means  of  a  glass  tube  7  millimetres  wide  and 
15  centimetres  from  the  top  of  the  cork  to  the  bend.  At  a  distance 
of  5  centimetres  above  the  cork  is  a  bulb  5  centimetres  in  diameter. 
The  flask  is  supported  on  a  circular  piece  of  asbestos  12  centimetres 
in  diameter,  having  a  hole  in  the  centre  5  centimetres  in  diameter, 
and  is  first  heated  by  a  very  small  flame,  to  fuse  the  insoluble  fatty 
acids,  but  the  heat  must  not  be  sufficient  to  cause  the  liquid  to  boil. 
The  heat  is  increased,  and  when  fusion  is  complete  110  c.c.  are 
distilled  off  into  a  graduated  flask,  the  distillation  lasting  about 
thirty  minutes  (say  from  twenty-eight  to  thirty- two  minutes),  the 
distillate  is  shaken,  100  c.c.  filtered  off,  transferred  to  a  beaker, 
0*5  c.c.  of  phenolphthalein  solution  (1  gm.  in  100  c.c.  alcohol)  added, 
and  the  nitrate  titrated  with  decinormal  soda  or  baryta  solution. 
Precisely  the  same  procedure  (with  the  same  reagents),  omitting 
the  fat,  should  be  followed,  and  the  amount  of  decinormal  alkali 
required  to  neutralize  the  distillate  ascertained.  This  should  not 
exceed  0-3  c.c.  The  volume  of  decinormal  solution  of  alkali  used, 
less  the  figure  obtained  by  blank  experiment,  is  multiplied  by  !•!. 
The  number  so  obtained  is  the  "Beichert-Wollny  Number." 

NOTES  ON  THE  METHOD  :  The  sample  is  melted  and  filtered  from  curd  and 
water  through  a  dry  filter.  From  the  filtrate  the  5  gm.  of  fat  for  the 
process  are  taken.  The  soda  solution  is  filtered  clear  from  carbonate  formed  in 
its  preparation,  and  kept  in  a  special  bottle.  The  S  o  x  h  1  e  t  spherical  condenser  is 
a  convenient  one  for  the  reflux  distillation.  This  is  fixed  near  the  water-bath 
in  which  the  saponification  is  to  take  place,  and  is  connected  with  the  flask  by 
means  of  a  T-piece  and  india-rubber  tubes  inclined  at  an  angle  of  45°.  During 
the  saponification  the  free  limb  of  the  T-piece  is  directed  upwards,  and  its  end 
closed  by  a  short  piece  of  india-rubber  and  glass  rod.  At  the  end  of  fifteen  minutes 
this  limb  is  turned  downwards,  and  the  piece  of  glass  rod,  replaced  by  a  tube 
carrying  away  the  alcohol. 

One  hundred  c.c.  of  hot  distilled  water  are  added,  and  the  flask  frequently 
shaken  until  the  soap  is  dissolved.  The  L  i  e  b  i  g  is  a  convenient  form  of  condenser. 
One  containing  a  column  of  water  30  to  35  centimetres  in  length  gives  sufficient 
condensing  surface.  After  shaking  the  distillate,  about  5  c.c.  are  filtered  through 
a  dry  paper  into  a  100  c.c.  flask.  This  serves  to  wash  out  the  flask.  When  the 
100  c.c.  are  transferred  to  a  beaker,  the  flask  is  not  washed  out,  but  the  main 
quantity  is  neutralized  with  the  standard  solution  of  alkali  and  returned  to  the 
flask,  then  again  transferred  to  the  beaker  and  the  titration  completed." 

The  somewhat  lengthy  process  of  saponification  in  the  above 
method  may  advantageously  be  replaced  by  the  following,  due  to 
Leffmann  &  Beam:* 

*  Analyst,  1891, 16.  153  ;  1892, 17,  65. 


POLENSKE    METHOD. 


409 


First,  prepare  a  solution  of  100  gm.  caustic  soda  (98-99  %  powdered  caustic 
answers  well)  in  water  and  make  up  to  200  c.c.  Put  the  solution  in  a  bottle  with 
a  rubber  stopper  and  allow  to  stand  till  quite  clear.  25  c.c.  of  the  clear  solution 
are  then  pipetted  into  125  c.c.  of  pure  glycerin  in  a  flask  and  well  mixed.  Next 
5  gm.  of  the  fat  are  placed  in  a  300  c.c.  flask,  10  c.c.  of  the  glycerol-soda  added 
and  the  flask  heated  cautiously  over  a  small  B  u  n  s  e  n  flame.  Much  foaming  takes 
place,  and  the  flask  should  be  vigorously  shaken.  Complete  saponification  takes 
place  in  less  than  five  minutes,  the  process  being  complete  when  the  foaming 
has  entirely  ceased.  The  hot  soap  is  then  dissolved  at  once  in  90  c.c.  of  water  that 
has  recently  boiled,  adding  it  drop  by  drop  at  first.  Then  40  c.c.  of  dilute  sulphuric 
acid  are  added,  together  with  a  few  pieces  of  pipe-clay,  and  the  distillation  proceeded 
with  as  above.  The  usual  blanks  are  0'2— 0'3  c.c.  of  N/±o  alkali  for  the  110  c.c. 
of  distillate. 

The  glycerol-soda  solution  should  be  kept  in  a  flask  closed  with  a  rubber  stopper 
and  measured  by  means  of  a  rather  wide  glass  tube  marked  to  deliver  10  c.c. 

Lewkowitsch*  testifies  to  the  fact  that  the  values  obtained  by  the  above 
method  are  practically  identical  with  that  obtained  by  the  R,  e  i  c  h  e  r  t- W  o  1 1  n  y 
process. 

The  water-insoluble  Volatile  Fatty  Acids.     (P  o  1  e  n  s  k  e  .) 

This  determination  is  best  made  by  Polenske's  method  which 
is  carried  out  as  follows  : — 

Saponify  5  gm.  of  the  filtered 
butter-fat  by  the  Leffmann- 
Beam|  process  described  above, 
taking  care  not  to  overheat  the 
mixture  of  fat  and  glycerol-soda. 
Allow  to  cool  below  100°  C.  before 
adding  90  c.c.  of  water,  and 
dissolve  the  mass  by  warming 
on  the  water-bath  to  about  50°  C. 
The  solution  must  be  clear  and 
almost  colourless ;  if  of  a  brown 
colour,  the  test  must  be  rejected. 
To  the  hot  soap  solution  add  40 
c.c.  of  the  dilute  sulphuric  acid 
and  0-1  gm.  of  finely  powdered 
pumice  and  attach  immediately 
to  the  condenser.  The  apparatus 
used  must  agree  in  all  details  with 
the  dimensions  given  in  fig.  57a. 
Regulate  the  heat  so  that  in 
19-20  minutes  110  c.c.  are  distilled 
over,  and  the  flow  of  water 
through  the  condenser  so  that  the 
distillate  does  not  drop  into  the 
110  c.c.  flask  at  a  higher  tempera- 
ture than  20-23°  C.  When  110 
c.c.  have  distilled  over,  remove 
the  burner  and  replace  the  flask 


Fig.  57a. 


*  Oils,  fats  and  waxes,  4th  edition,  Vol.  I..  334. 
t  Alcohol  being  inadmissible  in  this  process. 


410  POLENSKE   METHOD. 

by  a  20  c.c.  measuring  cylinder.  The  110  c.c.  flask  which  must 
not  be  shaken,  is  immersed  almost  completely  in  water  at  15°  C. 
After  five  minutes'  standing  the  neck  is  gently  tapped  so  that  the 
oily  drops  floating  on  the  surface  may  adhere  to  the  walls  of  the 
flask.  After  a  further  ten  minutes  the  consistence  of  the  insoluble 
acids  is  noted,  with  a  view  to  ascertaining  whether  they  form  a 
semi-solid  mass  or  oily  drops.  The  flask  is  then  corked  and  its 
contents  mixed  by  turning  it  upside  down  several  times,  avoiding, 
however,  any  violent  shaking.  100  c.c.  are  then  filtered  through  an 
8  cm.  dry  filter  and  titrated  withN/10  barium  or  potassium  hydroxide, 
as  in  the  R  e  i  c  h  e  r  t- W  o  1 1  n  y  process .  The  insoluble  vol  atile  acids 
on  the  filter  are  washed  three  times  in  succession  with  15  c.c.  of 
water  that  has  previously  been  passed  successively  through  the 
tube  of  the  condenser  the  20  c.c.  measuring  cylinder,  and  the  110 
c.c.  flask.  These  wash  waters  are  rejected.  The  water-insoluble 
volatile  acids  are  then  collected  by  rinsing  the  condenser,  cylinder 
and  flask  three  times  in  succession  with  15  c.c.  of  neutralized  90 
per  cent,  alcohol,  and  the  alcoholic  washings  poured  on  to  the  filter 
— each  being  allowed  to  pass  completely  through  before  the  next 
washing  is  added.  The  alcoholic  filtrate  is  then  titrated  with  N/10 
alkali.  The  figure  so  obtained  is  called  the  Polenske  value. 
For  butter  it  varies  from  1-5  to  3*0  c.c.  and  for  cocoanut  oil  from 
16-8-17-8  c.c.  In  order  to  obtain  concordant  results  with  this 
method,  it  is  absolutely  essential  to  follow  the  procedure  given 
above  in  all  its  minutest  details. 

Lewkowitsch  found  that  the  Polenske  values  of  genuine  French  and 
Finnish  butters  ranged  from  2'24 — 4'1,  and  those  of  cocoanut  oil  from  15'5 — 20'45, 
while  palm  kernel  oil  gives  values  lying  between  10  and  12.  Several  attempts  have 
been  made  to  determine  the  proportions  of  cocoanut  oil  in  mixtures  of  cocoa- 
nut  oil  and  butter  fat  by  means  of  the  Polenske  number,  but  in  view  of  the 
above  range  of  values  quantitative  results  should  be  received  with  great  caution, 
especially  with  regard  to  alleged  small  additions  of  cocoanut  oil  to  butter  fat. 
T  a  1 1  o  c  k  and  Thomson*  have  also  critically  examined  this  process  and  express 
the  opinion  that  "  the  possibility  of  the  detection  of  even  10  per  cent,  of  cocoa- 
nut  oil  in  a  butter  by  the  Polenske  method  is  very  doubtful."  But  in  the  case 
of  margarine  "the  Polesnke  number  appears  to  be  quite  reliable  within  limits 
of  say  5  per  cent."  They  obtained  results  within  3  per  cent,  of  the  truth,  where 
there  was  35  per  cent,  of  cocoanut  oil  present.  Margarine  containing  no  cocoa- 
nut  oil  gave  0'4  Reicher.t-Wollny  and  0-5  Polenske  values. 

The  Acetyl  Value. — This  indicates  the  number  of  milligrams  of 
KOH  required  for  the  neutralization  of  the  acetic  acid  obtained  on 
saponifying  1  gm.  of  an  acetylated  oil,  fat,  or  wax.  This  treatment 
of  fats  was  introduced  by  Benedikt,  and  a  process  by  himself  and 
Ulzer  for  acetylation  and  determination  was  arranged;  but  as  the 
results  were  not  consistent  with  modern  ideas,  Lewkowitschf 
modified  the  method  and  proposed  to  determine  the  true  acetyl 
value  by  actually  titrating  the  amount  of  acetic  acid 'assimilated 
by  the  hydroxylated  acid  in  the  form  of  acetyl  C2H30  and  given 
up  on  saponification  as  acetic  acid  to  the  standard  alkali. 

*  J.  S.  C.  I.  1909,  69.  f  J.  S.  C.  I.  1897,  503. 


ACETYL   VALUE.  411 

The  method  is  as  follows  : — 

METHOD  OF  PROCEDURE  :  10  gm.,  or  any  other  convenient  quantity,  are  boiled 
with  twice  the  amount  of  acetic  anhydride  for  two  hours  in  a  round-bottomed  flask 
attached  to  an  inverted  condenser.  The  solution  is  then  transferred  to  a  beaker 
of  about  1  litre  capacity ;  mixed  with  500 — 600  c.c.  <*f  boiling  water  and  heated 
for  half  an  hour,  whilst  a  slow  current  of  carbon  dioxide  is  passed  into  the  liquid 
through  a  finely  drawn  out  tube  reaching  nearly  to  the  bottom  of  the  beaker  ; 
this  is  done  to  prevent  bumping.  The  mixture  is  then  allowed  to  separate  into 
two  layers,  the  water  is  siphoned  off,  and  the  oily  layer  again  boiled  out  in  the 
same  manner  three  successive  times.  The  last  trace  of  acetic  acid  is  thus  removed 
— this  being  ascertained  by  testing  with  litmus  paper.  Prolonged  washing  beyond 
the  required  limit  causes  slight  dissociation  of  the  acetyl  product.  This  would 
lead  to  too  low  an  acetyl  value.  The  acetylated  product  is  then  filtered  through 
a  dry  filter-paper  in  a  drying  oven  to  remove  water. 

This  operation  may  be  carried  out  quantitatively,  and  in  that  case  the  washing 
is  best  done  with  boiling  water  on  a  weighed  filter.  On  weighing  the  acetylated 
oil  or  fat,  an  increase  of  weight  would  prove  that  assimilation  of  acetyl  groups 
had  taken  place.  This  method  may  be  found  useful  to  ascertain  preliminarily 
whether  a  notable  amount  of  hydroxylated  acids  is  present  in  the  sample  under 
examination. 

About  5  gm.  of  the  acetylated  product  are  then  saponified  by  means  of  alcoholic 
potash  solution  as  in  the  well-known  determination  of  the  saponification  value. 
If  the  "  distillation  process  "  be  adopted  it  is  not  necessary  to  work  with  an 
accurately  measured  quantity  of  standardized  alcoholic  potash.  In  case  the 
"filtration  process"  be  used,  the  alcoholic  potash  must  be  measured  exactly. 
(It  is,  however,  advisable  to  employ  in  either  case  a  known  volume  of  standard 
alkali,  as  one  is  then  enabled  to  determine  the  saponification  value  of  the 
acetylated  oil  or  fat).  Next,  the  alcohol  is  evaporated  and  the  soap  dissolved  in 
water.  From  this  stage  the  determination  is  carried  out  either  by  (a)  the 
"  distillation  process  "  or  (6)  the  "  filtration  process." 

(a)  DISTILLATION  PROCESS. — Add  dilute  sulphuric  acid  (1  :   10)  more  than 
is  required  to  saturate  the  potash  used,   and  distil  the  liquid  as  is  usual  in 
Reichert's  distillation  process.     Since  a  large  quantity  of  water  must  be  distilled 
off,  either  a  current  of  steam  is  blown  through  the  suspended  fatty  acids  or  water 
is  run  into  the  distilling  flask,  from  time  to  time,  through  a  stoppered  funnel  fixed 
in  the  cork,  or  any  other  convenient  device  is  adopted.     It  will  be  found  quite 
sufficient  to  distil  over  500  to  700  c.c.,  as  the  last  100  c.c.  contain  practically  no 
acid.     Then  filter  the  distillates  to  remove  any  insoluble  acids  carried  over  by 
the  steam,  and  titrate  the  filtrate  with  N/io  potash,  phenolphthalein  being  the 
indicator.      Multiply  the  number  of  c.c.  by  5*61,  and  divide  the  product  by  the 
weight  of  substance  taken.     This  gives  the  acetyl  value. 

(b)  FILTRATION    PROCESS.  —  Add    to    the    soap    solution    a    quantity    of 
standardized  sulphuric  acid  exactly  corresponding  to  the  amount  of  alcoholic 
potash  employed,  and  warm  gently,  when  the  fatty  acids  will  readily  collect  on 
the  top  as  an  oily  layer.     (If  the  saponification  value  has  been  determined,  it  is, 
of  course,  necessary  to  take  into  account  the  volume  of  acid  used  for  titrating 
back  the  excess  of  potash.)     Filter  off  the  liberated  fatty  acids,  wash  with  boiling 
water  until  the  washings  are  no  longer  acid,  and  titrate  the  filtrate  with  N/io 
potash,  using  phenolphthalein  as  indicator.      The  acetyl  value  is  calculated ,  in 
the  manner  shown  above. 

Both  methods  give  identical  results,  but  (6)  will  be  found  shorter  and  more 
convenient  than  (a). 

The  distilled  water  used  in  determining  the  value  by  either  the  distillation  or 
the  filtration  process  should  be  carefully  freed  from  CO2  by  previous  boiling, 
as  otherwise  serious  errors  may  be  made.  Even  the  water  used  for  generating 
steam  in  the  distillation  process  should  be  brought  to  violent  ebullition  before 
the  steam  is  passed  into  the  distilling  flask.  This  source  of  error  many  easily 
occur  in  the  case  of  very  hard  water.  Check  experiments  with  pure  acetic  acid 
will  readily  guide  the  operator,  if  necessary.  In  order  to  facilitate  the  separation 


412  OILS,   FATS,   AND   WAXES. 

of  the  insoluble  fatty  acids  in  the  nitration  process,  it  will  be  found  useful  to 
add  a  slight  excess  of  mineral  acid.  Of  course  this  amount  must  be  measured 
accurately,  and  deducted  from  the  alkali  required  for  determining  the  dissolved 
acids. 

A  full  discussion  as.  to  the  meaning  of  the  acetyl  value  in  fat 
analysis  will  be  found  in  a  lengthy  paper  byLewkowitsch  in  The, 
Analyst,  1899,  319. 

The  Bromine  Value.  —  This  indicates  the  percentage  of  bromine 
absorbed  by  a  fat  or  wax.  This  determination  was  proposed  by 
Cailletet  in  1857;  but  the  method  of  carrying  it  out  is  due  to 
Mills*  and  his  collaborators  Snodgrass  and  Akitt.  Mills  found 
that  it  was  of  the  utmost  importance  rigidly  to  exclude  moisture 
when  making  the  determination,  since  in  the  presence  of  water 
the  value  obtained  was  too  high.  He  dissolved  the  sample  of  dried 
and  filtered  fat  in  carbon  tetrachloride,  added  a  standard  solution 
of  bromine  in  carbon  tetrachloride  in  excess,  and  titrated  back 
the  excess  with  a  standard  solution  of  /3-naphthol  in  carbon 
tetrachloride,  when  monobromonaphthol  is  formed.  As,  however, 
this  determination  has  now  been  entirely  superseded  by  the  deter- 
mination of  Iodine  value,  the  reader  is  referred,  for  further  infor- 
mation, to  the  original  papers  on  the  subject  already  mentioned.! 

The  Iodine  Value.  —  This  indicates  the  percentage  of  iodine  chloride, 
expressed  in  terms  of  iodine,  absorbed  by  a  fat  or  wax.  "  This  value 
is  a  measure  of  the  proportion  of  unsaturated  fatty  acids,  which, 
both  in  their  free  state  and  in  combination  with  glycerol,  have 
the  property  of  assimilating  halogens  with  formation  of  additive 
compounds."  —  (Lewkowitsch.)  The  method  by  which  the  deter- 
mination was  carried  out  was  originated  by  Hiibl,J  who  proved 
that  from  an  alcoholic  solution  of  iodine,  in  the  presence  of  mercuric 
chloride,  glycerides  of  the  unsaturated  fatty  acids  absorb  iodine 
in  a  very  regular,  well-defined  manner,  when  kept  at  the  ordinary 
temperature,  so  that  quantitative  results  can  be  obtained. 

The  following  solutions  are  required  for  Hiibl's  process  :  — 

1.  Standard    iodine    solution.  —  This    is    made    by    dissolving 
respectively  5  gm.  of  iodine  and  6  gm.  of  mercuric  chloride,  each 
as  pure  as  possible,  in  separate  portions  of  100  c.c.  each  of  95  % 
alcohol,  then  mixing  the  two  liquids,  and  allowing  to  stand  for  12  to 
24  hours  before  use.     This  solution  must  always  be  standardized 
at  the  time  of  using,  and  it  is  advisable  not  to  mix  a  large  quantity 
unless  it  is  required  for  immediate  use. 

2.  Solution  of  Sodium  Thiosulphate.  —  Prepared  by  dissolving 
about  24  grams  of  the  crystallized  salt  in  a  litre  of  water.     It  is 
standardized  either  by  dissolving  about  0-25  gram  of  re-sublimed 
iodine,  most  accurately  weighed,  in  potassium  iodide  solution  and 

*  J.  S.  C.  I.  1883,  435  ;  1884,  366. 

?r(X^sl  fo£    determining    "  the    bromine    addition  "    and    "  the    bromine 

>  668 


%  Dingier'  a  Polyt.  Journal  1884,  281. 


IODINE   VALUE.  413 

running  in  the  thiosulphate  solution  from  a  burette  till  the  solution 
has  only  a  light  yellow  colour,  then  adding  starch  solution  and 
continuing  the  addition  of  thiosulphate  till  the  blue  colour  just 
disappears;  or  by  the  following  method,*  due  to  Volhard:  — 
Dissolve  exactly  3-8657  gm.  of  pure  potassium  dichromate  in  a  litre 
of  water.  Place  in  a  stoppered  bottle  10  c.c.  of  a  10  per  cent. 
solution  of  potassium  iodide  and  5  c.c.  of  hydrochloric  acid,  and 
run  in  from  a  burette  exactly  20  c.c.  of  the  dichromate  solution. 
In  this  way  0-2  gm.  precisely  of  iodine  will  be  set  free,  which  is 
then  titrated  as  described  above.  The  dichromate  solution  maintains 
its  strength  indefinitely,  hence  is  always  ready  for  standardizing 
a  thiosulphate  solution. 

3.  Chloroform  or  Carbon  Tetrachloride.  —  These  should  stand 
the  following  test  for  purity.     Mix  10  c.c.  with  10  c.c.  of  the  iodine 
solution,  allow  to  stand  for  2  or  3  hours,  and  titrate.      The  volume 
of  thiosulphate  required  should  be  the  same  as  that  required  for 
10  c.c.  of  the  iodine  solution  alone. 

4.  Potassium  iodide  Solution.  —  A  10  per  cent,  solution  of  the 
pure  salt  is  used. 

5.  Starch    solution.  —  This    should    always    be    freshly    made. 
About  0-5  gm.  of  starch  is  shaken  with  a  little  cold  water  in  a  test- 
tube,  poured  into  about  50  c.c.  of  hot  water  in  a  larger  tube,  the 
liquid  raised  to  the  boiling  point,  then  cooled  for  use. 

METHOD  OF  PROCEDURE  :  From  015  to  0'2  gm.  of  a  drying  or  fish  oil  ;  0'2  to 
0'3  gm.  of  a  semi-drying  oil  ;  0-3  to  0-4  gm.  of  a  non-drying  oil  ;  or  0*8  to  1  gm. 
of  a  solid  fat  is  dissolved  in  10  c.c.  of  chloroform  or  carbon  tetrachloride  in  a  well- 
stoppered,  wide-mouthed  bottle  of  about  300-400  c.c.  capacity,  and  25  c.c.  of 
the  iodine  solution  run  in  from  a  pipette,  which  should  be  drained  for  exactly 
15  seconds.  After  not  less  than  four  hours'  standing  in  a  dark  place  the  liquid 
should  possess  a  dark  brown  tint  ;  in  any  circumstances  it  is  necessary  to  have 
a  considerable  excess  of  iodine  (at  least  double  the  amount  absorbed  ought  to  be 
present)  and  the  period  of  standing  should  be  from  four  to  six  hours.  At  the 
end  of  that  time  some  strong  solution  of  potassium  iodide  should  be  added  and 
about  150  c.c.  of  water  (more  iodide  being  added  if  the  liquid  does  not  remain  clear), 
and  standard  thiosulphate  run  in  at  once  from  a  burette,  with  constant  shaking, 
till  the  liquid  becomes  yellow.  Starch  solution  is  then  added  and  the  titration 
finished  in  the  usual  way. 

If  after  standing,  say  two  hours,  the  solution  has  lost  its  deep  colour  brown,  it 
is  best  to  make  a  fresh  experiment  with  either  less  fat  or  more  iodine  solution. 

A  blank  experiment  should  in  every  case  be  made  side  by  side 
with  the  sample,  using  the  same  proportions  of  chloroform  and 
iodine  solution. 

EXAMPLE.  —  To  0*4120  gm.  of  olive  oil  10  c.c.  of  chloroform  and  25  c.c.  of  H  ii  b  1  '  s 
solution  were  added  and  allowed  to  stand  for  four  hours.     At  the  end  of  that 
time  18'9  c.c.  of  thiosulphate  were  required.     The  blank,  consisting  of  10  c.c.  of 
chloroform  and  25  c.c.  of  Hiibl's  solution,  required  46*9  c.c.  thiosulphate.     Also 
(i.)     0'2704  gm.  Iodine  required  21  '3  c.c.  thiosulphate. 
(ii.)    0-2076    „        „  „         16-35  c.c. 


Hence  1  c.c.  thiosulphate  ='0127  gm.  Iodine. 
*Lewkowitsch,  Oils,  Fats  and  Waxes,  4th  Edition,  Vol.  I.  p.  312. 


414  OILS,    FATS,    AND    WAXES. 

(46 -9- 18-9)  x -0127x100 
Iodine  value  =          — Q-412 — 

_28xl-27 

0-412 
=  86-3 

The  values  obtained  by  Hiibl  for  various  oils  and  fats  are  given  in  J.  S.  C.  I., 
1884,  642. 

The  relative  proportions  in  which  two  oils  are  mixed  can  be  deduced  from  the 
iodine  value  of  the  mixture,  thus 

Let  la  be  the  mean  iodine  value  of  an  oil  a. 
Ib         „  „  „  b. 

I  the  iodine  value  of  the  mixture. 
Then 

,     ..         100  (I -Ib) 
percentage  of  oil  a  =  — = ^u — 

EXAMPLE. — A  mixture  of  cotton  seed  and  olive  oils  gave  the  iodine  value  91, 
hence 

cotton  seed  oil=10?n(n91  ~85)  =25  per  cent. 

10*7   —  OO 

The  valuable  improvement  made  by  Wijs*  produces  an  iodine 
solution  which  holds  its  standard  strength  for  a  very  much  longer 
period  than  the  original  Hiibl  solution,  and  also  acts  much  more 
rapidly.  The  same  results  are  eventually  obtained  as  in  the 
original  Hiibl  process  when  the  latter  is  carefully  performed. 

The  method  proposed  by  Wijs  is  the  use  of  a  solution  of  iodine 
monochloride  in  strong  acetic  acid,  in  place  of  the  mixture  of  iodine 
and  mercuric  chloride.  The  original  acid  used  was  of  95  per  cent, 
strength,  but  a  much  better  solution  is  obtained  by  using  acid  of  not 
less  than  99  per  cent. 

Wijs  admits  a  decrease  of  about  0-3  per  cent,  in  96  hours  when 
using  very  pure  95  per  cent,  acid,  but  Lewkowitsch  found  it 
amounted  to  4  per  cent,  in  64  hours.  This  Wijs  attributed  to  the 
use  of  a  less  pure  acid  than  was  used  by  himself.  However,  the 
substitution  of  the  stronger  acid  seems  to  settle  the  difficulty 
completely.  Lewkowitsch  states  that  he  has  found  with  99  per 
cent,  acid  the  same  strength  remains  for  two  months  ;  other  operators 
have  not  found  it  quite  so  permanent  as  this,  but  all  agree  that  it 
does  not  alter  so  as  to  cause  inconvenience.  So  far  as  the  weakening 
of  the  acetic  acid  iodine  solution  is  concerned,  A.  Marshall^  is  of 
opinion  that  it  must  largely  depend  upon  the  amount  of  chloracetic 
acid  formed  in  preparing  the  solution.  Wijs  himself  is  of  opinion 
that  if  the  acid  is  pure,  and  especially  free  from  oxidizable  matters, 
there  should  theoretically  be  no  possibility  of  any  diminution  of 
strength. 

The  preparation  of  Wi  j  s'  iodine  solution  is  carried  out  as  follows  : 
13  gm.  of  pure  iodine  are  dissolved  in  a  litre  of  99  per  cent,  acetic 
acid ;  the  strength  is  then  determined  by  standard  thiosulphate,  then 
chlorine  gas,  washed  and  dried,  is  passed  into  it  until  the  titer  is 
doubled.  With  a  little  practice  the  proper  ending  of  the 
chlorination  is  ascertained  by  the  change  of  colour  from  dark  brown 

*  Berichte,  1898,  750.  f  J.  S.  C.  /.,  1900,  213 


PHENOLS   AND    CBESOLS.  415 

to  light  yellow.  If  the  gas  is  passed  in  until  this  just  occurs  the 
first  titration  may  be  dispensed  with.  The  process  of  titrating 
the  fat  is  carried  out  precisely  as  above  described,  with  the  exception 
that  the  time  required  for  the  absorption  is  very  greatly  curtailed. 
When  small  quantities  of  fat  are  used  which  are  of  low  iodine  value 
the  action  is  complete  in  less  than  five  minutes,  and  with  values 
below  100  half  an  hour  is  quite  sufficient.  Lewkowitsch*  strongly 
recommends  Wijs'  process.  He  finds  it  preferable  to  the  Hub! 
iodine  solution  in  almost  every  case,  as  it  is  infinitely  superior  to 
the  latter  as  regards  stability.  He  has  known  a  solution  to  keep 
its  strength  practically  unchanged  for  five  months.  It  can  be  pre- 
pared rapidly,  and  the  time  spent  on  the  test  is  very  much 
shortened. 


PHENOLS    AND    CRESOLS. 

Phenol  C6H5OH  =  94-05. 
Cresol  C6H4  (CH3)  OH  =  108-06. 

THE  chief  method  claiming  accuracy  for  the  determination  of 
phenols  volume  trie  ally  was  originated  by  Koppeschaarf,  and 
consists  in  precipitatirfg  the  phenol  from  its  aqueous  or  dilute 
alcoholic  solution  with  bromine  water  in  the  form  of  tribromphenol. 

The  strength  of  the  bromine  water  was  established  by 
Koppeschaar  by  titration  with  thiosulphate  and  potassium 
iodide  with  starch. 

Allen  modifies  the  method  as  follows:^ — 

METHOD  OF  PROCEDURE  :  a  certain  weight  of  the  sample  is  dissolved  in  water, 
as  much  as  corresponds  to  O'l  gin.  of  phenol  is  taken  out  and  put  into  a  stoppered 
bottle  holding  250  c.c.  Further,  to  7  c.c.  of  normal  soda  solution  (  =0*04  gm. 
NaOH  per  c.c.)  bromine  is  gradually  added  till  a  yellow  colour  appears  and  remains  ; 
the  liquid  is  then  boiled  till  it  has  become  colourless  again.  It  now  contains  5  mole- 
cules of  sodium  bromide  and  1  of  sodium  bromate.  When  completely  cooled,  it 
is  put  into  the  phenol  solution,  after  which  5  c.c.  concentrated  hydrochloric  acid 
are  at  once  added,  and  the  bottle  stoppered  and  shaken  for  some  time.  The 
reactions  are : — 

I.     SNaBr  +  NaBr03  +  6HC1  =  6NaCl  +  3Br2  +  3H2O. 

II.  C6H60  +  6Br  =  C6H3BrjO  +  3HBr. 

The  bromine  set  free  in  the  first,  and  not  fixed  by  phenol  in  the  second  reaction 
must  be  still  free,  and  is  determined  by  adding  potassium  iodide  and  titrating  the 
iodine  liberated  by  N/io  thiosulphate  : — 

III.  2KI  +  Br2  =  2KBr  + 12. 

IV.     I2  +2Na2S203  =Na2S406  +2NaI. 

For  this  purpose  the  bottle  is  allowed  to  stand  for  15  or  20  minutes  ;  a  solution 
of  about  1'25  gm.  potassium  iodide  (free  from  ioclate)  is  added,  the  bottle  is 
stoppered,  shaken  up,  and  allowed  to  rest.  Its  contents  are  now  poured  into  a 
beaker  ;  the  bottle  is  rinsed  out,  a  little  starch  solution  is  added,  and  thiosulphate 
is  run  in  from  a  burette  till  the  blue  colour  is  gone.  (It  will  be  best  not  to  add 
the  starch  till  the  colour  of  the  liquid  has  diminished  to  light  yellow.) 

*  Oils,  fats  and  waxes,  4th  Edition,  Vol.  I.  323. 
t  Z.  a.  C.  16,  233. 


416  PHENOLS   AND    CRESOLS. 

Where  a  number  of  determinations  have  to  be  made  a  standard 
solution  of  sodium  bromide  and  bromate  should  be  made  by 
measuring  out  100  c.c.  of  N.  NaOH  into  a  beaker,  adding  bromine 
till  the  liquid  becomes  brown  in  colour  and  smells  distinctly  of 
bromine,  then  heating  the  solution  till  quite  colourless.  It  is  then 
cooled  and  diluted  to  1  litre. 

1  c.c.  =0-007992  gm.  bromine 
=  0-001568  gm.  phenol. 
=  0-001801  gm.  cresol. 
Also  1  Br.  corresponds  to  0*19613  phenol. 
1    „  „  ,,   0-22537  cresol. 

Hence  79-92  Br.  =  15-68  Phenol. 
=  18-01  Cresol. 

/  15-68     \ 

and  Iodine  x  I  =  1-1235=  phenol. 

\izo*yz    / 


EXAMPLE.  —  5  gin.  carbolic  powder  are  put  into  a  250  c.c.  flask,  HC1  added,  and 
made  up  to  the  mark  with  water.  The  flask  is  shaken  and  the  contents  filtered. 
50  c.c.  of  the  filtrate  (  =  1  gm.  of  sample)  are  measured  into  a  bottle  and  100  c.c. 
of  the  bromide  and  bromate  solution  added,  together  with  a  little  more  HC1,  and 
allowed  to  stand  for  15  minutes.  KI  solution  is  then  added  and  the  Iodine  set 
free  titrated  with  N/io  thiosulphate.  Suppose  that  13*8  c.c.  of  the  latter  are 
required,  and  that  titration  with  Iodine  snowed  that  1  c.c.  of  it  =  0-0121  gm. 
iodine  ;  also  that  10  c.c.  of  the  bromide  and  bromate  solution  and  KI  required 
10*5  c.c.  thiosulphate.  Then  100  c.c.  of  the  bromide  and  bromate  solution  =105 
c.c.  N/io  thiosulphate  of  which  105—13-8  =  81-2  c.c.  represent  the  bromine 
absorbed,  which  has  been  determined  in  terms  of  Iodine. 

Hence  81-2  x  -0121  x  -1235  =  -1214  gm.  phenol  or  12-14  per  cent. 

For  the  determination  of  phenol  in  raw  products,  Toth*  modifies 
the  bromine  method  as  follows  :— 

METHOD  OF  PROCEDURE  :  20  c.c.  of  the  impure  carbolic  acid  are  placed  in  a 
beaker  with  20  c.c.  of  caustic  potash  solution  of  1  -3  sp.  gr.,  well  shaken  and  allowed 
to  stand  for  half  an  hour,  then  diluted  to  about  £  litre  with  water.  By  this  treat- 
ment the  tarry  constituents  are  set  free,  and  may  mostly  be  removed  by  filtration  ; 
the  filter  is  washed  with  warm  water,  until  all  alkali  is  removed.  The  filtrate  and 
washings  are  acidulated  slightly  with  HC1,  and  diluted  to  3  litres.  50  c.c.  are  then 
mixed  with  150  c.c.  of  standard  bromide  solution  and  then  5  c.c.  of  concentrated 
HC1.  After  twenty  minutes,  with  frequent  shaking,  10  c.c.  of  iodide  solution  are 
added,  mixed,  and  allowed  to  rest  three  to  five  minutes,  then  starch,  and  the 
free  iodine  titrated  with  a  sodium  thiosulphate  solution  containing  9-779  grams 
of  the  crystals  per  litre  (exactly  corresponding  to  5  grams  iodinef.) 

EXAMPLE.  —  20  c.c.  raw  carbolic  oil  were  treated  as  above  described.  50  c.c.  of 
the  solution,  with  150  c.c.  bromide  solution  (made  by  dissolving  2-04  gm.  sodium 
bromate  and  6-959  gm.  sodium  bromide  to  the  litre),  then  5  c.c.  of  HC1,  required 
17-8  c.c.  of  thiosulphate  for  titration.  The  150  c.c.  bromide  =0-237  gm.  Br. 
The  17-8  c.c.  thiosulphate  required  for  residual  titration  =OO52  gm.  Br.  leaving 
0-J85  gm.  Br.  for  combination  with  the  phenol. 

*  Z.  a.  O.  25,  160,  also  Analyst,  1886,  11,  92. 

t  Of  course  this  solution  would  not  maintain  its  strength  on  keeping,  and  it  is 
extremely  doubtful  whether  the  solution  could  be  depended  on  even  when  freshly 
made  to  have  exactly  the  iodine  equivalent  given.  It  is  best  to  standardize  it  against 
pure  iodine  (see  Iodine  Value  of  Oils). 


PHENOLS    AND    CRESOLS.  417 

As  50  c.c.  have  been  taken  out  of  a  total  volume  of  3000  c.c.  (representing  20 
c.c.  of  the  sample),  we  have  0-185  x  0-19613  x  60  =  2-177  gm.  phenol  or  2-177x5 
=  10-89  per  cent. 

Kleinert*  suggests,  and  his  experiments  appear  to  prove,  that 
in  titrating  acid  creosote  oilbyKoppeschaar's  method  for  phenol, 
a  serious  error  occurs  in  virtue  of  such  oil  containing  substances 
of  higher  boiling-point  than  phenol,  which  are  soluble  in  water, 
and  behave  with  bromine  in  the  same  manner  as  true  phenol. 

Messinger  and  Vortmannt  describe  a  method  of  determining 
phenol  based  on  the  fact  that  iodine  combines  with  phenol  in  alkaline 
solution  in  the  proportion  of  6  atoms  I  to  1  mol.  phenol. 

METHOD  OF  PROCEDURE  :  2  to  3  gm.  phenol  are  dissolved  in  caustic  soda  solution 
(3  eq.  NaHO  to  I  eq.  phenol)  and  made  up  to  500  c.c.  with  water  ;  10  c.c.  of  this 
are  placed  in  a  flask,  warmed  to  60°  C.,  and  N/io  iodine  added  until  the  solution  is 
faintly  yellow,  with  formation  of  a  red  precipitate.  When  cold,  the  solution  is 
acidified  with  dilute  H2S04,  made  up  to  500  c.c.  and  filtered.  In  100  c.c.  of  the 
nitrate,  the  excess  of  I  is  titrated  with  N/io  thiosulphate ;  this  amount,  deducted 
from  the  total  I  used,  gives  the  amount  absorbed  by  phenol,  which,  when  multiplied 
by  0-1235,  gives  amount  of  phenol  in  the  sample. 

A  method  for  the  examination  of  commercial  phenols  has  been 
described  by  S.  B.  Schry  verj,  and  is  based  on  the  interaction  of 
sodamide  and  bodies  containing  a  hydroxyl  group,  which  takes 
place  according  to  the  typical  reaction  : 

NaNH2 + C6H5OH = C6H5ONa + NH3. 

METHOD  OF  PROCEDURE  :  A  200  c.c.  wide-necked  flask  is  fitted  with  a  separating 
funnel,  the  tube  of  which  passes  to  the  bottom,  and  an  inverted  condenser  con- 
nected at  its  upper  end  with  an  absorbing  vessel,  and  thence  with  an  aspirator. 
About  1  gm.  of  sodamide  is  finely  ground,  washed  two  or  three  times  with  benzene 
by  decantafion,  then  introduced  into  the  flask,  and  50  or  60  c.c.  of  benzene  (free 
from  thiophen)  added.  The  mixture  is  heated  on  a  water-bath  in  a  current  of 
dry  air  freed  from  C02  for  some  ten  minutes  till  the  last  traces  of  ammonia  are 
expelled.  About  20  c.c.  of  normal  sulphuric  acid  are  next  placed  in  the  receiver, 
and  the  phenol  dissolved  in  six  times  its  weight  of  benzene,  is  brought  into  the 
funnel  and  allowed  to  drop  into  the  flask.  The  funnel  is  rinsed  with  more  benzene, 
and  the  current  of  air  is  maintained  through  the  boiling  liquid  for  75  minutes. 
The  excess  of  sulphuric  acid  is  finally  titrated  with  normal  sodium  carbonate  and 
methyl  orange.  With  phenol,  cresol,  and  guaiacol  (alone),  the  process  gives  correct 
results,  provided  (1)  the  apparatus  and  phenol  are  perfectly  dry  (sodamide  acts 
upon  water),  (2)  sufficient  benzene  is  employed  to  hold  the  sodium  salt  in  solution, 
(3)  the  benzene  is  free  from  thiophen,  and  (4)  air  is  aspirated  for  a  sufficient  length 
of  time.  Toluene  or  xylene  may  replace  the  benzene,  but  in  that  case  a  sand-bath 
must  be  used  instead  of  the  water-bath. 

The  process  is  obviously  not  applicable  to  the  determination  of 
the  relative  proportions  of  more  than  two  phenols  ;  but  it  has  been 
tested  on  mixtures  of  phenol  and  cresol,  on  wood-tar  guaiacol,  which 
is  a  mixture  of  guaiacol  and  creosol,  on  thymol  in  oil  of  thyme,  and 
on  eugenol  in  oil  of  cloves.  Calling  the  number  of  c.c.  of  standard 
acid  that  are  necessary  to  neutralize  the  ammonia  given  off  when 
1  gm.  of  a  phenol  (either  simple  substance  or  mixture)  is  treated 
with  an  excess  of  sodamide  under  the  experimental  conditions 

*  Z.  a.  C.  33,  1.        t  Ber.  1890,  2753,  and  J.  S.  C.  I.  1890,  1070. 
t  J.  S.  C.  I,  1899,  553. 

2  E 


418  PHENOLS    AND    CBESOLS. 

the    "  hydroxyl   value  "  —  which   in   the   case   of   pure   phenol   is 

10-63,  and  in  that  of  pure  cresol  is  (^  =  )  9-26,  etc.- 

' 


9'4 

a  table  may  be  prepared  for  converting  the  hydroxyl  value  obtained 
when  a  mixture  of  two  known  phenols  is  operated  upon  directly 
into  the  relative  proportion  between  the  two  ingredients,  and  the 
results  calculated  in  this  manner  from  the  analysis  of  materials  of 
the  above-mentioned  composition  appear  to  be  fairly  satisfactory. 
The  method  is  equally  available  for  determining  the  amount  of 
water  in  any  particular  phenol,  because  the  reaction  between 
sodamide  and  water  is  analogous  to  that  between  the  amide  and 
a  phenol.  Fused  sodium  acetate  is  the  best  substance  to  remove 
the  last  traces  of  moisture  from  ordinary  phenol,  and  the 
determination  of  moisture  can  be  made  by  two  experiments,  one 
before  and  one  after  drying  —  the  difference  in  ammonia  represents 
the  moisture.  The  process  has  an  advantage  over  methods  involving 
the  use  of  bromine  or  iodine,  as  the  results  are  not  affected  by  the 
presence  of  hydrocarbons,  for  which  reason  it  should  be  useful  for 
determining  phenols  in  a  large  number  of  essential  oils,  etc.  Soda- 
mide acts  upon  ketones,  amines,  etc.*,  but  these  bodies  can  be  readily 
removed  by  various  suitable  reagents. 


SALICYLIC   ACID. 

C6H4  (OH)  COOH-  138-05. 

SEVERAL  methods  have  been  proposed  for  the  determination  of 
this  substance  as  existing  in  the  form  of  salts  or  in  a  free  state, 
and  a  critical  examination  of  the  most  hopeful  of  these  has  been 
carefully  made  byW.  Fresenius  and  L.  Grunhutf.  The  experi- 
ments were  made  on  pure  sodium  salicylate.  The  method  proposed 
by  Messinger  and  Vortmann  in  which  it  is  said  by  the  authors 
that  1  mol.  of  salicylic  acid  consumes  6  atoms  of  iodine  was  not 
confirmed  in  these  experiments,  and  they  came  to  the  conclusion 
that  the  method  could  not  be  relied  on  to  give  even  approximately 
correct  results.  On  the  other  hand  their  experience  of  Freyer's 
bromine  method  was  that  it  gave  satisfactory  results. 

The  method  described  by  FreyerJ  is  based  on  the  facts  that, 
on  mixing  a  solution  of  salicylic  acid  with  bromine  water  in  excess, 
a  yellowish-white  precipitate  is  formed. 

+  4Br2  =  C6HBr3OBr  +  4HBr  +  CO2  ; 

and  that,  on  adding  a  solution  of  potassium  iodide,  not  only  does  the 
excess  of  bromine  liberate  an  equivalent  amount  of  iodine,  but  the 
tribromphenol  bromide  also  reacts  as  in  the  equation  : 
C6H2Br3.OBr  +  2KI  =  C6H2Br3.OK  +  KBr  +  12. 

*Titherley,  J.  C.S.  Trans.  1897,  460. 
t  Z.  a.  C.  1899,  292,  also  Analyst,  1900,  19.  \  Chem.  Zeits.  20,  820. 


SALICYLIC   ACID.  419 

Hence,*in  calculating  the  results,  only  6  atoms  of  bromine  correspond 
to  one  molecule  of  salicylic  acid. 

Freyer  states  that  an  excess  of  about  100  per  cent,  of  bromine  is 
necessary,  but  the  authors  have  proved  that  an  excess  of  from  75  to 
80  per  cent,  is  sufficient.  They  have  tested  the  method  with  con- 
centrated bromine  solutions,  using  considerable  quantities  of  sodium 
salicylate,  and  have  obtained  as  satisfactory  results  as  Freyer 
himself.  They  give  the  following  details  of  their  method  of  working, 
in  which,  like  Freyer  and  Koppeschaar,  they  used  solutions  of 
potassium  bromate  and  bromide,  and  liberated  the  bromine  by  the 
addition  of  hydrochloric  acid. 

METHOD  OF  PROCEDURE  :  The  required  quantity  of  the  bromide  and  bromate 
solution  is  diluted  with  300  c.c.  of  water,  and  decomposed  with  30  c.c.  of  dilute 
HC1  (sp.  gr.  1-10).  Into  this  mixture  is  introduced  with  continual  stirring  a 
solution  of  about  1  per  cent,  in  strength  of  the  substance  under  examination.  A 
white  precipitate  is  immediately  formed,  and,  after  this  has  been  allowed  to 
stand  for  about  five  minutes  with  occasional  agitation,  30  to  40  c.c.  of  a  10  per 
cent,  solution  of  KI  are  introduced  and  the  separated  iodine  titrated  with  N/1O 
thiosulphate. 

In  the  most  successful  results  the  bromide  solution  contained  2-5  gm.  of  potassium 
bromate,  and  about  10  gm.  of  potassium  bromide  in  a  litre  of  water.  25  c.c.  of 
this  solution  corresponded  with  25'5  c.c.  of  thiosulphate  solution,  and  1  c.c.  of 
the  latter  to  0-01098  gm.  of  iodine  or  0*00199  gm.  of  salicylic  acid. 

The  percentage  of  salicylic  acid  thus  found  in  the  same  sample  of 
pure  sodium  salicylate  varied  in  four  determinations  from  86-21  to 
86-43  per  cent.  The  theoretical  amount  is  86-23  per  cent. 

This  method  is  not  applicable  to  the  analysis  of  mixtures  of  starch 
and  sodium  salicylate  such  as  occur  in  medicinal  tabloids.  In  such 
cases  the  substance  should  be  dissolved  in  90  per  cent,  alcohol,  the 
solution  brought  to  a  definite  volume,  filtered  from  the  undissolved 
starch,  and  an  aliquot  part  of  the  filtrate  used  for  the  analysis.  In 
a  mixture  of  90-91  per  cent,  of  sodium  salicylate  and  9-09  per  cent, 
of  starch,  the  authors  found  in  this  way  89-97  per  cent,  of  the  former. 

In  the  analysis  of  wines  which  contain  sulphurous  acid,  aldehyde, 
and  other  substances  which  act  upon  bromine,  the  best  method  of 
determining  salicylic  acid,  when  present,  is  to  make  the  liquid 
alkaline,  concentrate  it,  render  it  acid,  and  extract  it  with  a  mixture 
of  ether  and  petroleum  spirit.  The  extract  thus  obtained  is  shaken 
with  alkaline  water,  which  removes  the  salicylic  acid,  and  this 
aqueous  solution  can  then  be  used  in  the  bromine  method. 

As  regards  the  colorimetric  method  of  determining  salicylic  acid 
by  means  of  ferric  chloride,  the  author  states  that  it  can  only  be 
used  when  the  amount  of  salicylic  acid  is  less  than  2  mgm.* 

The  Departmental  Committee  in  their  Report  on  Preservatives 
and  Colouring  Matters  in  Food  (1901)  recommend  that  salicylic 
acid  be  not  used  in  a  greater  proportion  than  1  grain  per  pint  in 
liquid  food  and  1  grain  per  pound  in  solid  food  and  that  its  presence 
in  all  cases  be  declared. 

Methyl  salicylate.  C6H4  (OH).COO(CH3).— E.Kremers  and  M.  M. 

*  For  the  colorimetrio  determination  of  salicylic  acid  in  Foodstuffs,  see  Analyst, 
1905,  124. 

2  E  2 


420  SALICYLIC   ACID. 

James*  have  slightly  modified  the  method  proposed  by  Ewing, 
and  boil  a  weighed  quantity  of  the  substance  with  a  known  volume 
of  normal  alkali  for  5  minutes.  The  excess  of  alkali  is  then  titrated 
with  normal  acid,  and  the  alkali  consumed,  multiplied  by  0*152, 
represents  the  weight  in  grams  of  methyl  salicylate. 

The  method  proposed  by  Messinger  and  Vortmann  is  also 
recommended.  5  gm.  of  the  sample  are  saponified  with  excess  of 
alkali,  and  when  cold  diluted  to  500  c.c.  ;  10  c.c.  of  this  are  heated, 
50  c.c.  of  N/10  iodine  solution  added,  and  the  liquid  diluted  to  500 
c.c.  ;  in  100  c.c.  of  this,  the  excess  of  iodine  is  determined  by  means 
of  N/10  sodium  thiosulphate.  1  c.c.  of  iodine  solution  absorbed, 
when  multiplied  by  0*002535,  represents  the  amount  of  methylic 
salicylate,  as  1-mol.  of  the  ethereal  salt  absorbs  6  atoms  of  iodine. 

*  Chem.  Centr.  1898,  1070. 


URINE.  421 


PART    VI. 

SPECIAL    APPLICATIONS    OF    THE    VOLUMETRIC 

SYSTEM    TO    THE    ANALYSIS    OF    URINE,    POTABLE 

WATERS,    SEWAGE,    ETC. 

ANALYSIS    OF    URINE. 

THE  complete  and  accurate  determination  of  the  normal  and 
abnormal  constituents  of  urine  presents  more  than  ordinary 
difficulty  to  even  experienced  chemists,  and  is  a  hopeless  task  in 
the  hands  of  any  other  than  such.  Fortunately,  however,  the  most 
important  matters,  such  as  urea,  glucose,  phosphates,  sulphates, 
and  chlorides,  can  all  be  determined  volumetrically  with  accuracy 
by  ordinary  operators,  or  by  medical  men  who  cannot  devote  much 
time  to  practical  chemistry.  The  researches  ofLiebig,Neubauer, 
Bence  Jones,  Vogel,  Beale,  Hassall,  Pavy,  Allen,  and  others, 
have  resulted  in  a  truer  knowledge  of  this  important  secretion  ; 
and  to  the  two  first  mentioned  chemists  we  are  mainly  indebted 
for  the  simplest  and  most  accurate  method  of  determining  its  con- 
stituents. With  the  relation  which  the  proportion  of  these 
constituents  bears  to  health  or  disease,  the  present  treatise  has 
nothing  to  do,  its  aim  being  simply  to  point  out  the  readiest  and 
most  useful  methods  of  determining  them  quantitatively. 

The  gram  system  of  weights  and  measures  will  be  adopted 
throughout  this  section,  while  those  who  desire  to  use  the  grain 
system  will  have  no  difficulty  in  working,  when  once  the  simple 
relation  between  them  is  understood*  (see  p.  27).  The  question  of 
weights  and  measures  is,  however,  of  very  little  consequence,  if  the 
analyst  considers  that  he  is  dealing  with  relative  parts  or  proportions 
only  ;  and  as  urine  is  generally  described  as  containing  so  many 
parts  of  urea,  chlorides,  or  phosphates,  per  1000,  the  absolute  weight 
may  be  left  out  of  the  question.  The  grain  system  is  more  readily 
calculated  into  English  ounces  and  pints,  and  therefore  is  generally 
more  familiar  to  the  medical  profession  of  this  country. 

One  thing,  however,  is  necessary  as  a  preliminary  to  the  exami- 
nation of  urine,  which  has  not  generally  been  sufficiently  considered  ; 
that  is  to  say,  the  relation  between  the  quantity  of  secretion  passed 
in  a  given  time  and  the  amount  of  solid  matters  found  in  it  by 
analysis.  From  a  medical  point  of  view  it'  is  a  mere  waste  of  time, 
generally  speaking,  to  determine  the  constituents  in  half-a-pint  or 
so  of  urine  passed  at  any  particular  hour  of  the  day  or  night  without 

*  In  a  word,  whenever  c.c.  occurs,  dm.  may  be  substituted  :  and  in  case  of  using 
grains  for  grams,  move  the  decimal  point  one  place  to  the  right ;  thus  7'0  grams 
would  be  changed  to  70  grains.  Of  course  it  is  understood  that  where  grams  are 
taken  c.c.  must  be  measured,  and  with  grains  dm.,  the  standard  solution  being  the 
same  for  both  systems. 


422  URINE. 

ascertaining  the  relation  which  that  quantity,  with  its  constituents, 
bears  to  the  whole  quantity  passed  during,  say,  24  hours ;  and  this 
is  the  more  necessary,  as  the  amount  of  fluid  secreted  varies  very 
considerably  in  healthy  persons  ;  besides  this,  the  analyst  should 
register  the  colour,  peculiarity  of  smell  (if  any),  consistence,  presence 
or  absence  of  a  deposit  (if  the  former,  it  should  be  collected  for 
separate  analysis,  filtered  urine  only  being  used  in  such  cases  for 
examination),  and  lastly  its  reaction  to  litmus  should  be  observed. 

1.    Specific  Gravity. 

This  may  be  taken  by  measuring  10  c.c.  with  an  accurate  pipette 
into  a  tared  beaker  or  flask.  The  observed  weight  say  is  10-265  gm.; 
therefore  1026-5  will  be  the  specific  gravity,  water  being  1000. 
Where  an  accurate  balance,  pipette,  or  weights  are  not  at  hand, 
a  good  urinometer  may  be  used.  These  instruments  are  now  to  be 
had  with  enclosed  thermometer  and  of  accurate  graduation.  . 

2.    Determination  of  Chlorides  (calculated  as  Sodium  Chloride). 

This  may  be  done  in  various  ways.  Liebig's  method  is  by  far 
the  simplest,  but  the  end  point  is  generally  so  obscure  that  the 
liability  to  error  is  very  great,  and  therefore  the  details  of  the  process 
are  omitted.  Mohr's  method  is  modified  by  the  use  of  ammonium 
in  place  of  potassium  nitrate,  owing  to  the  solvent  effect  which  the 
latter  has  been  found  to  produce  on  silver  chromate.  By  ignition 
the  ammonia  salt  is  destroyed. 

(a)  By  Silver  Nitrate  and  Chromate  Indicator  (Mohr). — 10  c.c. 
of  the  urine  are  measured  into  a  thin  porcelain  capsule,  and  1  gm. 
of  pure  ammonium  nitrate  in  powder  added  ;  the  whole  is  then 
evaporated  to  dryness,  and  gradually  heated  over  a  small  spirit 
lamp  to  low  redness  till  all  vapours  are  dissipated  and  the  residue 
becomes  white*  ;  it  is  then  dissolved  in  a  small  quantity  of  water, 
and  the  carbonates  produced  by  the  combustion  of  the  organic 
matter  neutralized  by  dilute  acetic  acid  ;  a  few  grains  of  pure  calcium 
carbonate  are  then  added,  to  remove  all  free  acid,  and  one  or  two 
drops  of  solution  of  potassium  chromate. 

The  mixture  is  then  titrated  with  N/10  silver,  as  on  p.  142. 

Each  c.c.  of  silver  solution  represents  0-005846  gm.  of  salt, 
consequently  if  12-5  c.c.  have  been  used,  the  weight  of  salt  in  the 
10  c.c.  of  urine  is  0-073075  gm.,  and  as  10  c.c.  only  were  taken,  the 
weight  multiplied  by  10,  or  what  amounts  to  the  same  thing,  the 
decimal  point  moved  two  places  to  the  right,  gives  7*3075  gm.  of 
salt  for  1000  c.c.  of  urine'. 

If  5-9  c.c.  of  the  urine  are  taken  for  titration,  the  number  of  c.c.  of  N/iO 
silver  used  will  represent  the  number  of  parts  of  salt  in  1000  parts  of  urine. 

*  Dr.  Edmunds  has  called  my  attent  on  to  the  fact  that  there  is  great  danger 
or  losing  chlorine  if  the  ignition  is  made  at  a  high  temperature,  and  there  is  no  doubt 
ne  w  right.  He  prefers  to  char  the  urinary  residue  thoroughly  over  a  spirit  lamp, 
and  wash  out  the  chlorides  with  hot  water.  The  filtered  liquid  is  then  available 
for  direct  determination  with  silver  and  chromate  or  by  the  V  o  1  h  a  r  d  method. 


URINE.  423 

(6)  By  V  o  1  h  a  r  d  '  s  Method. — This  is  a  direct  determination 
of  Cl  by  excess  of  silver  and  the  excess  found  by  ammonium  or 
potassium  thiocyanate  (p.  145),  which  gives  very  good  results  in  the 
absence  of  much  organic  matter,  and  is  carried  out  as  follows  : — 

10  c.c.  of  urine  are  placed  in  a  100  c.c.  flask  and  diluted  to  about  60  c.c.  2  c.c. 
of  pure  nitric  acid  and  15  c.c.  of  standard  silver  solution  (1  c.c.  =0'01  gm.  NaCl) 
are  then  added  ;  the  closed  flask  is  well  shaken,  and  the  measure  made  up  to  100  c.c. 
with  distilled  water. 

The  mixture  is  then  passed  through  a  dry  filter,  and  either  70  or  80  c.c.  of  the 
clear  fluid  titrated  with  standard  thiocyanate  for  the  excess  of  silver,  using  the 
ferric  indicator  described  on  page  146.  The  relative  strength  of  the  silver  and 
thiocyanate  being  known,  the  measure  of  the  former  required  to  combine  with 
the  chlorine  in  the  7  or  8  c.c.  of  urine  is  found  and  calculated  into  NaCl. 

Arnold*  carries  out  this  process  as  follows  : — 

10  c.c.  of  urine  are  mixed  with  10  drops  to  20  of  nitric  acid  sp.  gr.  T2,  2  c.c. 
of  ferric  indicator,  and  10  to  15  drops  of  solution  of  permanganate  to  oxidize 
organic  matter.  The  liquid  is  then  filtered  and  titrated  as  described  above. 

3.    Determination  of  Urea. 

Carbamide  CO(NH2)2. 

If  a  solution  of  urea  is  mingled  with  an  alkaline  solution  of 
sodium  hypochlorite  or  hypobromite,  the  urea  is  rapidly  decomposed 
and  nitrogen  evolved,  which  can  be  collected  and  measured  in  any 
of  the  usual  forms  of  gas  apparatus  described  in  the  section  on  gas 
analysis. 

CO(NH2)2+3  Na  Br  O  =  3Na  Br+2H20+CO2+N2. 
Urea.  Sodium  Sodium 

hypobromite.  bromide. 

Test  experiments  with  pure  urea  have  shown  that  the  whole  of 
the  nitrogen  contained  in  it  is  eliminated  in  this  process,  with  the 
exception  of  a  constant  deficit  of  8  per  cent.  The  carbon  dioxide 
set  free  in  the  reaction  is  absorbed  by  the  excess  of  caustic  alkali 
present,  so  that  nitrogen  gas  alone  is  evolved.  In  the  case  of  urine 
there  are  other  nitrogenous  constituents  present,  such  as  uric  acid, 
hippuric  acid,  and  creatinine,  which  render  up  a  small  proportion  of 
their  nitrogen  in  the  process,  but  the  quantity  so  obtained  is 
insignificant,  and  may  be  disregarded.  Consequently,  for  all 
medical  purposes,  this  method  of  determining  urea  in  urine  is 
sufficiently  exact. 

In  the  case  of  diabetic  urines,  however,  Menu  and  others  have 
pointed  out  that  this  deficiency  is  diminished,  and  if,  in  addition  to 
the  glucose  present,  cane  sugar  be  also  added,  it  will  almost  entirely 
disappear.  Mehuf  therefore  recommends  that  in  the  analysis  of 
saccharine  urines  cane  sugar  be  added  to  the  extent  of  ten  times  the 
amount  of  urea  present,  when  the  difference  between  the  actual 
and  theoretical  yield  of  nitrogen  will  not  exceed  1  per  cent. 

Russell  and  WestJ  have  described  a  very  convenient  apparatus 

*  Pflvyer's  Archiv.  30,  541.  t  Bull.  Soc.  Chim.  [2]  33,  410. 

J  J.  C.  S.  [2]  12,  749. 


424 


URINE. 


for  working  the  process,  which  gives  very  good  results  in  a  short 
space  of  time.  This  method  has  given  rise  to  endless  forms  of 
apparatus  devised  by  various  operators,  including  Mehu,  Yvon, 
Dupre,  Ap John,  Maxwell  Simpson,  Doremus,  O'Keefe,  etc., 
etc. ;  the  principles  of  construction  are  all,  however,  the  same.  Those 
who  may  wish  to  construct  simple  forms  of  apparatus  from  ordinary 
laboratory  appliances,  will  do  well  to  refer  to  the  arrangements  of 
Dupre*  or  Max  well  Simpsonf.  The  nitrometer,  with  side  flask, 
and  using  mercury,  is  perhaps  the  best  of  all  for  the  gasometric 
determination  of  urea.  Each  c.c.  of  N  produced,  after  correction 
for  temperature,  pressure,  and  moisture,  being  equal  to  0-002913 
gm.  of  urea  on  the  assumption  that  92  %  is  evolved. 

The  apparatus  devised  by  Russell  and  West  is  shown  in  fig.  58, 
and  may  be  described  as  follows  : — 

The  tube  for  decomposing  the  urine  is 
about  9  inches  long,  and  about  half  an 
inch  inside  diameter.  At  2  inches  from 
its  closed  end  it  is  narrowed,  and  an 
elongated  bulb  is  blown,  leaving  the  orifice 
at  its  neck  f  of  an  inch  in  diameter  ;  the 
bulb  should  hold  about  12  c.c.  The 
mouth  of  this  tube  is  fixed  into  the 
bottom  of  a  tin  tray  about  If  inch  deep, 
which  acts  as  a  pneumatic  trough ;  the  tray 
is  supported  on  legs  long  enough  to  allow 
of  a  small  spirit  lamp  being  held  under 
the  bulb  tube.  The  measuring  tube  for 
collecting  the  nitrogen  is  graduated  into 
cubic  centimetres,  and  is  of  such  size  as 
to  fit  over  the  mouth  of  the  decomposing 
tube  ;  one  holding  about  40  c.c.  is  a  con- 
venient size.  Russell  and  West  have 
fixed  by  experiment  the  proportions,  so 
as  to  obviate  the  necessity  for  correction 
for  pressure  and  temperature,  namely,  37'1  c.c.  =0-1  gm.  of  urea, 
since  they  found  that  5  c.c.  of  a  2  per  cent,  solution  of  urea  constantly 
gave  37-1  c.c.  of  nitrogen  at  ordinary  temperatures  and  pressures. 
The  entire  apparatus  can  be  purchased  of  most  operative  chemists 
for  a  moderate  sum. 

Hypobromite  Solution. — This  is  best  prepared  by  dissolving  100 
gm.  of  caustic  soda  in  250  c.c.  of  water,  and  at  the  time  required 
25  c.c.  of  the  (cold)  solution  are  mixed  with  2-5  c.c.  of  bromine  ; 
this  mixture  gives  a  rapid  and  complete  decomposition  of  the  urea. 
Strong  solution  of  sodium  or  calcium  hypochlorite  answers  equally 
well. 

METHOD  OF  PROCEDURE  :  5  c.c.  of  the  urine  are  measured  into  the  bulb-tube, 
hxed  in  its  proper  position,  and  the  sides  of  the  tube  washed  down  with  distilled 
water  so  that  the  bulb  is  filled  up  to  its  constriction. 


Fig.  58. 


*  J.  C.  S.  1877,534. 


A  glass  rod,  having  a  thin 
t  Ibid.  538. 


URINE.  425 

band  of  india-rubber  on  its  end,  is  then  passed  down  into  the  tube  so  as  to  plug 
up  the  narrow  opening  of  the  bulb.  The  hypobromite  solution  is  then  poured 
into  the  upper  part  of  the  tube  until  it  is  full,  and  the  trough  is  afterwards  half 
filled  with  water. 

The  graduated  tube  is  filled  with  water,  the  thumb  placed  on  the  open  end, 
and  the  tube  is  inverted  in  the  trough.  The  glass  rod  is  then  pulled  out,  and 
the  graduated  tube  slipped  over  the  mouth  of  the  bulb-tube. 

The  reaction  commences  immediately,  and  a  torrent  of  gas  rises  into  the 
measuring  tube.  To  prevent  any  of  the  gas  being  forced  out  by  the  reaction, 
the  upper  part  of  the  bulb-tube  is  slightly  narrowed,  so  that  the  gas  is  directed 
to  the  centre  of  the  graduated  tube.  With  the  strength  of  hypobromite  solution 
above  described,  the  reaction  is  complete  in  the  cold  in  about  ten  or  fifteen 
minutes ;  but  in  order  to  expedite  it,  the  bulb  is  slightly  warmed.  This  causes 
the  mixing  to  take  place  more  rapidly,  and  the  reaction  is  then  complete  in  five 
minutes.  The  reaction  will  be  rapid  and  complete  only  when  there  is  consider- 
able excess  of  the  hypobromite  present.  After  the  reaction  the  liquid  should 
still  have  the  characteristic  colour  of  the  hypobromite  solution. 

The  amount  of  constriction  in  the  tube  is  by  no  means  a  matter 
of  indifference,  as  the  rapidity  with  which  the  reaction  takes  place 
depends  upon  it.  If  the  liquids  mix  too  quickly,  the  evolution  of 
the  gas  is  so  rapid  that  loss  may  occur.  On  the  other  hand,  if  the 
tube  is  too  much  constricted,  the  reaction  takes  place  too  slowly. 

The  simplest  means  of  supporting  the  measuring  tube  is  to  have 
the  bulb-tube  corked  into  a  well,  which  projects  from  the  bottom  of 
the  trough  about  one  inch  downwards.  The  graduated  tube  stands 
over  the  bulb-tube,  and  rests  upon  the  cork  in  the  bottom  of  the 
well.  It  is  convenient  to  have,  at  the  other  end  of  the  trough, 
another  well,  which  will  form  a  support  for  the  measuring  tube 
when  not  in  use. 

To  avoid  all  calculations,  the  measuring  tube  is  so  graduated  that 
the  amount  of  gas  read  off  expresses  at  once  what  may  be  called  the 
percentage  amount  of  urea  in  the  urine  experimented  upon;  i.e.,  the 
number  of  grams  in  100  c.c.,  5  c.c.  being  the  quantity  of  urine  taken 
in  each  case.  The  gas  collected  is  nitrogen  saturated  with  aqueous 
vapour,  and  the  bulk  will  obviously  be  more  or  less  affected  by 
temperature  and  pressure.  Alterations  of  the  barometer  produce  so 
small  an  alteration  in  the  volume  of  the  gas  that  it  may  generally 
be  neglected  ;  e.g.,  if  there  are  30  c.c.  of  nitrogen,  the  quantity 
preferred,  an  alteration  of  one  inch  in  the  height  of  barometer  would 
produce  an  error  in  the  amount  of  urea  of  about  0-003  ;  but  for  more 
exact  experiments,  the  correction  for  pressure  should  be  introduced. 

In  the  wards  of  hospitals,  and  in  rooms  where  the  experiments  are 
most  likely  to  be  made,  the  temperature  will  not  vary  much  from 
65°  F.,  and  a  fortunate  compensation  of  errors  occurs  with  this 
form  of  apparatus  in  these  circumstances.  The  tension  of  the 
aqueous  vapour,  together  with  the  expansion  of  the  gas  at  this 
temperature,  almost  exactly  counterbalances  the  loss  of  nitrogen 
in  the  reaction. 

The  authors  found  from  experience  that  5  c.c.  of  urine  is  the  most 
advantageous  quantity  to  employ,  as  it  usually  evolves  a  convenient 
bulk  of  gas  to  experiment  with,  i.e.,  about  30  c.c.  They  have 
shown  that  5  c.c.  of  a  standard  solution  containing  2  per  cent,  of 


426 


URINE. 


urea  evolve  37-1  c.c.  of  nitrogen,  and  have  consequently  taken  this 
as  the  basis  of  the  graduation  of  the  measuring  tube,  viz.,  37'1  c.c. 
of  the  gas  as  measured  represent  0*1  gm.  of  urea.  This  bulk  of  gas 
is  read  off  at  once  as  2  per  cent,  of  urea,  and  in  the  same  way  the  other 
graduations  on  the  tube  represent  percentage  amounts  of  urea. 

If  the  urine  experimented  with  is  very  rich  in  urea,  so  that  the 
5  c.c.  evolve  a  much  larger  volume  of  gas  than  30  c.c.,  then  it  is  best 
at  once  to  dilute  the  urine  with  its  own  bulk  of  water  ;  take  5  c.c. 
of  this  diluted  urine,  and  multiply  the  volume  of  gas  obtained 
by  two. 

If  the  urine  contains  much  albumen,  this  interferes  with  the 
process  in  so  far  that  it  takes  a  long  time  for  the  bubbles  of  gas  to 
subside  before  the  volume  of  gas  obtained  can  be  accurately  read 
off.  It  is  therefore  better  in  such  cases  to  remove  as  much  as  possible 
of  the  albumen  by  heating  the  urine  with  two  or  three  drops  of 
acetic  acid,  filtering,  and  then  using  the  nitrate  in  the  usual  manner. 
Another  form  of  apparatus  much  used  in  making  this  deter- 
mination is  that  devised  by  A.  W.  Gerrard*  (fig.  59). 

It  consists  of  a  graduated  tube, 
which  is  connected  with  a  second 
tube,  serving  as  a  reservoir,  by 
means  of  india-rubber  tubing. 
The  graduated  tube  is  closed  at 
the  top  by  a  rubber  stopper, 
through  which  passes  a  T  tube, 
one  opening  of  which  is  fitted  with 
a  short  piece  of  flexible  tubing 
closed  by  a  clip,  while  the  other  is 
connected  by  a  second  piece  of 
tubing  with  a  bottle  fitted  with 
a  perforated  cork.  In  making 
a  test,  25  c.c.  of  the  hypobromite 
solution  are  measured  into  this 
bottle,  then  a  small  test-tube 
containing  5  c.c.  of  the  urine  to 
be  tested  is  carefully  placed  in  it 
in  such  a  manner  as  to  avoid 
contact  between  the  urine  and 
the  reagent.  The  bottle  and  gra- 
duated tube  are  now  connected 
as  shown  in  the  figure,  the  clip 
opened,  and  water  poured  into 
the  reservoir  (c)  until,  by  suitably  adjusting  its  height,  the 
water  stands  at  the  zero-point  in  the  measuring  tube  and  at  the 
same  level  in  the  reservoir— taking  care  that  when  this  is  effected 
the  latter  contains  but  little  water. 

The  clip  is  then  closed,  and  the  bottle  so  tilted  that  the  urine 
gradually  mixes  with  the  hypobromite  solution,  the  bottle  being 

*  Pharm.  Journ.  [3]  15,  464. 


URINE.  427 

gently  shaken  to  promote  the  evolution  of  gas,  which  commences 
immediately  and  is  complete  in  a  few  minutes.  After  five  (or  pre- 
ferably ten)  minutes  the  reservoir  is  lowered  until  the  water  in  it 
and  in  the  graduated  tube  stands  at  exactly  the  same  level,  when 
the  volume  of  gas  is  read  off  at  once  as  percentage  of  urea  contained 
in  the  urine.  If  the  urine  contains  more  than  3  per  cent,  of  urea, 
it  is  necessary  to  take  2-5  instead  of  5  c.c.,  dilute  it  with  an  equal 
volume  of  water,  and  double  the  result  obtained. 


4.    Determination  of  Phosphoric  Acid  (see  also  p.  307  et  seq.). 

The  principle  of  this  method  is  fully  described  at  page  307. 
The  following  solutions  are  required  : — 

(1)  Standard  uranium  acetate  or  nitrate.     1   c.c.  =0*005  gm. 
P205  (see  p.  309). 

(2)  Standard  phosphoric  acid  (see  p.  309). 

(3)  Solution  of  sodium  acetate  (see  p.  309). 

(4)  Solution  of  potassium  ferrocyanide. — About  1  part  to  20  of 
water,  freshly  prepared. 

METHOD  OP  PROCEDURE  :  50  c.c.  of  the  clear  urine  are  measured  into  a  small 
beaker,  together  with  5  c.c.  of  the  solution  of  sodium  acetate  (if  uranium  nitrate 
is  used.)  The  mixture  is  then  warmed  in  the  water-bath,  or  otherwise,  and  the 
uranium  solution  delivered  in  from  the  burette,  with  constant  stirring,  so  long  as 
a  precipitate  is  seen  to  form.  A  small  portion  of  the  mixture  is  then  removed 
with  a  glass  rod  and  tested  as  described  (p.  309) ;  so  long  as  no  brown  colour 
is  produced,  the  addition  of  uranium  may  be  continued;  when  the  faintest 
indication  of  this  reaction  is  seen,  the  process  must  be  stopped,  and  the  amount 
of  colour  observed.  If  it  coincides  with  the  original  testing  *of  the  uranium 
solution  with  a  similar  quantity  of  fluid,  the  result  is  satisfactory,  and  the  quantity 
of  solution  used  may  be  calculated  for  the  total  phosphoric  acid  contained  in  the 
50  c.c.  of  urine  ;  if  the  uranium  has  been  used  accidentally  in  too  great  quantity, 
10  or  20  c.c.  of  the  same  urine  may  be  added,  and  the  testing  concluded  more 
cautiously.  Suppose,  for  example,  that  the  solution  has  been  added  in  the  right 
proportion,  and  19 '2  c.c.  used,  the  50  c.c.  will  have  contained  0'096  gm.  phosphoric 
acid  (  =  1  '92  per  100).  With  care  and  some  little  practice  the  results  are  very 
satisfactory. 

Earthy  Phosphates. — The  above  determination  gives  the  total  amount  of 
phosphoric  acid,  but  it  may  sometimes  be  of  interest  to  know  how  much  of  it  is 
combined  with  lime  and  magnesia.  To  this  end  100  or  200  c.c.  of  the  urine  are 
measured  into  a  beaker,  and  rendered  freely  alkaline  with  ammonia ;  the  vessel 
is  then  set  aside  for  ten  or  twelve  hours,  for  the  precipitate  of  earthy  phosphates 
to  settle  :  the  clear  liquid  is  then  decanted  through  a  filter,  the  precipitate  brought 
upon  it  and  washed  with  ammoniacal  water ;  a  hole  is  then  made  in  the  filter 
and  the  precipitate  washed  through  ;  the  paper  moistened  with  a  little  acetic 
acid,  and  washed  into  the  vessel  containing  the  precipitate,  which  latter  is  dissolved 
in  acetic  acid  (some  sodium  acetate  added  if  uranium  nitrate  is  used),  and  the 
mixture  diluted  to  about  50  c.c.  and  titrated  as  before  described  ;  the  quantity 
of  phosphoric  acid  so  found  is  deducted  from  the  total  previously  determined, 
and  the  remainder  gives  the  quantity  existing  in  combination  with  alkalies. 

5.     Determination  of  Sulphuric  Acid. 

Standard  barium  chloride. — A  quantity  of  crystallized  barium 
chloride  is  to  be  powdered  and  dried  between  folds  of  blotting- 


428  URINE. 

paper.     Of  this,  30 -5  gm.  are  dissolved  in  distilled  water,  and  the 
liquid  made  up  to  a  litre.     1  c.c.  =0'01  gm.  of  SO3. 
Solution  of  sodium  sulphate. — 1  part  to  10  of  water. 

METHOD  OF  PROCEDURE  :  100  c.c.  of  the  urine  are  poured  into  a  beaker,  a 
little  hydrochloric  acid  added,  and  the  whole  placed  on  a  small  sand-bath,  to 
which  heat  is  applied.  When  the  solution  boils,  the  barium  chloride  is  allowed 
to  flow  in  very  gradually  as  long  as  the  precipitate  is  seen  distinctly  to  increase. 
The  heat  is  removed,  and  the  vessel  allowed  to  stand,  so  that  the  precipitate 
may  subside.  Another  drop  or  two  is  then  added,  and  so  on,  until  the  whole 
of  the  S03  is  precipitated.  Much  time,  however,  is  saved  by  using  Be  ale's 
filter,  represented  in  fig.  23.  A  little  of  the  fluid  is  thus  filtered  clear,  poured 
into  a  test-tube,  and  tested  with  a  drop  from  the  burette  ;  this  is  afterwards 
returned  to  the  beaker,  and  more  of  the  test  solution  added,  if  necessary.  The 
operation  is  repeated  until  the  precipitation  is  complete.  In  order  to  be  sure 
that  too  much  of  the  baryta  solution  has  not  been  added,  a  drop  of  the  clear 
fluid  is  added  to  the  solution  of  sodium  sulphate  placed  in  a  test-tube  or  upon 
a  small  mirror  (see  p.  351 ).  If  no  precipitate  appears,  more  barium  must  be  added  ; 
if  a  slight  cloudiness  takes  place,  the  analysis  is  finished  ;  but  if  much  precipitate 
is  produced,  too  large  a  quantity  of  the  test  has  been  used,  and  the  analysis  must 
be  repeated. 

For  instance,  suppose  that  18-5  c.c.  have  been  added,  and  there 
is  still  a  slight  cloudiness  produced  which  no  longer  increases  after 
the  addition  of  another  j  c.c.,  we  know  that  between  18J  and  19  c.c. 
of  solution  have  been  required  to  precipitate  the  whole  of  the 
sulphuric  acid  present,  and  that  accordingly  the  100  c.c.  of  urine 
contain  between  0-185  and  0-19  gm.  of  SO3. 

6.     Determination  of  Glucose. 

Fehling's  original  method  is  precisely  the  same  as  described  on 
p.  327,  but  the  most  suitable  methods  for  urine  are  Gerrard's 
cyano-cupric  (p.  337),  or  the  Pavy-Fehling. 

PROCESS  FOR  THE  CYANO-CUPRIC  SOLUTION. — 10  c.c.  of  the  clear  urine  are 
diluted  by  means  of  a  measuring  flask  to  200  c.c.  with  water,  and  a  large  burette 
filled  with  the  fluid.  To  10  c.c.  of  the  cyano-cupric  solution  prepared  as  directed 
(p.  337)  are  then  measured  another  10  c.c.  of  Fehling's  copper  solution  and 
the  liquid  brought  to  boiling  ;  the  diluted  urine  is  then  delivered  cautiously  from 
the  burette  into  the  still  boiling  liquid,  and  with  constant  stirring,  until  the  bluish 
colour  has  nearly  disappeared.  The  addition  of  the  urine  must  then  be  continued 
more  carefully,  until  the  colour  is  all  removed,  the  burette  is  then  read,  and  the 
quantity  of  sugar  in  the  urine  calculated  as  follows  : — 

Suppose  that  40  c.c.  of  the  diluted  urine  have  been  required  to  reduce  the  10  c.c. 
of  copper  solution,  that  quantity  will  have  contained  0*05  gm.  of  sugar  ;  but,  the 
urine  being  diluted  20  times,  the  40  c.c.  represent  only  2  c.c.  of  the  original 
urine  ;  therefore  2  c.c.  of  it  contain  0'05  gm.  of  glucose,  or  25  parts  per  1000. 

If  the  Pavy-Felhing  solution  is  used,  it  is  prepared  as  described 
on  p.  335. 

METHOD  OF  PROCEDURE  :  10  c.c.  of  clear  urine  are  diluted  as  just  described, 
and  delivered  cautiously  from  the  burette  into  50  or  100  c.c.  of  the  Pavy- 
Fehling  liquid  (previously  heated  to  boiling)  until  the  colour  is  discharged. 
Ihe  calculation  is  the  same  as  before.  100  c.c.  of  Pavy-Fehling  solution 
=0-05  gm.  glucose. 

The  ammoniacal  fumes  are  best  absorbed  by  leading  an  elastic  tube  from 


URINE.  429 

the  reduction  flask  into  a  beaker  of  water  ;  the  end  of  the  tube  should  be  plugged 
with  a  piece  of  solid  glass  rod,  and  a  transverse  slit  made  in  the  elastic  tube  just 
above  the  plug.  This  valve  allows  the  vapours  to  escape,  but  prevents  the  return 
of  the  liquid  in  case  of  a  vacuum. 

7.    Determination  of  Uric  Acid. 

C5H4N403. 

A  method  for  the  accurate  determination  of  this  constituent  of 
urine  has,  up  to  the  present,  not  been  found  ;  that  is  to  say,  although 
good  results  may  be  obtained  with  chemically  prepared  pure  uric 
acid,  there  is  no  certainty  that  the  same  correctness  will  be  attained 
with  the  urinary  acid  as  separated  in  the  usual  way.  The  difficulty 
is  caused  by  the  complicated  character  of  the  urine  itself,  and 
however  accurate  the  process  may  be  with  the  acid  itself  in  a  pure 
state,  it  becomes  far  less  reliable  when  such  method  is  applied  to 
normal  or  abnormal  urine.  The  precipitation  of  the  acid  in  com- 
bination with  some  metal,  such  as  silver  or  copper,  carries  with  it 
also  the  so-called  alloxuric  bases,  and  the  separation  by  hydrochloric 
acid  contaminates  the  precipitate  with  colouring  and  other  matters 
which  militate  against  its  accurate  determination  with  permanganate. 
I  am,  however,  of  the  opinion  that  the  latter  method  is,  even  now, 
one  of  the  best  for  a  rapid  comparative  determination  of  this 
constituent. 

METHOD  OF  PROCEDURE  :  200  c.c.  of  the  urine  are  put  into  an  evaporating 
basin  with  a  few  drops  of  concentrated  hydrochloric  acid,  and  evaporated  on  the 
water-bath  to  about  half  the  volume  ;  it  is  then  transferred  to  a  closely- stoppered 
flask,  together  with  any  slight  precipitate  which  may  have  formed.  5  c.c.  of 
concentrated  hydrochloric  acid  are  then  added,  and  the  mixture  violently  shaken 
for  a  few  minutes.  It  is  then  allowed  to  settle  for  half  an  hour  and  the  liquid 
passed  through  a  small  filter  of  smooth,  hard  texture,  taking  care  to  pass  as  little 
as  possible  of  the  sediment  on  to  the  filter.  About  20  c.c.  of  cold  water  are  then 
added  to  the  precipitate  in  the  flask,  which  is  in  turn  passed  through  the  filter. 
The  filter  is  then  also  washed  with  about  the  same  quantity  of  water  ;  a  hole  is 
then  made  at  its  apex,  and  the  small  quantity  of  adhering  precipitate  washed 
into  the  original  flask.  Finally  about  10  c.c.  of  concentrated  solution  of  caustic 
potash  (1  :  10)  are  added  to  the  contents  of  the  flask  and  slightly  warmed  until 
a  clear  solution  is  obtained.  The  mixture  is  then  diluted  with  about  100  c.c. 
of  cold  water,  20  c.c.  of  dilute  sulphuric  acid  added  (1:5).  and  the  titration  with 
N/io  permanganate  carried  out  in  the  usual  manner. 

Another  form  of  the  permanganate  process  is  to  precipitate  the 
phosphates  from  100  c.c.  of  urine  with  sodium  carbonate.  The 
filtrate  is  mixed  with  5  c.c.  of  a  4  per  cent,  solution  of  copper  sulphate 
and  20  c.c.  of  a  solution  containing  10  per  cent,  each  of  Rochelle 
salt  and  sodium  thiosulphate.  The  precipitate  so  formed  is  filtered 
off  and  well  washed  with  distilled  water,  then  transferred  to  a  flask 
with  about  400  c.c.  of  water,  5  c.c.  of  sulphuric  acid  added,  and  the 
uric  acid  titrated  with  permanganate. 

No  absolute  weight  of  uric  acid  can  be  calculated  from  the  results, 
but  Mohr  assumes  that  each  c.c.  of  N/10  permanganate  =0*0075  gm. 
of  uric  acid*  ;  the  process  may,  however,  be  made  available  for 

*  This  figure  has  besn  verified  by  F.  G.  Hopkins  (Allen's  Chemistry  of 
Urine,  p.  171). 


430  URINE. 

pathological  purposes  by  comparing  the  results  from  time  to  time 
with  the  urine  from  the  same  person. 

The  following  method  has  a  good  claim  to  accuracy  as  regards 
the  actual  amount  of  uric  acid  present  in  any  given  specimen  of 
urine,  but  is  tedious.  It  is  based  on  the  fact  that  an  alkaline  solution 
of  uric  acid  reduces  Fehling  solution  in  the  same  way  as  glucose. 
The  method  is  worked  out  by  E.  Riegler*,  who  found  that  an 
average  of  many  experiments  gave  0*8  gm.  of  reduced  copper  for 
1  gm.  of  uric  acid.  The  acid  is  first  separated  from  the  urine  as 
ammonium  urate  by  Hopkins's  method  :— 

METHOD  OF  PROCEDURE  :  200  c.c.  of  urine  are  mixed  with  10  c.c.  of  a  saturated 
solution  of  sodium  carbonate,  allowed  to  stand  for  half  an  hour,  and  filtered  from 
the  precipitated  phosphates.  The  precipitate  is  washed  with  50  c.c.  of  hot  water, 
and  to  the  filtrate  and  wash- water  20  c.c.  of  a  saturated  solution  of  ammonium 
chloride  added.  The  liquid  is  well  stirred,  and  after  five  hours  filtered,  preferably 
through  a  Schleicher  and  Schull  filter,  No.  597,  11  cm.  The  precipitate  of 
ammonium  urate  is  washed  with  50  c.c.  of  water,  and  then  introduced  by  means 
of  a  jet  from  a  washing-bottle  into  a  300  c.c.  beaker.  Several  drops  of  potash 
are  added  to  clear  the  liquid,  then  60  c.c.  of  Fehl ing's  solution,  and  the  whole 
well  stirred.  The  beaker  is  then  heated  on  wire  gauze  until  the  liquid  boils,  the 
boiling  being  continued  for  five  minutes.  When  the  precipitate  has  subsided, 
the  liquid  is  filtered  through  a  small  tough  filter  (Schleicher  and  Schull, 
No.  590,  9  cm.),  the  precipitate  well  washed,  and  dissolved  in  20  c.c.  of  nitric 
acid  (sp.  gr.  1*1),  the  filter  being  washed  with  60  c.c.  of  water. 

To  this  solution  dry  powdered  sodium  carbonate  is  added  little  by  little  until 
there  is  a  permanent  turbidity.  The  liquid  is  then  cleared  by  the  cautious 
addition  of  dilute  sulphuric  acid,  and  made  up  to  100  c.c.  25  c.c.  of  this  are 
placed  in  a  100  c.c.  flask,  1  gm.  of  potassium  iodide  in  10  c.c.  of  water  added, 
allowed  to  stand  for  ten  minutes,  then  titrated  with  standard  thiosulphate  solution 
(1  c.c.  =0'002  gm.  uric  acid),  using  starch  as  the  indicator.  To  the  total  amount 
of  uric  acid  found  in  the  200  c.c.  of  urine,  an  additional  0*020  gm.  should  be  added 
to  allow  for  the  solubility  of  the  ammonium  urate  in  urine. 

The  standard  thiosulphate  solution  is  made  by  diluting  126  c.c. 
of  N/10  solution  to  500  c.c.  The  reaction  is  : — 

2Cu(NO3)2 = 4KI + Cu2I2 + 4KN03  + 12. 

The  reduced  cuprous  oxide  may  also  be  weighed  directly  or 
reduced  to  metallic  copper,  as  in  the  determination  of  sugar.  In  the 
latter  case  the  amount  of  copper,  multiplied  by  the  factor  1  -25,  gives 
the  corresponding  amount  of  uric  acid. 

E.  H.  Bartleyf  points  out  with  reason  that  the  object  for  which 
the  determination  of  uric  acid  is  generally  made  does  not  require  ex- 
treme accuracy.  The  most  acceptable  process  ought  to  be  one  which 
will  give  consistent  results  and  which  can  be  quickly  accomplished, 
and  though  not  absolutely  exact  is  nevertheless  comparatively  so. 
The  method  proposed  by  Bartleyis  based  to  some  extent  upon 
previous  ones  by  Salkowski,  Haycraft,  etc.,  that  is  to  say,  the 
uric  acid  is  precipitated  from  the  urine  by  silver  nitrate  in  the 
presence  of  an  excess  of  ammoniacal  magnesia  mixture. 

METHOD  OF  PROCEDURE  :  To  50  or  100  c.c.  of  the  clear  urine  add  5  c.c.  of 
rdinary  magnesia  mixture  such  as  is  used  for  phosphates,  and  about  10  c.c.  of 

*  Z.  A.  C.  1896,  31.  f  J.'Am.C.  S.  1897,  649. 


URINE.  431 

ammonia  of  sp.  gr.  0*96, — this  must  be  in  excess.  Warm  the  mixture  on  the 
water-bath  and  add  from  a  burette  N/so  silver  nitrate  until  a  drop  of  the  nitrate 
when  brought  into  contact  with  a  drop  of  weak  sodium  sulphide  solution  on 
a  white  plate  shows  a  dark  ring  or  cloud.  The  nitration  can  be  carried  out  with 
Beale's  filter  (fig.  23)  or  a  dropping  pipette  can  be  used,  the  end  of  which  is  tied 
over  with  cotton  wool.  The  clear  liquid  only  must  be  tested.  Each  c.c.  of  silver 
solution  represents  0 '00336  gm.  of  uric  acid,  and  the  number  of  c.c.  used  (less  one 
half  of  a  c.c.  for  each  50  c.c.  of  urine)  when  multiplied  by  this  factor  will  give  the 
amount  of  uric  acid  in  the  urine  examined.  The  half  c.c.  is  deducted  because 
it  takes  that  amount  of  silver  solution  to  give  the  colour  with  50  c.c.  of  plain 
water. 

As  soon  as  the  process  is  complete  the  precipitate  settles  freely,  and  it  is  advisable 
to  test  a  drop  of  the  clear  solution  again.  The  ending  can  also  be  checked  by 
adding  a  drop  of  the  silver  to  the  clear  supernatant  liquid  to  see  whether  a  cloudi- 
ness appears. 

There  being  no  excess  of  silver  in  the  hot  liquid  at  any  time  there  can  be  no 
reduction  of  the  silver.  If  after  the  titration  is  complete  the  mixture  be  allowed 
to  cool  to  ordinary  temperature  it  will  be  found  that  1  to  3  c.c.  more  silver  will 
be  required  to  give  the  colour  test,  and  this  Hartley  attributes  to  the  precipita- 
tion of  xanthin  bases  by  the  silver  in  a  cold  solution,  which  does  not  take  place 
when  the  solution  is  heated. 

J.W.  Tunnicliffeand  O.Rosenheim*  publish  a  method  which 
may  be  rapidly  performed  when  once  the  uric  acid  is  obtained  as 
ammonium  salt  by  Hopkins's  process.  The  crystals  obtained  by 
decomposing  the  urate  with  HC1  are  washed  free  from  the  latter  on 
a  small  filter  with  repeated  small  proportions  of  water  to  remove 
all  HC1,  the  uric  acid  is  then  rinsed  into  a  flask  with  20  or  30  c.c. 
of  hot  water,  through  a  hole  made  in  the  filter,  and  is  ready  for 
titration. 

METHOD  OF  PROCEDURE  :  This  depends  on  the  fact  that  piperidine  combines 
with  uric  acid  in  molecular  proportions  (4*25  gm.  of  base  equal  8'4  gm.  of  acid)  to 
form  a  soluble  salt.  A  N/2o  solution  of  the  former  is  prepared  by  dissolving 
about  4 '2  gm.  of  piperidine  in  1  litre  of  water,  standardizing  it  on  hydrochloric 
acid  of  equivalent  strength,  phenolphthalein  being  used  as  indicator.  The 
sample  of  uric  acid  separated  from  ammonium  urate  as  above  described  is 
suspended  in  water,  heated  nearly  to  the  boiling-point,  and  the  reagent  run 
in ;  neutrality  being  shown  either  by  the  liquid  becoming  clear  or  by  the  use 
of  phenolphthalein  as  before.  Although  the  solubility  of  the  urate  at  15°  is 
5 '3  per  cent.,  it  is  better  to  employ  hot  solutions  ;  and  there  is  no  danger  of  losing 
any  piperidine  by  volatilization,  as  the  reaction  is  instantaneous. 

Dr.  Edmunds  sends  me  the  following  remarks  as  to  the  deter- 
mination of  uric  acid. 

1.  Chemical  uric  acid  differs  entirely  in  its  habitudes  from  urinary  uric  acid. 
Its  crystalline  form  is  always  uniform  as  chemical  uric  acid — colourless — and 
quite  different  from  urinary  uric  acid,  which,  as  got  from  urine,  is  always  coloured 
yellow-brown,  and  is  protean  in  its  crystalline  forms. 

2.  The  problem  of  titrating  chemical  uric  acid — or  pure  uric  acid — is  not 
quite  the  same  as  that  of  titrating  the  uric  acid  in  urine.     I  am  not  yet  able 
to  say  in  what  the  difference  consists,  and  I  have  often  crystallized  pure  uric 
acid  out  of  iron  and  other  solutions,  but  have  never  been  able  to  colour  uric  acid, 
nor  to  get  it  to  crystallize  again  like  urinary  uric  acid.     The  only  way  in  which 
I  have  succeeded  is  to  add  an  alkaline  solution  of  chemical  urate  of  potash  to 
a  urine  out  of  which  I  had  precipitated  all  its  uric  acid  with  HC1.     In  that  way 
I  found  that  the  uric  acid  took  up  from  the  urine  something  which  gave  it  the 
colouration  and  the  protean  crystalline  form  of  urinary  uric  acid.     I  have  thought 

*  Cenlralbl.  Physiol.    1897. 11.  434. 


432  URINE. 

that  urinary  uric  acid  is  really  a  combination  of  chemical  uric  acid  with  some 
animal  base  or  colouration  of  urine. 

3.  To  purify  urinary  uric  acid  it  should  be  dissolved  (and  thrown  out  by 
dilution)  in  H2S04  three  successive  times.     In  titrating  this  with  permanganate 
I  am  not  prepared  to  give  you  the  reaction,  but  the  practical  point  is  that,  as 
the  permanganate  goes  in  by  drops,  it  is  instantly  decolourized  as  long  as  there 
is  any  uric  acid  present,  and  the  end-point  is  marked  quite  distinctly  (if  you  are 
on  the  look  out  for  it)  by  a  certain  hang  or  hesitation  in  the  decolourization  of 
the  permanganate. 

4.  Fokker's  process,  as  modified  by  Hopkins,  is,  I  think,  the  best.     The 
saturation  with  pure  NH4C1  of  an  acid  urine  (which  should  be  freshly  passed 
and  filtered  at  120°)  throws  out  all  the  uric  acid  as  ammonium  urate.     This  is 
well  set  out  in  Allen's  Chemistry  of  Urine,  p.  168.    But  much  of  the  work  does 
not  say  whether  the  processes  have  been  worked  out  on  the  chemical  uric  acid 
or  on  the  natural  uric  acid,  freshly  obtained  from  urine.     What  we  have  to  deal 
with  in  medicine  is  that  coloured  protean  crystalline  substance  which  comes  out 
constantly  from  urines  on  adding  pure  strong  HC1  and  setting  aside  for  forty- 
eight  hours.     That  is  what  we  get  in  the  uric  acid  diathesis,  in  gout,  and  in 
calculi. 

For  the  determination  of  uric  acid  I  set  aside  100  c.c.  of  fresh  urine,  filtered  at 
about  120°  F.,  and  acidify  it  with  5  %  of  pure  strong  hydrochloric  acid.  At  the 
end  of  forty-eight  hours  a  deposit  of  uric  acid  will  be  seen  at  the  bottom  of  the 
tube,  and  from  this  a  very  good  idea  is  gained  of  the  amount  of  uric  acid  in  the 
urine.  If  closer  quantification  be  wanted,  the  uric  acid  is  collected  on  a  small 
fine  filter  paper,  washed  with  a  few  centimetres  of  ice-cold  distilled  water,  then 
dried  and  weighed,  with  deduction  for  the  filter  paper,  and  with  addition  for  the 
uric  acid  dissolved  in  the  105  c.c.  of  acid  urinary  mother-liquor.  The  amount 
of  uric  acid  contained  in  the  105  c.c.  of  liquid  would  depend  upon  the 
temperature  before  and  at  the  time  of  filtration.  At  33°  F.  it  would  contain 
only  some  2  mgm.,  at  68°  F.  it  would  contain  6  mgm.,  at  212°  F.  it  would  contain 
62-5  mgm. 

8.    Determination  of  Lime  and  Magnesia. 

METHOD  OF  PROCEDTJBE  :  100  c.c.  of  the  urine  are  precipitated  with  ammonia, 
the  precipitate  re-dissolved  in  acetic  acid,  and  sufficient  ammonium  oxalate 
added  to  precipitate  all  the  lime  present  as  oxalate.  The  precipitate  is  allowed 
to  settle  in  a  warm  place,  then  the  clear  liquid  passed  through  a  small  filter,  the 
precipitate  brought  upon  it,  washed  with  hot  water,  the  filtrate  and  washings 
set  aside,  then  the  precipitate,  together  with  the  filter,  pushed  through  the  funnel 
into  a  flask,  some  sulphuric  acid  added,  the  liquid  freely  diluted,  and  titrated 
with  permanganate,  precisely  as  on  p.  172  ;  each  c.c.  of  N/iO  permanganate 
required  represents  0*0028  gm.  of  CaO. 

Or  the  following  method  may  be  adopted  :— 

The  precipitate  of  calcium  oxalate,  after  being  washed,  is  dried,  and  together 
with  the  filter,  ignited  in  a  platinum  or  porcelain  crucible,  by  which  means  it 
is  converted  into  a  mixture  of  calcium  oxide  and  carbonate.  It  is  then  transferred 
to  a  flask  by  the  aid  of  the  washing-bottle,  and  an  excess  of  N/1O  nitric  acid 
delivered  in  with  a  pipette.  The  amount  of  acid,  over  and  above  what  is  required 
to  saturate  the  lime,  is  found  by  N/  caustic  alkali,  each  c.c.  of  acid  being 
equal  to  0-0028  gm.  of  CaO. 

In  examining  urinary  sediment  or  calculi  for  calcium  oxalate,  it  is 
first  treated  with  caustic  potash  to  remove  uric  acid  and  organic 
matter,  then  dissolved  in  sulphuric  acid,  freely  diluted,  and  titrated 
with  permanganate  ;  each  c.c.  of  N/10  solution  represents  0-0054  gm. 
of  calcium  oxalate. 

Magnesia.— The  filtrate  and  washings  from  the  precipitate  of 


URINE. 


433 


calcium  oxalate  are  evaporated  on  the  water-bath  to  a  small  bulk, 
then  made  alkaline  with  ammonia,  sodium  phosphate  added,  and 
set  aside  for  8  or  10  hours  in  a  cool  place  so  that  the  magnesia  may 
separate  as  ammonio-magnesium  phosphate.  The  supernatant 
liquid  is  then  passed  through  a  small  filter,  the  precipitate  brought 
upon  it,  washed  with  ammoniacal  water  in  the  cold,  and  dissolved 
in  acetic  acid,  then  titrated  with  uranium  solution,  as  on  p.  309  ; 
each  c.c.  of  solution  required  represents  0-002815  gm.  of  magnesia. 


9.     Ammonia. 

The  method  hitherto  applied  to  the  determination  of  free  ammonia 
and  its  salts  in  urine  is  that  of  Schlosing,  which  consists  in  placing 
a  measured  quantity  of  the  urine,  to  which  milk  of  lime  is  previously 
added,  under  an  air-tight  bell-glass,  together  with  an  open  vessel 
containing  a  measured  quantity  of  standard  acid.  In  the  course 
of  from  24  to  36  hours  all  the  ammonia  will  have  passed  out  of  the 
urine  into  the  acid,  which  is  then  titrated  with  standard  alkali  to 
find  the  amount  of  ammonia  absorbed. 

One  great  objection  to  this  method  is  the  length  of  time  required, 
since  no  heating  must  be  allowed,  urea  evolving  free  ammonia 
when  heated  with  alkali.  There  is  also  the  uncertainty  as  to  the 
completion  of  the  process  ;  and  if  the  vessel  be  opened  before  the 
absorption  is  complete,  the  analysis  is  spoiled. 


Fig.  60. 

Another  method  which  gives  good  results,  and  occupies  only 
a  short  time,  has  been  devised  by  C.  Wurster*.  The  apparatus 
necessary  for  it  is  shown  in  Fig.  60.  The  principle  of  the  method  is 
the  same  as  Schlosing's,but  the  liberation  of  ammoni a  is  hastened 
by  increase  of  temperature  under  reduced  atmospheric  pressure. 

As  is  well-known,  urea  is  decomposed  when  urine  is  boiled  with 
caustic  alkali  or  alkaline  earth  into  ammonium  carbonate,  but  if  the 


Centralblatt.  /.  Physiologic,  Dec.  1887. 


2   F 


434  URINE. 

operation  is  carried  on  at  50°  C.  and  in  a  vartial  vacuum,  practically 
no  such  decomposition  occurs.  In  fact  a  solution  of  artificial  urea 
gives  off  no  ammonia,  even  when  evaporated  nearly  to  dryness  with 
b  rium,  calcium,  or  magnesium  hydrate,  in  a  vacuum  at  50°  C. 
Owing  to  the  production  of  much  froth  when  urine  is  heated  with 
baryta  or  lime  under  reduced  pressure,  one  flask  for  distillation  is  not 
enough,  although  the  frothing  may  be  reduced  to  some  extent  by 
adding  some  high-boiling  hydro-carbon  such  as  paraffin  or  toluol ; 
but  a  much  safer  plan  is  to  use  two  flasks  as  shown  in  the  figure, 
or  by  using  only  a  small  quantity  of  urine  two  good-sized  boiling 
tubes  will  suffice.  The  boiling-flask  dips  a  small  way  into  the 
water  and  the  second  flask  rests  on  the  bottom  of  the  bath  ;  the  tube 
with  stop-cock  or  burette  clip,  which  simply  enters  below  the  rubber 
stopper  in  this  flask,  allows  air  to  enter  when  the  operation  is  ended 
so  as  to  clear  out  every  trace  of  ammonia.  The  absorption  tube 
containing  standard  acid  must  have  rather  long  side  tubes,  and  the 
whole  must  be  immersed  in  a  beaker  of  cold  water.  The  delivery 
end  of  this  tube  is  connected  with  an  efficient  water  pump,  and  of 
course  all  connections  must  be  perfectly  tight. 

METHOD  OF  PROCEDURE  :  10  to  20  c.c.  of  the  urine  with  10  to  20  c.c.  of 
saturated  barium  hydrate  or  lime  solution,  with  a  little  water,  are  placed  in  the 
distilling  flask,  and  the  water-bath  gradually  heated  up  to  50°  C.,  and  the  whole 
apparatus  is  covered  with  a  cloth  to  avoid  regurgitation  from  cold  air.  When 
two-thirds  of  the  distilling  liquid  have  passed  over,  the  ammonia  will  have  all 
been  absorbed  by  the  standard  acid,  and  the  valve  or  stop-cock  may  be  opened 
while  the  pump  is  still  working  so  as  to  clear  away  the  last  traces  of  vapour. 

The  method  can  be  used  for  liquids  other  than  urine. 
The  following  method  is  available  in  some  cases  : — 
When  a  solution  containing  salts  of  ammonia  is  mixed  with 
a  measured  quantity  of  free  fixed  alkali  of  known  strength,  and 
boiled  until  ammoniacal  gas  ceases  tq  be  evolved,  it  is  found  that 
the  resulting  liquid  has  lost  so  much  of  the  free  alkali  as  corresponds 
to  the  ammonia  evolved  (p.  76)  ;  that  is  to  say,  the  acid  which 
existed  in  combination  with  the  ammonia  in  the  original  liquid 
has  simply  changed  places,  taking  so  much  of  the  fixed  alkali  (potash 
or  soda)  as  is  equivalent  to  the  ammonia  it  has  displaced.  In  the 
case  of  urine  being  treated  in  this  way,  the  urea  will  also  be  decom- 
posed into  free  ammonia,  but  happily  in  such  a  way  as  not  to  interfere 
with  the  determination  of  the  original  amount  of  ammoniacal  salts. 
The  decomposition  is  such  that,  while  free  ammonia  is  evolved  from 
the  splitting  up  of  the  urea,  carbonate  of  fixed  alkali  (say  potash) 
is  formed  in  the  boiling  liquid,  and,  as  this  reacts  as  alkaline  as 
though  it  were  free  potash,  it  does  not  interfere  in  the  slightest 
degree  with  the  determination  of  the  original  ammonia. 

METHOD  OP  PROCEDURE  :  100  c.c.  of  the  urine  are  exactly  neutralized  with 
*Yio  soda  or  potash,  as  for  the  determination  of  free  acid  ;  it  is  then  put  into 
a  flask  capable  of  holding  five  or  six  times  the  quantity  ;  10  c.c.  of  normal  alkali 
added,  and  the  whole  brought  to  boiling,  taking  care  that  the  abundant  froth 
which  is  at  first  formed  does  not  come  over.  After  a  few  minutes  this  subsides, 


URINE.  435 

and  the  boiling  proceeds  quietly.  When  all  ammoniacal  fumes  are  dissipated, 
the  lamp  is  removed,  and  the  flask  allowed  to  cool  slightly ;  the  contents  then 
emptied  into  a  beaker,  and  normal  nitric  acid  delivered  in  from  the  burette  with 
constant  stirring,  until  a  fine  glass  rod  or  small  feather  dipped  in  the  mixture 
and  brought  in  contact  with  violet-coloured  litmus  paper  produces  neither  a  blue 
nor  a  red  spot.  The  number  of  c.c.  of  normal  acid  are  deducted  from  the  10  c.c. 
of  alkali,  and  the  rest  calculated  as  ammonia.  1  c.c.  of  normal  alkali  =0'017 
gm.  of  ammonia. 

It  must  be  borne  in  mind  that  the  plan  just  described  is  not  applicable  to  urine 
which  has  already  suffered  decomposition  by  age  or  other  circumstances  so  as  to 
contain  ammonium  carbonate  ;  in  this  case  it  would  be  preferable  to  adopt  the 
Wurster  or  Schlosing  method. 

10.  Determination  of  Free  Acid. 

The  acidity  of  normal  urine  is  doubtless  due  to  various 
substances,  among  the  most  prominent  of  which  appear  to  be  acid 
sodium  phosphate  (Na  H2  PO4)  and  lactic  acid.  Other  free  organic 
acids  are  probably  in  many  cases  present.  In  these  circumstances, 
the  degree  of  acidity  cannot  be  placed  to  the  account  of  any 
particular  body  ;  nevertheless,  it  is  frequently  desirable  to  ascertain 
its  amount,  which  is  best  done  as  follows  : — 

100  c.c.  of  urine  are  measured  into  a  beaker,  and  N/5p  alkali  delivered  in  from 
a  small  burette,  until  a  thin  glass  rod  or  feather,  moistened  with  the  mixture 
and  streaked  across  some  well-prepared  violet  litmus  paper,  produces  no  change 
of  colour ;  the  degree  of  acidity  is  then  registered  as  being  equal  to  the  quantity 
of  alkali  used. 

Accurate  results  are  obtained  by  the  method  of  Gautier,  in 
which  the  urine  is  made  alkaline  by  standard  caustic  soda  in  known 
quantity,  and  the  phosphates  and  other  salts  precipitated  by  neutral 
barium  chloride.  The  liquid  is  made  up  to  a  definite  volume  with 
distilled  water,  and  when  settled  an  aliquot  portion  is  titrated  with 
standard  acid  and  phenolphthalein. 

11.  Determination  of  Albumen. 

The  accurate  determination  of  this  substance  is  difficult  and 
troublesome.  The  best  process  is  perhaps  that  recommended  by 
Menu. 

METHOD  OF  PROCEDURE  :  100  c.c.  of  the  urine  are  slightly  acidified  with  acetic 
acid,  2  c.c.  of  strong  nitric  acid  are  added  and  the  mixture  thoroughly  agitated. 
10  c.c.  of  a  mixture  of  crystallized  carbolic  acid  1  part,  acetic  acid  1  part,  and 
alcohol  2  parts  are  then  added,  and  the  whole  well  stirred  for  a  few  minutes. 
The  precipitate  is  collected  on  a  small  filter  and  washed  with  a  cold  aqueous 
solution  of  4  per  cent,  of  carbolic  acid  ;  when  fully  washed  the  filter  is  dried,  and 
together  with  the  paper  the  precipitate  is  treated  by  the  Kjeldahl  process,  and 
the  nitrogen  obtained  is  multiplied  by  6*3  for  albumen.  The  presence  of  sugar 
or  much  saline  matter  does  not  affect  the  accuracy  of  results. 

12.    Determination  of  Soda  and  Potash. 

r  50  c.c.  of  urine  are  mixed  with  the  same  quantity  of  baryta  solution,  allowed 
to  stand  a  short  time,  and  filtered  ;  then  80  c.c.  (  =40  c.c.  urine)  measured 

2  F  2 


436  URINE. 

into  a  platinum  dish  and  evaporated  to  dryness  in  the  water-bath  ;  the  residue 
is  then  ignited  to  destroy  all  organic  matter,  and  when  cold  dissolved  in  a  small 
quantity  of  hot  water,  ammonium  carbonate  added  so  long  as  a  precipitate  is 
produced,  filtered  through  a  small  filter,  the  precipitate  washed,  the  filtrate 
acidified  with  hydrochloric  acid  and  evaporated  to  dryness,  then  cautiously 
heated  to  expel  all  ammoniacal  salts.  The  residue  is  then  treated  with  a  little 
water  and  a  few  drops  each  of  ammonia  and  ammonium  carbonate,  filtered,  the 
filter  thoroughly  washed,  the  filtrate  and  washings  received  into  a  tared  platinum 
dish,  then  evaporated  to  dryness,  ignited,  cooled,  and  weighed. 

By  this  means  the  total  amount  of  mixed  sodium  and  potassium 
chlorides  is  obtained.  The  proportion  of  each  is  found  by  titrating 
for  the  chlorine  as  on  p.  142,  and  calculating  as  directed  on  page  144, 
or  the  soda  maybe  determined  direct  by  Fen  ton's  method  (p.  65). 

13.    Determination  of  Total  Nitrogen. 

This  can  now  be  easily  accomplished  by  Kjeldahl's  method 
(p.  83)  and  is  especially  serviceable,  since  it  has  been  found  that 
the  results  of  the  titration  method  for  urea  by  Liebig's  process, 
either  iri  its  original  way  or  by  subsequent  modifications,  cannot 
give  the  true  data  for  calculating  the  total  nitrogen  in  any  given 
specimen  of  urine. 

METHOD  OF  PROCEDURE  :  5  c.c.  of  urine  of  average  concentration  are  measured 
into  a  flask  holding  about  300  c.c.,  together  with  10  c.c.  of  sulphuric  acid,  then 
gradually  heated  to  boiling,  and  the  heat  continued  until  all  vapour  and  gases 
are  given  off  and  the  fluid  possesses  a  clear  yellow  tint.  25  to  30  minutes  generally 
suffice  unless  sugar  is  present  in  tolerable  quantity,  in  which  case  mercuric  oxide 
and  potassium  sulphate  must  be  used,  and  perhaps  more  sulphuric  acid.  The 
flask  is  then  suffered  to  cool,  the  liquid  diluted,  and  distilled  with  caustic  soda 
and  zinc  as  described  on  p.  87. 


WATER   AND    SEWAGE.  437 

ANALYSIS   OP   NATURAL   WATERS   AND   SEWAGE. 

THE  analysis  of  natural  waters  and  sewage  has  from  an  early 
period  received  the  attention  of  chemists,  but  for  long  no  methods 
of  examination  were  produced  which  could  be  said  to  satisfy  the 
demands  of  those  interested  in  the  subject  from  various  points  of 
view,  The  researches  of  Frankland  and  Armstrong,  Miller, 
Wanklyn,  Tidy,  Crookes,  Dewar,  and  others,  have,  however, 
now  brought  the  whole  subject  into  a  more  satisfactory  form,  so  that 
it  may  fairly  be  said  that,  as  regards  accuracy  of  chemical  processes 
or  interpretation  of  results  from  a  chemical  and  sanitary  point  of 
view,  very  little  addition  is  required.  Considerable  space  will  be 
devoted  to  the  matter  here  ;  and  as  most  of  the  processes  are  now 
volumetric,  and  admit  of  ready  and  accurate  results,  the  general 
subject  naturally  falls  within  the  scope  of  this  work.  Care  has  been 
taken  to  render  the  treatment  of  the  matter  practical  and  trust- 
worthy. 

The  bacteriological  examination  of  waters  has  now  been  largely 
developed  and  undoubtedly  is  of  the  greatest  importance,  especially 
with  the  filtered  waters  derived  from  rivers,  lakes,  and  other  sources 
liable  to  be  contaminated  with  unoxidized  surface  or  drainage 
impurities.  This  book  has,  however,  nothing  to  describe  but 
chemical  methods,  and  therefore  no  further  mention  of  bacterial 
investigation  will  be  made. 

THE  PREPARATION  OP  REAGENTS. 

A.    Reagents    required    for    the    Determination    of    Nitrogen 
present  as  Ammonia,  Free  and  Saline,  and  Albuminoid. 

(i)  Nessler's  Solution. — Dissolve  62-5  gm.  of  potassium 
iodide  in  about  250  c.c.  of  distilled  water,  set  aside  a  few  c.c.  and 
add  gradually  to  the  larger  part  a  cold  saturated  solution  of 
mercuric  chloride  (of  which  about  500  c.c.  will  be  required)  until  the 
mercuric  iodide  precipitated  ceases  to  be  redissolved  on  stirring. 
When  a  permanent  precipitate  is  obtained,  restore  the  reserved 
potassium  iodide  so  as  to  redissolve  it,  and  continue  adding  mercuric 
chloride  very  gradually  until  a  slight  precipitate  remains  undissolved. 
(The  small  quantity  of  potassium  iodide  is  set  aside  merely  to 
enable  the  mixture  to  be  made  rapidly  without  danger  of  adding 
an  excess  of  mercury.) 

Next  dissolve  150  gm.  of  solid  potassium  hydrate  (that  usually 
sold  in  sticks  or  cakes)  in  150  c.c.  of  distilled  water,  allow  the  solution 
to  cool,  add  it  gradually  to  the  above  solution,  and  make  up  with 
distilled  water  to  one  litre. 

On  standing,  a  brown  precipitate  is  deposited,  and  the  solution 
becomes  clear,  and  of  a  pale  greenish- yellow  colour.  It  is  ready  for 
use  as  soon  as  it  is  perfectly  clear,  and  should  be  decanted  into 
a  smaller  bottle  as  required.  The  reagent  improves  on  keeping. 


438  WATER  AND   SEWAGE. 

(ii)  Standard  solution  of  ammonium  chloride. — Dissolve  1-9093 
gm.  of  pure  dry  ammonium  chloride  in  a  litre  of  distilled  water  ; 
of  this  take  100  c.c.,  and  make  up  to  a  litre  with  distilled  water. 
The  latter  solution  will  contain  ammonia  corresponding  to  0-00005 
gm.  of  nitrogen  in  each  c.c.  In  use  it  should  be  measured  from  a 
narrow  burette  of  10  c.c.  capacity  divided  into  tenths,  or  from  a 
1  c.c.  pipette. 

[If  it  is  desired  to  determine  "ammonia"  rather'than  "nitrogen  as  ammonia  " 
take  1-5708  gm.  of  ammonium  chloride  instead  of  1'9093  gm.  ;  1  c.c.  will  then 
correspond  to  0*00005  gm.  of  ammonia  (NH3)]. 

(Hi)  Sodium  carbonate. — Heat  anhydrous  sodium  carbonate  in 
a  platinum  crucible  for  about  an  hour,  taking  care  not  to  fuse  it. 
While  still  warm  rub  it  in  a  clean  mortar  so  as  to  break  any  lumps 
which  may  have  been  formed,  and  transfer  to  a  clean  dry  wide- 
mouthed  stoppered  bottle. 

(iv)  Water  free  from  Ammonia. — If,  when  1  c.c.  of  Nessler's 
solution  (A.  i)  is  added  to  100  c.c.  of  distilled  water  in  a  glass 
cylinder,  standing  on  a  white  surface  (see  Determination  of 
Ammonia),  no  trace  of  a  yellow  tint  is  visible  after  five  minutes, 
the  water  is  sufficiently  pure  for  use.  As,  however,  this  is  rarely 
the  case,  the  following  process  must  usually  be  adopted.  Distil 
from  a  large  glass  retort  (or  better,  from  a  copper  or  tin  vessel 
holding  15 — 20  litres)  ordinary  distilled  water  which  has  been 
rendered  distinctly  alkaline  by  addition  of  sodium  carbonate. 
A  glass  Lie  big  condenser  or  a  clean  tin  worm  should  be  used  to 
condense  the  vapour  ;  it  should  be  connected  to  the  still  by  a  short 
india-rubber  joint.  Test  the  distillate  from  time  to  time  with 
Nessler's  solution,  as  above  described,  and  when  free  from  ammonia 
collect  the  remainder  for  use.  The  distillation  must  not  be  carried 
to  dryness.  Ordinary  water  may  be  used  instead  of  distilled  water, 
but  it  occasionally  continues  for  some  time  to  give  off  traces  of 
ammonia  by  the  slow  decomposition  of  the  organic  matter  present  in  it. 

J.  Barnes*  has  pointed  out  that  distilled  water  can  be  completely 
freed  from  ammonia  by  adding  a  little  bromine  and  boiling  for  a  few 
minutes.  More  rapid  is  the  action  of  alkaline  hypobromite  in  the 
cold.  Enough  bromine  is  added  to  the  water  to  give  it  a  perceptible 
tint,  and  then  a  drop  of  sodium  hydroxide  solution  ;  after  ten  minutes 
a  little  potassium  iodide  is  added  to  remove  the  undecomposed 
hypobromite,  and  the  water  is  then  fit  for  use  in  the  determination 
of  ammonia  by  Nessler's  test. 

(v)  Alkaline  Permanganate  Solution.— Dissolve  200  grams  of 
stick  potash  in  water  in  a  large  porcelain  dish  and  add  a  solution 
of  8  grams  of  potassium  permanganate  in  water,  using  1100  c.c. 
altogether.  Boil  rapidly  until  concentrated  to  about  900  c.c., 
add  about  200  c.c.  of  hot  distilled  water,  and  continue  boiling  till 
the  volume  is  reduced  to  a  litre.  When  cool  pour  at  once  into 
a  bottle.  Every  fresh  lot  of  solution  made  should  be  carefully 
tested  before  being  used. 

*J.S.C.  I.I  896, 15,  254-255. 


REAGENTS.  439 

B.    Reagents  required  for  the  Determination  of  Organic 
Carbon  and  Nitrogen. 

(i)  Water  free  from  Ammonia  and  Organic  Matter. — Distilled 
water  to  which  1  gm.  of  potassium  hydrate  and  0*2  gm.  of  potassium 
permanganate  per  litre  have  been  added,  is  boiled  gently  for  about 
twenty-four  hours  in  a  similar  vessel  to  that  used  in  preparing 
water  free  from  ammonia  (A.  iv),  a  reflux  condenser  being  so  arranged 
as  to  return  the  condensed  water.  At  the  end  of  that  time  the 
condenser  is  adjusted  in  the  usual  way,  and  the  water  carefully 
distilled,  the  distillate  being  tested  at  intervals  for  ammonia,  as  in 
preparing  A.  iv.  When  ammonia  is  no  longer  found  the  remainder 
of  the  distillate  may  be  collected,  taking  care  to  stop  short  of  dryness. 
The  neck  of  the  retort  or  still  should  point  slightly  upwards,  so 
that  the  joint  which  connects  it  with  the  condenser  is  the  highest 
point.  Any  particles  carried  up  mechanically  will  then  run  back 
to  the  still,  and  not  contaminate  the  distillate.  The  water  thus 
obtained  should  then  be  rendered  slightly  acid  with  sulphuric  acid, 
and  re-distilled  from  a  clean  vessel  for  use,  again  stopping  short 
of  dryness. 

(ii)  Solution  of  sulphurous  acid. — Sulphurous  anhydride  is  pre- 
pared by  the  action  of  pure  sulphuric  acid  upon  clippings  of  clean 
metallic  copper,  which  have  been  digested  in  the  cold  with  concen- 
trated sulphuric  acid  for  twenty-four  hours  and  then  washed  with 
water.  The  gas  is  made  to  bubble  through  water  to  remove 
impurities  mechanically  carried  over,  and  then  conducted  into 
water  free  from  ammonia  and  organic  matter  (B.  i)  until  a  saturated 
solution  is  obtained. 

(iii)  Solution  of  hydrogen  sodium  sulphite. — Sulphurous 
anhydride,  prepared  and  washed  as  above,  is  passed  into  a  solution 
of  sodium  carbonate  made  by  dissolving  ignited  sodium  carbonate 
(A.  iii)  in  water  free  from  ammonia  and  organic  matter  (B.  i), 
The  gas  is  passed  until  carbonic  anhydride  ceases  to  be  evolved. 

(iv)  Solution  of  ferrous  chloride. — Pure  crystallized  ferrous 
sulphate  is  dissolved  in  water,  precipitated  by  sodium  hydrate,  the 
precipitate  well  washed  (using  pure  water  B.  i  for  the  last  washings) 
and  dissolved  in  the  smallest  possible  quantity  of  pure  hydrochloric 
acid.  Two  or  three  drops  must  not  contain  an  appreciable  quantity 
of  ammonia.  It  is  convenient  to  keep  the  solution  in  a  bottle 
with  a  ground  glass  cap  instead  of  a  stopper,  so  that  a  small  dropping 
tube  may  be  kept  in  it  always  ready  for  use. 

(v)  Cupric  oxide. — Prepared  by  heating  to  redness  with  free 
access  of  air,  on  the  hearth  of  a  reverberatory  furnace  or  in  a  muffle, 
copper  wire  cut  into  short  pieces,  or  copper  sheets  cut  into  strips. 
That  which  has  been  made  by  calcining  the  nitrate  cannot  be  used, 
as  it  appears  to  be  impossible  to  expel  the  last  traces  of  nitrogen. 
After  use,  the  oxide  should  be  extracted  by  breaking  the  combustion 
tube,  rejecting  the  portion  which  was  mixed  with  the  substance 


440  WATER   AND    SEWAGE. 

examined.  As  soon  as  a  sufficient  quantity  has  been  recovered,  it 
should  be  recalcined.  This  is  most  conveniently  done  in  an  iron 
tube  about  30  mm.  in  internal  diameter,  and  about  the  same  length 
as  the  combustion  furnace.  One  end  should  be  closed  with  a  cork, 
the  cupric  oxide  poured  in,  the  tube  placed  in  the  combustion 
furnace  (which  is  tilted  at  an  angle  of  about  15°,  so  as  to  produce 
a  current  of  air),  the  cork  removed,  and  the  tube  kept  at  a  red  heat 
for  about  two  hours.  In  a  Hofmann's  gas  furnace,  with  five 
rows  of  burners,  two  such  tubes  may  be  heated  at  the  same  time  if 
long  clay  burners  are  placed  in  the  outer  rows,  and  short  ones  in 
the  three  inner  rows.  If  the  furnace  has  but  three  rows  of  burners, 
a  rather  smaller  iron  tube  must  be  used.  When  cold,  the  oxide 
can  easily  be  extracted,  if  the  heat  has  not  been  excessive,  by  means 
of  a  stout  iron  wire,  and  should  be  kept  in  a  clean  dry  stoppered 
bottle.  Each  batch  thus  calcined  should  invariably  be  assayed 
by  filling  with  it  a  combustion  tube  of  the  usual  size,  and  treating  it 
in  every  respect  as  an  ordinary  combustion.  It  should  yield  only 
a  very  minute  bubble  of  gas,  which  should  be  almost  wholly  absorbed 
by  potassium  hydrate.  (The  quantity  of  CO2  found  should  not 
correspond  to  more  than  0 '00005  gm.  of  C,  otherwise  the  oxide  must 
be  recalcined).  The  finer  portions  of  the  oxide  should,  after 
calcining,  be  sifted  out  by  means  of  a  sieve  of  clean  copper  gauze, 
and  reserved  for  use  as  described  hereafter. 

New  cupric  oxide  as  obtained  from  the  reverberatory  furnace 
should  be  assayed,  and  if  not  sufficinetly  pure,  as  is  most  usually 
the  case,  calcined  as  above  described,  and  assayed  again. 

(vi)  Metallic  copper. — Fine  copper  gauze  is  cut  into  strips  about 
80  mm.  wide,  and  rolled  up  as  tightly  as  possible  on  a  copper  wire 
so  as  to  form  a  compact  cylinder  80  mm.  long.  This  is  next  covered 
with  a  tight  case  of  moderately  thin  sheet  copper,  the  edges  of  which 
meet  without  overlapping.  The  length  of  the  strip  of  gauze,  and 
the  consequent  diameter  of  the  cylinder,  must  be  so  regulated  that 
it  will  fit  easily,  but  not  too  loosely,  in  the  combustion  tubes.  A 
sufficient  number  of  these  cylinders  being  prepared,  a  piece  of 
combustion  tube  is  filled  with  them,  and  they  are  heated  to  redness 
in  the  furnace,  a  current  of  atmospheric  air  being  passed  through 
them  for  a  few  minutes  in  order  to  burn  off  organic  impurity,  and 
coat  the  copper  gauze  superficially  with  oxide.  A  current  of 
hydrogen,  dried  by  passing  through  strong  sulphuric  acid,  is  then 
substituted  for  the  air,  and  a  red  heat  maintained  until  hydrogen 
issues  freely  from  the  end  of  the  tube.  It  is  then  allowed  to  cool, 
the  current  of  hydrogen  being  continued,  and  when  cold  the  copper 
cylinders  are  removed,  and  kept  in  a  stoppered  bottle.  After 
being  used  several  times  they  must  be  heated  in  a  stream  of  hydrogen 
as  before,  and  are  then  again  ready  for  use.  The  heating  in  air  need 
not  be  repeated. 

(vii)  Solution  of  potassium  dichromate. — This  is  used  as  a  test 
for  and  to  absorb  sulphurous  anhydride  which  may  be  present  in 


REAGENTS.  441 

the  gas  obtained  by  combustion  of  the  water  residue.  It  should 
be  saturated,  and  does  not  require  any  special  attention.  The 
yellow  neutral  chromate  may  also  be  used,  but  must  be  rendered 
slightly  acid,  lest  it  should  absorb  carbonic  as  well  as  sulphurous 
anhydride. 

(viii)  Solution  of  potassium  hydrate. — A  cold  saturated  solution, 
made  by  dissolving  stick  potash  in  distilled  water. 

(ix)  Solution  of  pyrogallic  acid. — A  cold  saturated  solution,  made 
by  dissolving  in  distilled  water  solid  pyrogallic  acid  obtained  by 
sublimation. 

(x)  Solution  of  cuprous  chloride. — A  saturated  solution  of  cupric 
chloride  is  rendered  strongly  acid  with  hydrochloric  acid,  a  quantity 
of  metallic  copper  introduced  in  the  form  of  wire  or  turnings,  and 
the  whole  allowed  to  stand  in  a  closely  stoppered  bottle  until  the 
solution  becomes  colourless. 

(xi)  Oxygen. — Blow  a  bulb  of  about  30  c.c.  capacity  at  the  end 
of  a  piece  of  combustion  tube,  and  draw  out  the  tube  so  that  its 
internal  diameter  for  a  length  of  about  30  mm.  is  about  3  mm. 
This  is  done  in  order  that  the  capacity  of  the  apparatus  apart  from 
the  bulb  may  be  as  small  as  possible.  Cut  the  tube  at  the  wide 
part  about  10  mm.  from  the  point  at  which  the  narrow  tube  com- 
mences, thus  leaving  a  small  funnel-shaped  mouth.  Then  introduce, 
a  little  at  a  time,  dried,  coarsely  powdered,  potassium  chlorate  until 
the  bulb  is  full.  Cut  off  the  funnel,  and,  at  a  distance  of  100  mm. 
from  the  bulb,  bend  the  tube  at  an  angle  of  45°,  and  at  10  mm. 
from  the  end  bend  it  at  right-angles  in  the  opposite  direction.  It 
then  forms  a  retort  and  delivery  tube  in  one  piece,  and  must  be 
adjusted  in  a  mercury  trough  in  the  usual  manner,  taking  care 
that  the  end  does  not  dip  deeper  than  about  20mm.  below  the  surface, 
as  otherwise  the  pressure  of  so  great  a  column  of  mercury  might 
destroy  the  bulb  when  softened  by  heat.  On  gently  heating,  the 
potassium  chlorate  fuses  and  evolves  oxygen.  The  escaping  gas 
is  collected  in  test  tubes  about  150  mm.  long  and  20  mm.  in  diameter, 
rejecting  the  first  60  or  80  c.c.,  which  contain  the  nitrogen  of  the 
air  originally  in  the  bulb  retort.  Five  or  more  of  these  tubes, 
according  to  the  quantity  of  oxygen  required,  are  collected  and 
removed  from  the  mercury  trough  in  very  small  beakers,  the  mercury 
in  which  should  be  about  10  mm.  above  the  end  of  the  test  tube. 
Oxygen  may  be  kept  in  this  way  for  any  desired  length  of  time, 
care  being  taken,  if  the  temperature  falls  considerably,  that  there 
is  sufficient  mercury  in  the  beaker  to  keep  the  mouth  of  the  test 
tube  covered.  About  10  c.c.  of  the  gas  in  the  first  tube  collected  is 
transferred  by  decantation  in  a  mercury  trough  to  another  tube, 
and  treated  with  potassium  hydrate  and  pyrogallic  acid,  when — 
if  after  a  few  minutes  it  is  absorbed,  with  the  exception  of  a  very 
small  bubble — the  gas  in  that  and  the  remaining  tubes  may  be-con- 
sidered  pure.  If  not,  the  first  tube  is  rejected,  and  the  second  tested 
in  the  same  way,  and  so  on. 


442  WATER_AND    SEWAGE. 

(xii)  Hydrogen  metaphosphate. — The  glacial  hydrogen  meta- 
phosphate,  usuaUy  sold  in  sticks,  is  generally  free  from  ammonia, 
or  very  nearly  so.  A  solution  should  be  made  containing  about 
100  gm.  in  a  litre.  It  should  be  so  far  free  from  ammonia  that 
10  c.c.  do  not  contain  an  appreciable  quantity. 

(xiii)  Calcium  phosphate. — Prepared  by  precipitating  common 
disodium  phosphate  with  calcium  chloride,  washing  the  precipitate 
with  water  by  decantation,  drying  and  heating  to  redness  for  an 
hour.  0 

C.    Reagents  required  for  the  Determination  of  Nitrogen  present 
as  Nitrates  and  Nitrites  (C rum's  Process). 

(i)  Concentrated  sulphuric  acid. — This  must  be  free  from  nitrates 
and  nitrites. 

(ii)  Potassium  permanganate.. — Dissolve  about  10  gm.  of  crys- 
tallized potassium  permanganate  in  a  litre  of  distilled  water. 

(iii)  Sodium  carbonate. — Dissolve  about  10  gm.  of  dry,  or  an 
equivalent  quantity  of  crystallized,  sodium  carbonate,  free  from 
nitrates,  in  a  litre  of  distilled  water. 

For  the  Determination  of  Nitrogen  as  Nitrates  and  Nitrites  in  Waters 
containing  a  very  large  quantity  of  Soluble  Matter,  but  little 
Organic  Nitrogen. 

(iv)     Metallic  aluminium. — As  thin  foil. 

(v)  Solution  of  sodium  hydrate. — Dissolve  100  gm.  of  stick 
soda  in  a  litre  of  distilled  water  ;  when  cold,  put  it  in  a  tall  glass 
cylinder,  and  introduce  about  100  sq.  cm.  of  aluminium  foil,  which 
must  be  kept  at  the  bottom  of  the  solution  by  means  of  a  glass  rod. 
When  the  aluminium  is  dissolved,  boil  the  solution  briskly  in  a 
porcelain  basin  until  about  one-third  of  its  volume  has  been  evapor- 
ated, allow  to  cool,  and  make  up  to  its  original  volume  with  water 
free  from  ammonia.  The  absence  of  nitrates  is  thus  ensured. 

(vi)  Broken  pumice. — Clean  pumice  is  broken  in  pieces  of  the 
size  of  small  peas,  sifted  free  from  dust,  heated  to  redness  for  about 
an  hour,  and  kept  in  a  closely  stoppered  bottle. 

^  (vii)  Hydrochloric  acid  free  from  ammonia. — The  acid  sold  as 
"  pure  for  analysis  "  is  nearly  always  quite  free  from  ammoniacal 
contamination.  Only  2  or  3  drops  are  required  for  each  experiment. 

For  the  Determination  of  Nitrites  by  G  r  i  e  s  s  '  s  Process. 

(viii)  Meta  phenylene-diamine. — A  half  per  cent,  solution  of  the 
base  in  very  dilute  sulphuric  or  hydrochloric  acid.  The  base  alone 
is  not  permanent.  If  too  highly  coloured,  it  may  be  bleached  by 
pure  animal  charcoal. 


REAGENTS.  443 

(ix)     Dilute  sulphuric  acid. — One  volume  of  acid  to  two  of  water. 

(x)  Standard  potassium  or  sodium  nitrite. — Dissolve  0-405  gm. 
of  pure  silver  nitrite  in  boiling  distilled  water,  and  add  pure  potassium 
or  sodium  chloride  till  no  further  precipitate  of  silver  chloride  is 
formed.  Make  up  to  a  litre  ;  let  the  silver  chloride  settle,  and  dilute 
100  c.c.  of  the  clear  liquid  to  a  litre.  It  should  be  kept  in  small 
stoppered  bottles  completely  filled,  and  in  the  dark. 

1  c.c.  =0*01  mgm.  N2O3. 

(By  using  T100  gm.  of  silver  nitrites  instead  of  0'405  gm. 
1  c.c.=0'01  mgm.  nitrogen.) 

The  colour  produced  by  the  reaction  of  nitrous  acid  on  meta- 
phenylene-diamine  is  triamidoazo-benzene,  or  "  Bismarck  brown." 

D.    Reagents   required   for   the   Determination    of   combined 

Chlorine. 

(i)  Standard  solution  of  silver  nitrate. — Dissolve  2-4  grams  of 
recrystallized  silver  nitrate  in  a  litre  of  distilled  water  and  standardize 
against  a  solution  of  pure  sodium  chloride  containing  0'8243  gm. 
per  litre  (1  c.c.=0'0005  gm.  chlorine).  1  c.c.  silver  nitrate  solution 
=0'0005  gm.  Cl,  or  when  50  c.c.  of  water  are  titrated,  1  c.c.= 
1  part  of  combined  chlorine  per  100,000. 

(ii)  Solution  of  potassium  chromate. — A  strong  solution  of  pure 
neutral  potassium  chromate  free  from  chlorine.  It  is  most  con- 
veniently kept  in  a  bottle  similar  to  that  used  for  the  solution  of 
ferrous  chloride  (B  iv). 

E.    Reagents  required  for  determination  of  Hardness  by  Clark's 

method. 

(i)  Standard  solution  of  calcium  chloride. — Dissolve  in  dilute 
hydric  chloride,  in  a  platinum  dish,  0'2  gm.  of  pure  crystallized 
calcite,  adding  the  acid  gradually,  and  having  the  dish  covered  with 
a  glass  plate,  to  prevent  loss  by  spirting.  When  all  is  dissolved, 
evaporate  to  dryness  on  a  water-bath,  add  a  little  distilled  water, 
and  again  evaporate  to  dryness.  Repeat  the  evaporation  several 
times  to  ensure  complete  expulsion  of  hydric  chloride.  Lastly, 
dissolve  the  calcium  chloride  in  distilled  water,  and  make  up  to 
one  litre. 

50  c.c.  correspond  to  O'Ol  gm.  CaCO3. 

(ii)  Standard  solution  of  potassium  soap. — Weigh  out  50  grams 
of  commercial  oleic  acid  in  a  beaker  and  add  100  c.c.  of  an  alcoholic 
potash  solution  made  by  dissolving  20  grams  of  stick  potash  in 
180  c.c.  of  industrial  methylated  spirit,  and  continue  adding  the 
same  solution  from  a  burette  till  a  drop  of  the  oleate  just  gives 
a  red  colour  with  phenolphthalein  spotted  on  a  white  plat  —about 


444  WATER   AND    SEWAGE. 

10  c.c.  more  being  required.  Measure  the  solution  and  make  the 
volume  to  400  c.c.  by  the  addition  of  industrial  methylated  spirit. 
45  c.c.  of  the  strong  soap  solution  thus  obtained  are  diluted  with 
methylated  spirit  (2  vols.)  and  water  (1  vol.)  to  a  litre,  allowed  to 
stand  for  about  24  hours,  filtered  through  a  double  Swedish  filter 
and  , standardized  against  standard  calcium  chloride  solution. 
The  solution  will  be  found  a  little  too  strong,  and  is  diluted  as  before 
to  exact  strength,  which  is  attained  when  14*25  c.c.  are  required 
to  form  a  permanent  lather  with  50  c.c.  of  the  standard  calcium 
chloride  solution.  The  process  is  carried  out  exactly  as  in  deter- 
mining the  hardness  of  a  water.  (When  diluting  the  soap  solution 
to  exact  strength,  add  the  requisite  amount  of  methylated  spirit 
and  water  mixed — not  separately.) 


F.    Reagents  required  for  the  determination  of  Oxygen  absorbed. 

Standard  solution  of  potassium  permanganate.  Dissolve  0'395 
gin.  of  pure  potassium  permanganate  in  1000  c.c.  of  water.  Each 
c.c.  contains  O'OOOl  gm.  of  available  oxygen. 

Potassium  iodide  solution. — One  part  of  the  pure  salt  dissolved  in 
ten  parts  of  distilled  water. 

Dilute  sulphuric  acid. — One  part  by  volume  of  pure  sulphuric 
acid  is  mixed  with  three  parts  by  volume  of  distilled  water,  and 
solution  of  potassium  permanganate  dropped  in  until  the  whole 
retains  a  very  faint  pink  tint,  after  warming  to  80°  F.  for  four  hours. 

Sodium  thiosulphate. — One  gram  of  the  pure  crystallized  salt 
dissolved  in  1000  c.c.  of  water. 

Starch  indicator. — The  best  form  in  which  to  use  this  is  the 
solution  described  on  page  131. 


THE  ANALYTICAL  PROCESSES. 

The  various  determinations  usually  required  in  the  analysis  of 
samples  of  water,  sewage,  and  sewage  effluents  will  be  dealt  with  in 
the  following  order. 

*1.     The  determination  of  organic  carbon  and  nitrogen  (p.  446). 

total  solid  matter  (p.  462). 

3.  „  ,,  Nitrogen  as  nitrates  and  nitrites  (p.  463). 

(i)  by  Crum's  method  (p.  463). 
(ii)  by  Schulze's  method  (p.  465). 
(iii)  by  the  Copper-zinc  couple  (p.  466). 
(iv)  by  Sprengel's  method  (p.  468). 

4.  ,,  ,,  Nitrogen  as  nitrite  (p.  470). 

(i)  by  Griess's  method  (p.  470). 
(ii)byGriess-Ilosvaymethod(p.  470). 
»  ,,  Suspended  matter  (p.  471). 

6.          ,,  ,,  Combined  chlorine  (p.  472). 

*  Seldom  used  at  the  present  time. 


INDEX   TO   PROCESSES.  445 

7.  The  determination  of  hardness  (p.  473). 

8.  ,,  ,,  mineral  constituents  and  metals  (p.  476). 

9.  ,,  ,,  oxygen  absorbed  (p.  484). 

10.  ,,  ,,  free  and  saline  ammonia  and  of  albumi- 

noid ammonia  (Wanklyn's  method) 
(p.  488). 

11.  ,,  „  organic  nitrogen  (Kjeldahl)  (p.  490). 

12.  „  ,,  chlorine,   nitrates,   etc.,  in  mossy  and 

peaty  waters  (p.  491). 

13.  ,,  ,,  dissolved  oxygen  in  waters  and  sewage 

effluents  (vide  ante  pp.  290-305). 

14.  Microscopical  examination  of  deposit  (p.  493). 

15.  Method  of  recording  water  and  sewage  examination  results 

(p.  493). 

16.  Interpretation  of  the  results  of  analysis  (p.  497). 

17.  Standards  for  sewage  effluents  (p.  495). 

18.  Examples  of  analyses  of  effluents  (p.  496). 

19.  Characteristics    of    waters    derived    from    various    geological 

formations  (p.  496). 

20.  Determining  the  hardness  of  water  (H  e  h  n e  r's  process  modified) 

(p.  479). 

21.  Examples  of  analyses  of  waters  of  various  kinds  (pp.  474,  5). 

NOTE. — All  tables  required  in  water  analysis  will  be  found  at  the  end  of  the  book. 

1.  Collection  of  Samples. — The  points  to  be  considered  under 
this  head  are,  the  vessel  to  be  used,  the  quantity  of  water  required, 
and  the  method  of  ensuring  a  truly  representative  sample. 

Stoneware  bottles  should  be  avoided,  as  they  are  apt  to  affect  the 
hardness  of  the  water,  and  are  more  difficult  to  clean  than  glass. 
Stoppered  glass  bottles  should  be  used  it  possible  ;  those  known  as 
"  Winchester  Quarts,"  which  hold  about  two  and  a  half  litres  each, 
are  very  convenient  and  easy  to  procure.  One  of  these  will  contain 
sufficient  for  the  general  analysis  of  sewage  and  largely  polluted 
rivers,  two  for  well  waters  and  ordinary  rivers  and  streams,  and 
three  for  lakes  and  mountain  springs.  If  a  more  detailed  analysis  is 
required,  of  course  a  larger  quantity  must  be  taken. 

If  corks  must  be  used,  they  should  be  new,  and  well  washed  with 
the  water  at  the  time  of  collection. 

In  collecting  from  a  well,  river,  or  tank,  plunge  the  bottle  itself, 
if  possible,  below  the  surface  ;  but  if  an  intermediate  vessel  must  be 
used,  see  that  it  is  thoroughly  clean  and  weU  rinsed  with  the  water. 
Avoid  the  surface  water  and  also  any  deposit  at  the  bottom. 

If  the  sample  is  taken  from  a  pump  or  tap,  take  care  to  let  the 
water  which  has  been  standing  in  the  pump  or  pipe  run  off  before 
collecting,  then  allow  the  stream  to  flow  directly  into  the  bottle. 
If  it  is  to  represent  a  town  water-supply,  take  it  from  the  service 
pipe  communicating  directly  with  the  street  main,  and  not  from 
a  cistern. 

In  every  case,  first  fill  the  bottle  completely  with  the  water,  thus 


446  WATER   AND    SEWAGE. 

expelling  all  gases  and  vapours,  empty  it  again,  rinse  once  or  twice 
carefully  with  water,  and  then  fill  it  nearly  to  the  stopper,  and 
tie  down  tightly. 

At  the  time  of  collection  note  the  source  of  the  sample,  whether 
from  a  deep  or  shallow  well,  a  river  or  spring,  and  also  its  local 
name,  so  that  it  may  be  clearly  identified. 

If  it  is  from  a  well,  ascertain  the  nature  of  the  soil,  subsoil,  and 
water-bearing  stratum  ;  the  depth  and  diameter  of  the  well,  its 
distance  from  neighbouring  cesspools,  drains,  or  other  sources  of 
pollution  ;  whether  it  passes  through  an  impervious  stratum  before 
entering  the  water-bearing  stratum,  and  if  so,  whether  the  sides  of 
the  well  above  this  are,  or  are  not,  water-tight. 

If  the  sample  is  from  a  river,  ascertain  the  distance  from  the  source 
to  the  point  of  collection  ;  whether  any  pollution  takes  place  above 
that  point,  and  the  geological  nature  of  the  district  through  which 
it  flows. 

If  it  is  from  a  spring,  take  note  of  the  stratum  from  which  it  issues. 

2.  Preliminary  Observations. — In  order  to  ensure  uniformity, 
the  bottle  should  invariably  be  well  shaken  before  taking  out 
a  portion  of  the  sample  for  any  purpose.     The  colour  should  be 
observed  through  a  two -foot  tube  with  plate  glass  ends  half -filled 
with  the  sample,  placed  in  a  horizontal  position,  and  a  well-illumi- 
nated white  surface  observed  through  it.     It  is  well  to  compare  it 
with  distilled  water  in  a  similar  vessel.     The  taste  and  odour  are 
most  easily  detected  when  the  water  is  heated  to  30°-35°  C. 

Before  commencing  the  quantitative  analysis  it  is  necessary  to 
decide  whether  the  water  shall  be  filtered  or  not  before  analysis. 
This  must  depend  on  the  purpose  for  which  the  examination  is 
undertaken.  As  a  general  rule,  if  the  suspended  matter  is  to  be 
determined,  the  water  should  be  filtered  before  the  determination 
of  organic  carbon  and  nitrogen,  nitrogen  as  ammonia,  and  total 
solid  residue  ;  if  otherwise,  it  should  merely  be  shaken  up.  If  the 
suspended  matter  is  not  determined,  the  appearance  of  the  water, 
as  to  whether  it  is  clear  or  turbid,  should  be  noted.  This  is  con- 
veniently done  when  measuring  out  the  quantity  to  be  used  for  the 
determination  of  carbon  and  nitrogen.  If  the  measuring  flask  be 
held  between  the  eye  and  a  good  source  of  light,  but  with  an  opaque 
object  such  as  a  window  bar,  in  the  line  drawn  from  the  eye  through 
the  centre  of  the  flask,  any  suspended  particles  will  be  seen  well 
illuminated  on  a  dark  ground. 

Water  derived  from  a  newly  sunk  well,  or  one  which  has  been 
rendered  turbid  by  the  introduction  of  innocuous  mineral  matter 
from  some  temporary  and  exceptional  cause,  should  be  filtered, 
but  the  suspended  matter  in  such  cases  need  not  usually  be 
determined.  The  introduction  of  organic  matter  of  any  kind  would 
almost  always  render  the  sample  useless. 

3.  Determination  of  Organic  Carbon  and  Nitrogen. — This  should 
be  commenced  as  soon  as  the  nitrogen  as  ammonia  has  been  deter- 


ORGANIC   CARBON   AND    NITROGEN. 


447 


mined.  If  that  is  less  than  0*05  part  per  100,000,  a  litre  should  be 
used  ;  if  more  than  0'05,  and  less  than  0*2,  half  a  litre  ;  if  more 
than  0-2,  and  less  than  TO,  a  quarter  of  a  litre  ;  if  more  than  1-0, 
a  hundred  c.c.,  or  less.  These  quantities  are  given  as  a  guide  in 
dealing  with  ordinary  waters  and  sewage,  but  subject  to  variation 
in  exceptional  cases.  A  quantity  which  is  too  large  should  be 
avoided  as  entailing  needless  trouble  in  evaporation,  and  an  incon- 
veniently bulky  residue  and  resulting  gas.  If  it  is  to  be  filtered 
before  analysis,  the  same  precaution  as  to  filter  paper  must  be  taken 
as  for  the  determination  of  nitrogen  as  ammonia,  the  same  filter 
being  generally  used. 


Fig.  61. 


Fig.  62. 


Having  measured  the  quantity  to  be  used,  add  to  it  in  a  capacious 
flask  15  c.c.  of  the  solution  of  sulphurous  acid  (B.  ii),  and  boil 
briskly  for  a  few  seconds,  in  order  to  decompose  the  carbonates 
present.  Evaporate  to  dry  ness  in  a  hemispherical  glass  dish,  about 
a  decimetre  in  diameter,  and  preferably  without  a  lip,  supported 
in  a  copper  dish  with  a  flange  (fig.  61  d  e).  The  flange  has  a  diameter 
of  about  14  centimetres,  is  sloped  slightly  towards  the  centre,  and 
has  a  rim  of  about  5  mm.  turned  up  on  its  edge,  except  at  one  point, 


448  WATER   AND    SEWAGE. 

where  a  small  lip  is  provided.  The  concave  portion  is  made  to  fit 
the  contour  of  the  outside  of  the  glass  dishes,  and  is  of  such  a  depth 
as  to  allow  the  edge  of  the  dish  to  rise  about  15  mm.  above  the 
flange.  The  diameter  of  the  concavity  at  /  is  about  90  mm.,  and 
the  depth  at  g  about  30  mm.  A  thin  glass  shade,  such  as  is  used  to 
protect  statuettes,  about  30  centimetres  high,  stands  on  the  flange 
of  the  copper  dish,  its  diameter  being  such  as  to  fit  without  difficulty 
on  the  flange,  and  leave  a  sufficient  space  between  its  interior 
surface  and  the  edge  of  the  glass  dish.  The  copper  dish  is  supported 
on  a  steam  or  water  bath,  and  the  water  as  it  evaporates  is  condensed 
on  the  interior  of  the  glass  shade,  runs  down  into  the  copper  dish, 
filling  the  space  between  it  and  the  glass  dish,  and  then  passes  off 
by  the  lip  at  the  edge  of  the  flange,  a  piece  of  tape  held  by  the  edge 
of  the  glass  shade,  and  hanging  over  the  lip,  guiding  it  into  a  vessel 
placed  to  receive  it. 

We  are  indebted  to  Bischof  for  an  improved  apparatus  for 
evaporation,  which  by  keeping  the  dish  always  full  by  a  self-acting 
contrivance  permits  the  operation  to  proceed  without  attention 
during  the  night,  and  thus  greatly  reduces  the  time  required.  This 
form  of  apparatus  is  shown  in  fig.  62.  The  glass  dish  d  is  supported 
by  a  copper  dish  e,  as  described  above,  and  resting  on  the  latter  is 
a  stout  copper  ring  h,  which  is  slightly  conical,  being  115  mm.  in 
diameter  at  the  top  and  130  at  the  bottom.  At  the  top  is  a  narrow 
flange  of  about  10  mm.  with  a  vertical  rim  of  about  5  mm.  The 
diameter  across  this  flange  is  the  same  as  the  diameter  of  the  dish  e, 
so  that  the  glass  shade  i  will  fit  securely  either  on  h  or  e.  The 
height  of  the  conical  ring  is  about  80  mm. 

The  automatic  supply  is  accomplished  on  the  well-known  principle 
of  the  bird  fountain,  by  means  of  a  delivery  tube  6,  the  upper  end  of 
which  is  enlarged  to  receive  the  neck  of  the  flask  a  containing  the 
water  to  be  evaporated,  the  joint  being  carefully  ground  so  as  to  be 
water-tight.  The  upper  vertical  part  of  6,  including  this  enlarge- 
ment, is  about  80  mm.  in  length,  and  the  sloping  part  about  260  mm., 
with  a  diameter  of  13  mm.  The  lower  end  which  goes  into  the  dish 
is  again  vertical  for  about  85  mm.  and  carries  a  side  tube  c  of  about 
3  mm.  internal  diameter,  by  which  air  enters  the  delivery  tube 
whenever  the  level  of  the  water  in  the  dish  falls  below  the  point  at 
which  the  side  tube  joins  the  delivery  tube.  The  distance  from 
this  point  to  the  end  of  the  tube  which  rests  on  the  bottom  of  the 
dish  at  g,  and  is  there  somewhat  contracted,  is  about  30  mm. 
The  side  tube  c  should  not  be  attached  on  the  side  next  the  flask, 
as  if  so  the  inclined  part  of  b  passes  over  its  mouth  and  renders  it 
very  difficult  to  clean.  Mills  prevents  circulation  of  liquid  in  the 
sloping  part  of  the  tube  by  bending  it  into  a  slightly  undulating 
form,  so  that  permanent  bubbles  of  air  are  caught  and  detained  at 
two  points  in  it.  The  flask  a  should  hold  about  1200  c.c.,  and 
have  a  rather  narrow  neck — about  20  mm. — and  a  flat  bottom. 
A  small  slot  is  cut  in  the  upper  edge  of  the  copper  ring  h  to 
accommodate  the  delivery  tube,  as  shown  in  fig.  60.  Its  size  and 


ORGANIC    CARBON    AND    NITROGEN.  449 

shape  should  be  such  that  the  tube  does  not  touch  the  edge  of  the 
glass  shade  i,  lest  water  running  down  the  inner  surface  of  the  shade 
should  find  its  way  down  the  outside  of  the  delivery  tube  into  the 
dish.  This  being  avoided,  the  opening  should  be  as  closely  adjusted 
to  the  size  of  the  delivery  tube  as  can  be.  The  copper  dish  e  should 
rest  on  a  steam  or  water  bath,  so  that  only  the  spherical  part  is 
exposed  to  the  heat. 

After  the  addition  of  the  15  c.c.  of  sulphuric  acid,  the  water  may 
either  be  boiled  in  the  flask  a,  or  in  another  more  capacious  one, 
and  then  transferred  to  a.  It  should  be  allowed  to  cool  before  the 
delivery  tube  is  adjusted,  otherwise  the  joint  between  the  two  is 
liable  to  become  loose  by  expansion  of  the  cold  socket  of  the  delivery 
tube,  after  being  placed  over  the  hot  neck  of  the  flask. 

The  glass  dish  having  been  placed  on  the  copper  dish  e,  the  conical 
ring  h  is  fitted  on,  and  the  flask  with  the  delivery  tube  attached 
inverted,  as  shown  in  fig.  61,  a  b.  This  should  not  be  done  too 
hurriedly,  and  with  a  little  care  there  is  no  risk  of  loss.  The  flask  is 
supported  either  by  a  large  wooden  filtering  stand,  the  ring  of  which 
has  had  a  slot  cut  in  it  to  allow  the  neck  of  the  flask  to  pass  or  by 
a  clamp  applied  to  the  upper  end  of  the  delivery  tube  where  the  neck 
of  the  flask  fits  in.  The  delivery  tube  having  been  placed  in  the 
slot  made  to  receive  it,  the  glass  shade  is  fitted  on,  and  the  evapora- 
tion allowed  to  proceed.  When  all  the  water  has  passed  from  the 
flask  into  the  dish,  the  flask  and  delivery  tube  and  the  conical  ring 
h  may  be  removed,  and  the  glass  placed  directly  on  the  dish  e  until 
the  evaporation  is  complete.  If  the  water  is  expected  to  contain 
a  large  quantity  of  nitrates,  two  or  three  drops  of  ferrous  chloride 
(B.  iv)  should  be  added  to  the  first  dishful ;  and  if  it  contains  little 
or  no  carbonate,  one  or  two  c.c.  of  hydrogen  sodium  sulphite  (B.  iii). 
The  former  facilitates  the  destruction  of  nitrates  and  nitrites,  and 
the  latter  furnishes  base  for  the  sulphuric  acid  produced  by  oxidation 
of  the  sulphurous  acid,  and  which  would,  if  free,  decompose  the 
organic  matter  when  concentrated  by  evaporation.  An  estimate  of 
the  quantity  of  carbonate  present,  sufficiently  accurate  for  this 
purpose,  may  generally  be  made  by  observing  the  quantity  of  pre- 
cipitate thrown  down  on  addition  of  sodium  carbonate  in  the  deter- 
mination of  nitrogen  as  ammonia. 

With  sewages  and  very  impure  waters  (containing  upwards  of  0*1 
part  of  nitrogen  as  ammonia  per  100,000  for  example)  such  great 
precaution  is  hardly  necessary,  and  the  quantity  to  evaporate  being 
small,  the  evaporation  may  be  conducted  in  a  glass  dish  placed 
directly  over  a  steam  bath,  and  covered  with  a  drum  or  disc  of  filter 
paper  made  by  stretching  the  paper  by  means  of  two  hoops  of  light 
split  cane,  one  thrust  into  the  other,  the  paper  being  between  them, 
in  the  way  often  employed  in  making  dialyzers.  This  protects  the 
contents  of  the  dish  from  dust,  and  also  to  a  great  extent,  from 
ammonia  which  may  be  in  the  atmosphere,  and  which  would  impair 
the  accuracy  of  the  results  *  As  a  glass  dish  would  be  in  some  danger 
of  breaking  by  the  introduction  of  cold  water,  the  flask  containing 

2  G 


450  WATER   AND    SEWAGE. 

the  water  being  evaporated  in  this  or  in  the  first  described  manner, 
must  be  kept  on  a  hot  plate  or  sand  bath  at  a  temperature  of  about 
60°  or  70°  C.,  and  should  be  covered  with  a  watch-glass.  This 
precaution  is  not  necessary  when  Bischof's  apparatus  is  used. 
If,  at  any  time,  the  water  in  the  flask  ceases  to  smell  strongly  of 
sulphurous  acid,  more  should  be  added.  The  preliminary  boiling 
may  be  omitted  when  less  than  250  c.c.  is  used.  When  the  nitrogen 
as  nitrates  and  nitrites  exceeds  0*5  part,  the  dish,  after  the  evapora- 
tion has  been  carried  to  dryness,  should  be  filled  with  distilled 
water  containing  ten  per  cent,  of  saturated  sulphurous  acid  solution, 
and  the  evaporation  again  carried  to  dryness.  If  it  exceeds  1*0 
part,  a  quarter  of  a  litre  of  this  solution  should  be  evaporated  on 
the  residue  ;  if  2-0  parts,  half  a  litre  ;  and  if  5  parts,  a  litre  If 
less  than  a  litre  has  been  evaporated,  a  proportionally  smaller 
volume  of  this  solution  may  be  used.  The  determination  of  nitrogen 
as  nitrates  and  nitrites  will  usually  be  accomplished  before  this  stage 
of  the  evaporation  is  reached. 

M.  W.  Williams*  proposes  to  avoid  the  use  of  sulphurous  acid, 
with  its  acknowledged  disadvantages  and  defects,  by  removing  the 
nitric  and  nitrous  acids  with  the  zinc-copper  couple  and  converting 
them  into  ammonia.  If  the  amount  is  large,  it  is  best  distilled  from 
a  retort  into  weak  acid  ;  if  small,  into  an  empty  Nessler  tube.  The 
amount  so  found  is  calculated  into  nitrogen  as  nitrates  and  nitrites, 
if  the  latter  are  found  in  the  water.  The  residue,  when  free  from 
ammonia,  is  further  concentrated,  the  separated  carbonates  re-dis- 
solved in  phosphoric  or  sulphurous  acid,  in  just  sufficient  quantity, 
then  transferred  to  a  glass  basin  for  evaporation  to  dryness  as  usual 
ready  for  combustion. 

In  the  case  of  sewage,  however,  it  is  advisable  to  employ  hydrogen 
metaphosphate  in  the  place  of  sulphurous  acid,  as  the  ammonium 
phosphate  is  even  less  volatile  than  the  sulphite.  This  can  only  be 
employed  for  sewage  and  similar  liquids  which  are  free  from  nitrates 
and  nitrites.  To  the  measured  quantity  of  liquid  to  be  evaporated 
add,  in  the  glass  dish,  10  c.c.  of  the  hydric  metaphosphate  (B.  xii), 
and,  in  order  to  render  the  residue  more  convenient  to  detach  from 
the  dish,  about  half  a  grain  of  calcium  phosphate  (B.  xiii),  and 
proceed  as  usual.  No  ferrous  chloride,  sulphurous  acid,  or  sodium 
sulphite  is  required ;  nor  is  it  necessary  to  boil  before  commencing 
the  evaporation. 

The  next  operation  is  the  combustion  of  the  residue.  The 
combustion  tube  should  be  of  hard,  difficultly  fusible  glass,  with  an 
internal  diameter  of  about  10  mm.  Cut  it  in  lengths  of  about 
430  mm.,  and  heat  one  end  of  each  in  the  blowpipe  flame  to  round 
the  edge.  Wash  well  with  water,  brushing  the  interior  carefully 
with  a  tube  brush  introduced  at  the  end  whose  edge  has  been 
rounded,  rinse  with  distilled  water,  and  dry  in  an  oven.  When  dry, 
draw  off  and  close,  at  the  blowpipe,  the  end  whose  edge  has  been 
left  sharp.  The  tube  is  then  ready  for  use. 

*  J.  C.  S.  1881,  144. 


ORGANIC    CARBON    AND    NITROGEN. 


451 


Pour  on  to  the  perfectly  dry  residue  in  the  glass  dish,  standing  on 
a  sheet  of  white  glazed  paper,  a  little  of  the  fine  cupric  oxide  (B.  v), 
and  with  the  aid  of  a  small  elastic  steel  spatula  (about  100  mm.  long 
and  15  mm.  wide)  carefully  detach  the  residue  from  the  glass  and 
rub  it  down  with  the  cupric  oxide.  The  spatula  readily  accom- 


*=  *• 


\ 


7 


Fig.  63. 

modates  itself  to  the  curvature  of  the  dish,  and  effectually  scrapes 
its  surface.  When  the  contents  of  the  dish  are  fairly  mixed,  fill 
about  30  mm.  of  the  length  of  the  combustion  tube  with  granulated 
cupric  oxide  (B.  v),  and  transfer  the  mixture  in  the  dish  to  the 

2  G  2 


452  WATER   AND    SEWAGE. 

tube.  This  is  done  in  the  usual  way  by  a  scooping  motion  of  the 
end  of  the  tube  in  the  dish,  the  last  portions  being  transferred  by 
the  help  of  a  bent  card  or  a  piece  of  clean  and  smooth  platinum 
foil.  Rinse  the  dish  twice  with  a  little  fine  cupric  oxide,  rubbing 
it  well  round  each  time  with  the  spatula,  and  transfer  to  the  tube 
as  before.  Any  particles  scattered  on  the  paper  are  also  to  be  put 
in.  Fill  up  to  a  distance  of  270  mm.  from  the  closed  end  with 
granular  cupric  oxide,  put  in  a  cylinder  of  metallic  copper  (B.  vi), 
and  then  again  20  mm.  of  granular  cupric  oxide.  This  last  is  to 
oxidize  any  traces  of  carbonic  oxide  which  might  be  formed  from 
carbonic  anhydride  by  the  reducing  action  of  iron  or  other  impurity 
in  the  metallic  copper.  Now  draw  out  the  end  of  the  tube  so  as  to 
form  a  neck  about  100  mm.  long  and  4  mm.  in  diameter,  fuse  the 
end  of  this  to  avoid  injury  to  the  india-rubber  connector,  and  bend 
it  at  right-angles.  It  is  now  ready  to  be  placed  in  the  combustion 
furnace  and  attached  to  the  Sprengel  pump. 

The  most  convenient  form  of  this  instrument  for  the  purpose  is 
shown  in  fig.  63.  The  glass  funnel  a  is  kept  supplied  with  mercury, 
and  is  connected  by  a  caoutchouc  joint  with  a  long  narrow  glass  tube 
which  passes  down  nearly  to  the  bottom  of  a  wider  tube  d,  900  mm. 
long  and  10  mm.  in  internal  diameter.  The  upper  end  of  d  is 
cemented  into  the  throat  of  a  glass  funnel  c  from  which  the  neck  has 
been  removed.  A  screw  clamp  b  regulates  the  flow  of  mercury  down 
the  narrow  tube.  A  piece  of  ordinary  glass  tube  /  g,  about  6  mm.  in 
diameter  and  600  mm.  in  length,  is  attached  to  g  to  a  tube  g  h  k, 
about  6  mm.  in  diameter,  1500  mm.  long,  wiiii  a  bote  of  1  mm. 
This  is  bent  sharply  on  itself  at  h,  the  part  h  k  being  1300  mm.  long, 
and  the  two  limbs  are  firmly  lashed  together  with  copper  wire  at 
two  points,  the  tubes  being  preserved  from  injury  by  short  sheaths 
of  caoutchouc  tube.  The  end  k  is  recurved  for  the  delivery  of  gas. 
At  the  top  of  the  bend  at  h,  a  piece  of  ordinary  tube  h  I,  about 
120  mm.  long  and  5  mm.  in  diameter,  is  sealed  on.  The  whole 
I  k  is  kept  in  a  vertical  position  by  a  loose  support  or  guide,  near 
its  upper  part,  the  whole  of  its  weight  resting  on  the  end  k,  so  that 
it  is  comparatively  free  to  move.  It  is  connected  at  /  with  the  lower 
end  of  d,  by  means  of  a  piece  of  caoutchouc  tube  covered  with  tape, 
and  furnished  with  a  screw  clamp  e.  At  I  it  is  connected  with  the 
combustion  tube  o,  by  the  connecting  tube  I  m  n,  which  is  made  of 
tube  similar  to  that  used  for  h  k.  A  cork  slides  on  h  I,  which  is 
fitted  into  the  lower  end  of  a  short  piece  of  tube  of  a  width  sufficient 
to  pass  easily  over  the  caoutchouc  joint  connecting  the  tubes  at  I. 
After  the  joint  has  been  arranged  (the  ends  of  the  tubes  just  touching) 
and  bound  with  wire,  the  cork  and  wide  tube  are  pushed  over  it 
and  filled  with  glycerine.  The  joint  at  n  is  of  exactly  the  same  kind, 
but  as  it  has  to  be  frequently  disconnected,  water  is  used  instead 
of  glycerine,  and  the  caoutchouc  is  not  bound  on  to  the  combustion 
tube  with  wire.  It  will  be  seen  that  the  joint  at  I  is  introduced 
chiefly  to  give  flexibility  to  the  apparatus.  At  m  is  a  smaU  bulb 
blown  on  the  tube  for  the  purpose  of  receiving  water  produced  in 


ORGANIC   CARBON   AND    NITROGEN.  453 

the  combustion.  This  is  immersed  in  a  small  water  trough  x. 
The  tube  h  k  stands  in  a  mercury  trough  p,  which  is  shown  in  plan 
on  a  larger  scale  at  B. 

This  trough  should  be  cut  of  a  solid  piece  of  mahogany,  as  it  is 
extremely  difficult  to  make  joints  to  resist  the  pressure  of  such 
a  depth  of  mercury.  It  is  200  mm.  long,  155  mm.  wide,  and  100 
mm.  deep,  outside  measurement.  The  edge  r  r  is  13  mm.  wide, 
and  the  shelf  s  65  mm.  wide,  174  mm.  long,  and  50  mm.  deep  from 
the  top  of  the  trough.  The  channel  t  is  25  mm.  wide  and  75  mm. 
deep,  having  at  one  end  a  circular  well  w,  42  mm.  in  diameter,  and 
90  mm.  deep.  The  recesses  u  u  are  to  receive  the  ends  of  two 
Sprengel  pumps.  They  are  each  40  mm.  long,  25  mm,  wide,  and 
of  the  same  depth  as  the  channel  t.  A  short  iron  wire  c,  turning  on 
a  small  staple,  and  resting  at  the  other  end  against  an  iron  pin, 
stretches  across  each  of  these,  and  serves  as  a  kind  of  gate  to  support 
the  test  tube,  in  which  the  gas  delivered  by  the  pump  is  collected. 
The  trough  stands  upon  four  legs,  75  mm.  high,  and  is  provided  at 
the  side  with  a  tube  and  screw  clamp  g,  by  which  the  mercury  may 
be  drawn  off  to  the  level  of  the  shelf  s. 

The  combustion  tube  being  placed  in  the  furnace,  protected  from 
the  direct  action  of  the  flame  by  a  sheet-iron  trough  lined  with 
asbestos,  and  the  water  joint  at  n  adjusted,  the  gas  is  lighted  at  the 
front  part  of  furnace  so  as  to  heat  the  whole  of  the  metallic  copper 
and  part  of  the  cupric  oxide.  A  small  screen  of  sheet  iron  is 
adjusted  astride  the  combustion  tube  to  protect  -the  part  beyond 
the  point  up  to  which  the  gas  is  burning  from  the  heat. 

At  the  same  time  a  stream  of  mercury  is  allowed  to  flow  from  the 
funnel  a,  which  fills  the  tubes  d  and  /  until  it  reaches  h,  when  it  falls 
in  a  series  of  pellets  down  the  narrow  tube  h  k,  each  carrying  before 
it  a  quantity  of  air  drawn  from  the  combustion  tube.  The  flow  of 
mercury  must  be  controlled  by  means  of  the  clamps  b  and  e,  so  as 
not  to  be  too  rapid  to  admit  of  the  formation  of  these  separate 
pistons,  and  especially,  care  should  be  taken  not  to  permit  it  to  go 
so  fast  as  to  mount  into  the  connecting  tube  I  m  n,  as  it  cannot  be 
removed  thence  except  by  disconnecting  the  tube.  During  the 
exhaustion,  the  trough  x  is  filled  with  hot  water  to  expel  from  the 
bulb  m  any  water  condensed  from  a  previous  operation.  In  about 
ten  minutes  the  mercury  will  fall  in  the  tube  h  k  with  a  loud,  sharp, 
clicking  sound,  showing  that  the  vacuum  is  complete.  As  soon  as 
this  occurs,  the  pump  may  be  stopped,  a  test  tube  filled  with  mercury 
inverted  over  the  delivery  end  of  the  tube  k,  cold  water  substituted 
for  hot  in  the  trough  x,  the  iron  screen  removed,  and  combustion 
proceeded  with  in  the  usual  way.  This  will  take  from  fifty  to  sixty 
minutes.  As  soon  as  the  whole  of  the  tube  is  heated  to  redness, 
the  gas  is  turned  off,  and  the  tube  immediately  exhausted,  the  gases 
produced  being  transferred  to  the  tube  placed  to  receive  them. 
When  the  exhaustion  is  complete,  the  test  tube  of  gas  may  be 
removed  in  a  small  beaker,  and  transferred  to  the  gas  analysis 
apparatus. 


454 


WATER  AND   SEWAGE. 


The  gas  collected  consists  of  carbonic  anhydride,  nitric  oxide, 
nitrogen,  and  (very  rarely)  carbonic  oxide,  which  can  readily  be 
separated  and  determined  by  the  ordinary  methods  of  gas  analysis. 
This  is  rapidly  accomplished  with  the  apparatus,  shown  in  the 


Fig.  64. 

accompanying  diagram,  Fig.  64,  which,  whilst  it  does  not  permit  of 
analysis  by  explosion,  leaves  nothing  to  be  desired  for  this  particular 
operation.  It  is  essentially  that  described  byFrankland*,  but 
is  slightly  modified  in  arrangement.  In  the  diagram,  a  c  d  is  a 
measuring  tube,  of  which  the  cylindrical  portion  a  is  370  mm.  long, 

*  J.  c.  s.  [2]  vi.  109. 


ORGANIC   CARBON   AND   NITROGEN.  455 

and  18  mm.  in  internal  diameter,  the  part  c  40  mm.  long  and  7  mm. 
in  diameter,  and  the  part  d  175  mm.  long  and  2'5  mm.  in  diameter. 
To  the  upper  end  of  d  a  tube,  with  a  capillary  bore  and  stop-cock 
/,  is  attached,  and  bent  at  right-angles.  Allowing  20  mm.  for  each 
of  the  conical  portions  at  the  joints  between  a  and  c,  and  c  and  d, 
and  25  mm.  for  the  vertical  part  of  the  capillary  tube,  the  vertical 
measurement  of  the  entire  tube  is  650  mm.  It  is  graduated  care- 
fully from  below  upward,  at  intervals  of  10  mm.,  the  zero  being 
100  mm.  from  the  end,  as  about  that  length  of  it  is  hidden  by  its 
support,  and  therefore  unavailable.  The  topmost  10  mm.  of  d 
should  be  divided  into  single  millimetres.  At  the  free  end  of  the 
capillary  tube  a  small  steel  cap,  shown  in  fig.  65,  B,  is  cemented 
gas  tight.  The  lower  end  of  a  is  drawn  out  to  a  diameter  of  5  mm. 


Fig.  65. 

The  tube  b  is  about  T2  metre  long,  and  6  mm.  internal  diameter,  is 
drawn  out  like  a  at  the  lower  end,  and  graduated  in  millimetres 
from  below  upward,  the  zero  being  about  100  mm.  from  the  end.* 
The  tubes  a  c  d  and  6  pass  through  a  caoutchouc  stopper  o,  which  fits 
into  the  lower  end  of  a  glass  cylinder  n  n,  intended  to  contain  water 
to  give  a  definite  temperature  to  the  gas  in  measuring.  The  zeros 
of  the  graduations  should  be  about  10  mm.  above  this  stopper. 
Immediately  below  this  the  tubes  are  firmly  clasped  by  the  wooden 
clamp  p  (shown  in  end  elevation  and  plan  at  fig.  64,  B,  C),  the  two 
parts  of  which  are  drawn  together  by  screws,  the  tubes  being 
protected  from  injury  by  a  piece  of  caoutchouc  tube  fitted  over  each. 
The  clamp  is  supported  on  an  upright  piece  of  wood,  screwed  firmly 
to  the  base  A.  It  the  stopper  o  is  carefully  fitted,  and  the  tubes 
tightly  clamped,  no  other  support  than  p  will  be  necessary.  The 
tubes  below  the  clamp  are  connected  by  joints  of  caoutchouc  covered 
with  tape,  and  strongly  bound  with  wire,  to  the  vertical  legs  of  the 
union  piece  q,  to  the  horizontal  leg  of  which  is  attached  a  long 
caoutchouc  tube  of  about  2  mm.  internal  diameter,  which  passes  to 
the  glass  reservoir  t.  This  tube  must  be  covered  with  strong  tape, 
or  (less  conveniently)  have  a  lining  of  canvas  between  two  layers  of 
caoutchouc,  as  it  will  be  exposed  to  considerable  pressure.  In  its 
course  it  passes  through  the  double  screw  steel  pinch-cock  r,  the 
lower  bar  of  which  is  fixed  to  the  side  of  the  clamp  p.  It  is  essential 
that  the  screws  of  the  pinch-cock  should  have  smooth  collars  like 
that  shown  in  fig.  65  A,  and  that  the  upper  surface  of  the  upper  bar 
of  the  pinch-cock  should  be  quite  flat,  the  surfaces  between  which 
the  tube  is  passed  being  cylindrical. 

*  The  graduation  is  not  shown  in  the  diagram. 


456  WATER   AND    SEWAGE. 

Frankland  has  introduced  a  form  of  joint  by  which  the  steel  caps 
and  clamp  are  dispensed  with.  The  capillary  tube  at  the  upper  end 
of  a  c  d  is  expanded  into  a  small  cup  or  funnel,  and  the  capillary  tube 
of  the  laboratory  vessel  bent  twice  at  right-angles,  the  end  being 
drawn  out  in  a  conical  form  to  fit  into  the  neck  of  the  above-named 
cup.  The  opposed  surfaces  are  fitted  by  grinding  or  by  covering  the 
conical  end  of  the  laboratory  vessel  with  thin  sheet  caoutchouc. 
The  joint  is  kept  tight  by  an  elastic  band  attached  at  one  end  to  the 
stand,  and  at  the  other  to  a  hook  on  the  horizontal  tube  of  the 
laboratory  vessel,  and  the  cup  is  filled  with  mercury. 

In  the  base  A  is  fixed  a  stout  iron  rod,  1  '4  metre  long,  with  a  short 
horizontal  arm  at  its  upper  end,  containing  two  grooved  pulleys. 
The  reservoir  t  is  suspended  by  a  cord  passing  over  these  pulleys, 
and  attached  to  an  eye  u  in  the  iron  rod,  the  length  of  the  cord  being 
such  that,  when  at  full  stretch,  the  bottom  of  the  reservoir  is  level 
with  the  bottom  of  the  clamp  p.  A  loop  is  made  on  the  cord, 
which  can  be  secured  by  a  hook  v  on  the  rod,  so  that  when  thus 
suspended,  the  bottom  of  t  is  about  100  mm.  above  the  stop-cock  /. 
A  stout  elastic  band  fitted  round  t  at  its  largest  diameter  acts 
usefully  as  a  fender  to  protect  it  from  an  accidental  blow  against 
the  iron  rod.  A  thermometer  e,  suspended  by  a  wire  hook  from  the 
edge  of  the  cylinder,  n  n,  gives  the  temperature  of  the  contained 
water,  the  uniformity  of  which  may  be  ensured  (though  it  is  scarcely 
necessary)  by  passing  a  slow  succession  of  bubbles  of  air  through 
it  or  by  moving  up  and  down  in  it  a  wire  with  its  end  bent  into  the 
form  of  a  ring.  The  jar  k  is  called  the  laboratory  vessel,  and  is 
100  mm.  high,  and  38  mm.  in  internal  diameter,  having  a  capillary 
tube,  glass  stop-cock,  and  steel  cap  g  h  exactly  like  /  g.  The  mercury 
trough  /  is  shown  in  figs.  66  and  67.  It  is  of  solid  mahogany,  265 
mm.  long,  80  mm.  broad,  and  90  mm.  deep,  outside  measurement. 
The  rim  a  a  a  a  is  8  mm.  broad  and  15  mm.  deep.  The  excavation 
b  is  230  mm.  long,  26  mm.  broad,  and  65  mm.  deep,  with  a  circular 
cavity  to  receive  the  laboratory  vessel  sunk  at  one  end,  45  mm.  in 
diameter,  and  20  mm.  in  depth  below  the  top  of  the  excavation. 
Two  small  lateral  indentations  c  c  (fig.  67)  near  the  other  end 


3 


Fig-  66.  Fig.  67. 

accommodate  a  capsule  for  transferring  to  the  trough  tubes  con- 
taining gas.  This  trough  rests  upon  a  telescope  table,  which  can 
be  fixed  at  any  height  by  means  of  a  screw,  and  is  supported  on  three 
feet.  It  must  be  so  arranged  that  when  the  laboratory  vessel  is 
in  its  place  in  the  trough  the  two  steel  caps  exactly  correspond 
face  to  face. 

The  difference  of  level  of  the  mercury  in  the  tubes  b  and  a  c  d, 


ORGANIC    CARBON    AND    NITROGEN.  457 

caused  by  capillary  action,  when  both  are  freely  open  to  the  air, 
must  be  ascertained  by  taking  several  careful  observations.  This 
will  be  different  for  each  of  the  portions  a  c  and  d,  and  must  be  added 
to  or  deducted  from  the  observed  pressure,  as  the  mercury  when 
thus  freely  exposed  in  both  tubes  to  the  atmospheric  pressure  stands 
in  a  c  or  d  above  or  below  that  in  6.  This  correction  will  include 
also  any  that  may  be  necessary  for  difference  of  level  of  the  zeros  of 
the  graduations  of  the  two  tubes,  and,  if  the  relative  positions  of 
these  be  altered,  it  must  be  redetermined.  A  small  telescope, 
sliding  on  a  vertical  rod,  should  be  used  in  these  and  all  other  read- 
ings of  the  level  of  mercury. 

The  capacity  of  the  measuring  tube  a  c  d  at  each  graduation  must 
now  be  determined.  This  is  readily  done  by  first  filling  the  whole 
apparatus  with  mercury,  so  that  it  drips  from  the  cap  g.  The  stop- 
cock /  is  then  closed,  a  piece  of  caoutchouc  tube  slipped  over  the 
cap,  and  attached  to  a  funnel  supplied  with  distilled  water.  The 
reservoir  t  being  lowered,  the  clamp  r  and  the  stop-cock  /  are  opened, 
so  that  the  mercury  returns  to  the  reservoir,  water  entering  through 
the  capillary  tube.  As  soon  as  it  is  below  the  zero  of  the  graduation, 
the  stop-cock  /  is  closed,  the  funnel  and  caoutchouc  tube  removed 
from  the  cap,  and  the  face  of  the  last  slightly  greased  in  order  that 
water  may  pass  over  it  without  adhering.  Now  raise  the  reservoir, 
open  the  stop-cock  /,  and  allow  the  water  to  flow  gently  out  until  the 
top  of  the  convex  surface  of  the  mercury  in  a  just  coincides  with 
the  zero  of  the  graduation.  The*  mercury  should  be  so  controlled 
by  the  clamp  r  that  the  water  issues  under  very  slight  pressure. 
Note  the  temperature  of  the  water  in  the  water-jacket,  and  proceed 
with  the  expulsion  of  the  water,  collecting  it  as  it  drops  from  the 
steel  cap,  in  a  small  carefully  weighed  glass  flask.  When  the 
mercury  has  risen  through  100  mm.  stop  the  flow  of  water,  and 
weigh  the  flask.  The  weight  of  water  which  was  contained  between 
the  graduations  0  and  100  on  the  tube  is  then  known,  and  if  the 
temperature  be  4°  C.,  the  weight  in  grams  will  express  the  capacity 
of  that  part  of  the  tube  in  cubic  centimetres.  If  the  temperature 
be  other  than  4°  C.,  the  volume  must  be  calculated  by  the  aid  of 
the  co-efficient  of  expansion  of  water  by  heat.  In  a  similar  way 
the  capacity  of  the  tube  at  successive  graduations  about  100  mm. 
apart  is  ascertained,  the  last  determination  in  a  being  at  the  highest, 
and  the  first  in  c  at  the  lowest  graduation  on  the  cylindrical  part  of 
each  tube  ;  the  tube  between  these  points  and  similar  points  on 
c  and  d  being  so  distorted  by  the  glass  blower  that  observations 
could  not  well  be  made.  The  capacity  at  a  sufficient  number  of 
points  being  ascertained,  that  at  each  of  the  intermediate  graduations 
may  be  calculated,  and  a  table  arranged  with  the  capacity  marked 
against  each  graduation.  As  the  calculations  in  the  analysis  are 
made  by  the  aid  of  logarithms,  it  is  convenient  to  enter  on  this 
table  the  logarithms  of  the  capacities  instead  of  the  natural  numbers. 

In  using  the  apparatus,  the  stop-cocks  on  the  measuring  tube  and 
laboratory  vessel  should  be  slightly  greased  with  a  mixture  of  resin 


458  WATER   AND    SEWAGE. 

cerate  and  oil,  or  vaseline,  the  whole  apparatus  carefully  filled  with 
mercury,  and  the  stop-cock  /  closed  ;  next  place  the  laboratory  vessel 
in  position  in  the  mercury  trough,  and  suck  out  the  air.  This  is 
readily  and  rapidly  done  by  the  aid  of  a  short  piece  of  caoutchouc 
tube,  placed  in  the  vessel  just  before  it  is  put  into  the  mercury 
trough,  and  drawn  away  as  soon  as  the  air  is  removed.  Suck  out 
any  small  bubbles  of  air  left  still  through  the  capillary  tube,  and  as 
soon  as  the  vessel  is  entirely  free  from  air  close  the  stop-cock.  Slightly 
grease  the  face  of  both  caps  with  resin  cerate  (to  which  a  little  oil 
should  be  added  if  very  stiff),  and  clamp  them  tightly  together. 
On  opening  both  stop-cocks  mercury  should  flow  freely  through  the 
capillary  communication  thus  formed,  and  the  whole  should  be 
quite  free  from  air.  To  ascertain  if  the  joints  are  all  in  good  order, 
close  the  stop-cock  ^,  and  lower  the  reservoir  t  to  its  lowest  position  ; 
the  joints  and  stop-cocks  will  thus  be  subjected  to  a  pressure  of 
nearly  half  an  atmosphere,  and  any  leakage  would  speedily  be 
detected.  If  all  be  right,  restore  the  reservoir  to  its  upper 
position. 

Transfer  the  tube  containing  the  gas  to  be  analyzed  to  an  ordinary 
porcelain  mercury  trough  ;  exchange  the  beaker  in  \vhich  it  has  been 
standing  for  a  small  porcelain  capsule,  and  transfer  it  to  the  mercury 
trough  I,  the  capsule  finding  ample  room  where  the  trough  is  widened 
by  the  recess  D. 

Carefully  decant  the  gas  to  the  laboratory  vessel,  and  add  a  drop 
or  two  of  potassium  dichromate  solution  (B.  vii)  from  a  small  pipette 
with  a  bent  capillary  delivery  tube,  to  ascertain  if  the  gas  contains 
any  sulphurous .  anhydride.  If  so,  the  yellow  solution  will  imme- 
diately become  green  from  the  formation  of  a  chromic  salt,  and 
the  gas  must  be  allowed  to  stand  over  the  chromate  for  four  or  five 
minutes,  a  little  more  of  the  solution  being  added  it  necessary.  The 
absorption  may  be  greatly  accelerated  by  gently  shaking  from  time 
to  time  the  stand  on  which  the  mercury  trough  rests,  so  as  to  cause 
the  solution  to  wet  the  sides  of  the  vessel.  With  care  this  may 
be  done  without  danger  to  the  apparatus.  Mercury  should  be 
allowed  to  pass  slowly  into  the  laboratory  vessel  during  the  whole 
time,  as  the  drops  falling  tend  to  maintain  a  circulation  both  in 
the  gas  and  in  the  absorbing  liquid.  The  absence  of  sulphurous 
anhydride  being  ascertained,  both  stop-cocks  are  set  fully  open, 
the  reservoir  /  lowered,  and  the  gas  transferred  to  the  measuring 
tube.  The  stop-cock  h  should  be  closed  as  soon  as  the  liquid  from 
the  laboratory  vessel  is  within  about  10  mm.  of  it.  The  bore  of 
the  capillary  tube  is  so  fine,  that  the  quantity  of  gas  contained  in  it 
is  too  small  to  affect  the  result.  Next  bring  the  top  of  the  meniscus 
of  mercury  seen  through  the  telescope  exactly  to  coincide  with  one 
of  the  graduations  on  the  measuring  tube,  the  passage  of  mercury 
to  or  from  the  reservoir  being  readily  controlled  by  the  pinch-cock 
Note  the  position  of  the  mercury  in  the  measuring  tube  and  in 
the  pressure  tube  6,  the  temperature  of  the  water-jacket,  and  the 
height  of  the  barometer,  the  level  of  the  mercury  in  the  pressure 


ORGANIC   CARBON   AND   NITROGEN.  459 

tube  and  barometer  being  read  to  the  tenth  of  a  mm.  and  the 
thermometer  to  0'1°  C.  This  done,  introduce  into  the  laboratory 
vessel  from  a  pipette  with  a  bent  point,  a  few  drops  of  potassium 
hydrate  solution  (B.  viii),  and  return  the  gas  to  the  laboratory  vessel. 
The  absorption  of  carbonic  anhydride  will  be  complete  in  about  three 
to  five  minutes,  and  if  the  volume  of  the  gas  is  large,  may  be  much 
accelerated  by  gently  shaking  the  stand  from  time  to  time,  so  as  to 
throw  up  the  liquid  on  the  sides  of  the  vessel.  If  the  small  pipettes 
used  to  introduce  the  various  solutions  are  removed  from  the  mercury 
trough  gently,  they  will  always  contain  a  little  mercury  in  the  bend, 
which  will  suffice  to  keep  the  solution  from  flowing  out,  and  they 
may  be  kept  in  readiness  for  use  standing  upright  in  glass  cylinders 
or  other  convenient  supports.  At  the  end  of  five  minutes  the  gas, 
which  now  consists  of  nitrogen  and  nitric  oxide,  is  again  transferred 
to  the  measuring  tube,  and  the  operation  of  measuring  repeated  ; 
the  barometer,  however,  need  not  be  observed,  under  ordinary 
circumstances,  more  than  once  for  each  analysis,  as  the  atmospheric 
pressure  will  not  materially  vary  during  the  twenty-five  to  thirty 
minutes  required.  Next  pass  into  the  laboratory  vessel  a  few  drops 
of  saturated  solution  of  pyrogallic  acid  (B.  ix),  and  return  the  gas 
upon  it.  The  object  of  adding  the  pyrogallic  acid  at  this  stage  is  to 
ascertain  if  oxygen  is  present,  as  sometimes  happens  when  the  total 
quantity  of  gas  is  very  small,  and  the  vacuum  during  the  combustion 
but  slightly  impaired.  Under  such  circumstances,  traces  of  oxygen 
are  given  off  by  the  cupric  oxide,  and  pass  so  rapidly  over  the  metallic 
copper  as  to  escape  absorption.  This  necessarily  involves  the  loss 
of  any  nitric  oxide  which  also  escapes  the  copper,  but  this  is  such 
a  very  small  proportion  of  an  already  small  quantity  that  its  loss 
will  not  appreciably  affect  the  result.  If  oxygen  be  present,  allow 
the  gas  to  remain  exposed  to  the  action  of  the  pyrogallate  until 
the  liquid  when  thrown  up  the  sides  of  the  laboratory  vessel  runs 
off  without  leaving  a  dark  red  stain.  If  oxygen  be  not  present, 
a  few  bubbles  of  that  gas  (B.  xi)  are  introduced  to  oxidize  the  nitric 
oxide  to  nitrogen  peroxide,  which  is  absorbed  by  the  potassium 
hydrate.  The  oxygen  may  be  very  conveniently  added  from  the 

gas  pipette  shown  in  fig.  68,  where 
a  b  are  glass  bulbs  of  about 
50  mm.  diameter,  connected  by  a 
glass  tube,  the  bore  of  which  is 
constricted  at  c,  so  as  to  allow 
mercury  to  pass  but  slowly  from 
one  bulb  to  the  other,  and  thus 
control  the  passage  of  gas  through 
Fig.  68.  the  narrow  delivery  tube  d.  The 

other  end   e  is  provided  with  a 

short  piece  of  caoutchouc  tube,  by  blowing  through  which  any 
desired  quantity  of  gas  may  be  readily  delivered.  Care  must  be 
taken  after  use  that  the  delivery  tube  is  not  removed  from  the 
trough  till  the  angle  d  is  filled  with  mercury. 


460  WATER   AND    SEWAGE. 

|  ' 

To  replenish  the  pipette  with  oxygen,  fill  the  bulb  6  and  the  tubes 
c  and  d  with  mercury  ;  introduce  the  point  of  d  into  a  tube  of  oxygen 
standing  in  the  mercury  trough,  and  draw  air  from  the  tube  e.  The 
gas  in  b  is  confined  between  the  mercury  in  c  and  that  in  d. 

When  the  excess  of  oxygen  has  been  absorbed  as  above  described, 
the  residual  gas,  which  consists  of  nitrogen,  is  measured,  and  the 
analysis  is  complete.* 

There  are  thus  obtained  three  sets  of  observations,  from  which, 
by  the  usual  methods,  we  may  calculate  A  the  total  volume,  B  the 
volume  of  nitric  oxide  and  nitrogen,  and  C  the  volume  of  nitrogen, 
all  reduced  to  0°  C.  and  760  mm.  pressure  ;  from  these  may  be 
obtained — 

A-B  =  vol.  of  CO2, 

B — G     ~     B  -|-  C         ,      f  -\r 
-y-  +C  =  — ±— =vol.  of  N, 

and  hence  the  weight  of  carbon  and  nitrogen  can  be  readily 
found. 

It  is  much  less  trouble,  however,  to  assume  that  the  gas  in  all 
three  stages  consists  wholly  of  nitrogen  ;  then,  if  A  be  the  weight 
of  the  total  gas,  B  its  weight  after  treatment  with  potassium  hydrate, 
and  C  after  treatment  with  pyrogallate,  the  weight  of  carbon  will 

O  T>  _|_  /~N 

be  (A  —  B)i  and  the  weight  of  nitrogen  — S—  ;  fr>r  the  weights  of 

carbon  and  nitrogen  in  equal  volumes  of  carbon  anhydride  and 
nitrogen,  at  the  same  temperature  and  pressure,  are  as  6  :  14  ;  and 
the  weights  of  nitrogen  in  equal  volumes  of  nitrogen  and  nitric  oxide 
are  as  2  :  1. 

The  weight  of  1  c.c.  of  nitrogen  at  0°  C.  and  760  mm.  is  0'0012562f 

0-0012562  xv  xp 
gm.,  and  the  formula  for  the  calculation  is  t0  =  — -- •        ^Efn&rzk 

(1  +U"OOob7/)7oU 

in  which  w  =  the  weight  of  nitrogen,  v  the  volume,  p  the  pressure 
corrected  for  tension  of  aqueous  vapour,  and  /  the  temperature  in 
degrees  centigrade.  To  facilitate  this  calculation,  there  is  given  in 

Table  2  the  logarithmic  value  of  the  expression         0-0012562 


(1+0-00367*)  760 
for  each  tenth  of  a  degree  from  0°  to  29-9°  C.,  and  in  Table  1  (Tables 
1  to  8  are  inserted  at  the  end  of  the  book)  the  tension  of  aqueous 
vapour  in  millimetres  of  mercury.  As  the  measuring  tube  is  always 
kept  moist  with  water,  the  gas  when  measured  is  always  saturated 
with  aqueous  vapour. 

*  When  the  quantity  of  carbon  is  very  large  indeed,  traces  of  carbonic  oxide  are 

occasionally  present  in  the  gas,  and  will  remain  with  the  nitrogen  after  treatment 

with  alkaline  pyrogallate.     When  such  excessive  quantities  of  carbon  are  found, 

tne  stop-cock  /should  be  closed  when  the  last  measurement  is  made,  the  laboratory 

vessel  detached,  washed,  and  replaced  filled  with  mercury.     Introduce  then  a  little 

solution    of   cuprous    chloride  (B.  x),  and  return  the  gas  upon  it.     Any  carbonic 

3  will  be  absorbed,  and  after  about  five  minutes  the  remaining  nitrogen  may 

measured.     In  more  than  twenty  consecutive  analyses  of  waters  of  very  varying 

kinds,  not  a  trace  of  carbonic  oxide  was  found  in  any  of  the  gases  obtained  on 

combustion. 


/iKn's  .value-     Tne   most  recent   determinations   by   Lord 
and  by  Gray  give  the  value  0*0012507  gm. 


ORGANIC    CARBON    AND    NITROGEN. 

461 

The  following  example  will  show  the  precise  mode  of  calculation  :  — 

A 

B 

c 

Total. 

After  Absorption 

Nitrogen. 

Of  CO2. 

Volume  of  gas 

4-4888  c.c. 

0-26227  c.c. 

0-26227  c.c. 

Temperature     .... 

13-5° 

13-6° 

13-7° 

mm. 

mm. 

mm. 

Height  of  Mercury  in  a,  c,  d, 

310-0 

480-0 

480-0 

193-5 

343-5 

328-2 

" 

Difference    . 

116-5 

136-5 

151-8 

Plus  tension  of  aqueous  vapour 

11-5 

11-6 

11-7 

128-0 

Add  for  \2-2 

2-2 

Deduct  correction  for  capillarity 

0-9 

capillarity  / 

127-1 

150-3 

165-7 

769-8 

769-8 

769-8 

Deduct  this  from  height  of  bar  . 

127-1 

150-3 

165-7 

Tension  of  dry  gas    . 

642-7 

619-5 

604-1 

Logarithm  of  volume  of  gas 

0-65213 

1-41875 

1-41875 

0-0012562 

„     (1+0-003672)760 

6-19724 

6-19709 

6-19694 

„        „     tension  of  dry  gas     . 

2-80801 

2-79204 

2-78111 

Logarithm  of  weight  of  gas  calcu- 

lated as  N.          . 

3-65738 

4-40788 

4~-39680 

= 

0-0045434 

i         0-0002558 

0-0002494  gm. 

From  these  weights,  those  of  carbon  and  of  nitrogen  are  obtained 
by  the  use  of  the  formulae  above  mentioned.     Thus  — 

A-B=0-0042876  B+C=0'0005052 


Weight  of  nitrogen,  Q-Q002526 


-^-7)0-0128628 
Weight  of  carbon,~CK)01837 

When  carbonic  oxide  is  found,  the  corresponding  weight  of 
nitrogen  may  be  found  in  a  similar  manner,  and  should  be  added 
to  that  corresponding  to  the  carbonic  anhydride  before  multiplying 

o 

by  7  and  must  be  deducted  from  the  weight  corresponding  to  the 

7, 
volume  after  absorption  of  carbonic  anhydride. 

As  it  is  impossible  to  attain  to  absolute  perfection  of  manipulation 
and  materials,  each  analyst  should  make  several  blank  experiments 
by  evaporating  a  litre  of  pure  distilled  water  (B.  i)  with  the  usual 
quantities  of  sulphurous  acid  and  ferrous  chloride,  and,  in  addition, 
0*1  gm.  of  freshly  ignited  sodium  chloride  (in  order  to  furnish  a 
tangible  residue).  The  residue  should  be  burnt  and  the  resulting 
gas  analyzed  in  the  usual  way,  and  the  average  amounts  of  carbon 
and  nitrogen  thus  obtained  deducted  from  the  results  of  all  analyses. 
This  correction,  which  may  be  about  0-0001  gm.  of  C  and  0-00005 
gm.  of  N,  includes  the  errors  due  to  the  imperfection  of  the  vacuum 
produced  by  the  Sprengel  pump,  nitrogen  retained  in  the  cupric 


462  WATER   AND    SEWAGE. 

oxide,  ammonia  absorbed  from  the  atmosphere  during  evaporation, 

etc. 

When  the  quantity  of  nitrogen  as  ammonia  exceeds  0'007  part 
per  100,000,  there  is  a  certain  amount  of  loss  of  nitrogen  during  the 
evaporation  by  dissipation  of  ammonia.  This  appears  to  be  very 
constant,  and  is  given  in  Table  3,  which  is  calculated  from  Table  5, 
which  has  been  kindly  furnished  by  the  late  Sir  E.  Frankland. 
The  number  in  this  table  corresponding  to  the  quantity  of  nitrogen 
as  ammonia  present  in  the  water  analyzed  should  be  added  to  the 
amount  of  nitrogen  found  by  combustion.  The  number  thus 
obtained  includes  the  nitrogen  as  ammonia,  and  this  must  be 
deducted  to  ascertain  the  organic  nitrogen.  If  "ammonia"  is 
determined  instead  of  "nitrogen  as  ammonia,"  Table  5  may  be  used. 

When,  in  operating  upon  sewage,  hydrogen  metaphosphate  has 
been  employed,  Tables  4  or  6  should  be  used. 

4.  Determination  of  Total  Solid  Matter. — This  is  done  by 
evaporating  a  known  quantity  of  the  water  to  dryness  in  a  weighed 
platinum  dish  on  the  water-bath.  When  the  residue  is  not  required 
for  any  subsequent  determination,  250  c.c.  are  generally  taken  ; 
but  when,  as  is  often  the  case,  the  residue  is  to  be  used  for  the 
determination  of  nitric  and  nitrous  nitrogen  by  Cr urn's  method, 
the  amounts  usually  taken  are  as  follows  :— For  water  supplies 
and  river  water — 500  c.c.  and  shallow  well  waters — 250  c.c. 

The  sample  used  should  be  filtered  or  unfiltered  according  to  the 
decision  made  in  that  respect  at  the  commencement  of  the  analysis. 

For  sewage  and  effluents  take  100  c.c. 

It  is  desirable  to  support  the  platinum  dish  during  evaporation 
in  a  glass  ring  with  a  flange,  shaped  like  the  top  of  a  beaker,  the 
cylindrical  part  being  about  20  mm.  deep.  This  is  dropped  into 
the  metal  ring  on  the  water-bath,  and  thus  lines  the  metal  with 
glass,  and  keeps  the  dish  clean.  A  glass  disc  with  a  hole  in  it  to 
receive  the  dish  is  not  satisfactory,  as  drops  of  water  conveying  solid 
matter  find  their  way  across  the  under  surface  from  the  metal  vessel 
to  the  dish,  and  thus  soil  it.  As  soon  as  the  evaporation  is  complete, 
the  dish  with  the  residue  is  removed,  its  outer  surface  wiped  dry 
with  a  clqth,  and  it  is  dried  in  a  water  or  steam  oven  for  about  three 
hours.  It  is  then  removed  to  a  desiccator,  allowed  to  cool,  weighed 
as  rapidly  as  possible,  returned  to  the  oven,  and  weighed  at  intervals 
of  an  hour,  until  between  two  successive  weighings  it  has  lost  less 
than  0-001  gm.* 

*  There  is  some  diversity  of  opinion  as  to  the  temperature  at  which  the  total 
solids  in  water  should  be  dried  previous  to  weighing.  The  Committee  appointed 
by  the  Society  of  Public  Analysts  (see  Analyst  al  1881,  137)  recommended  220°  F. 
(104-4°  C.).  Dr.  R  ideal  ("Water  and  its  Purification")  recommends  120°  C. 
Prof.  Still  man  ("Engineering  Chemistry")  gives  105°  C.  Dr.  Thresh  ("Ex- 
amination of  Waters  and  Water  Supplies")  dries  at  180°  C.,  "because  at  this 
temperature  magnesium  sulphate  retains  a  definite  proportion  of  water  and  calcium 
sulphate  loses  the  whole  of  its  water  of  crystallization,  the  results  consequently  being 
much  more  uniform  and  satisfactory  than  at  a  lower  temperature."  Dr.  Fowler 
(Sewage  Works  Analyses")  dries  the  solids  from  sewages  and  effluents  at  100- 
S  J^'-Jv*  Go  wan  (lee.  cit.)  states  that  the  total  solids  in  sewages  and 
effluents  should  be  dried  in  an  air-bath  at  110°  C.  till  the  weight  becomes  constant. 
Hence  it  is  important  for  analysts  to  state  on  water  and  sewage  certificates  the 
temperature  at  which  the  solids  have  been  dried 


NITROGEN  AS  NITRATES  AND  NITRITES.  463 

If  the  residue  is  not  wanted  for  any  other  purpose,  the  dish  should 
be  gradually  heated  to  redness  and  note  made  of  any  changes  that 
may  take  place,  especially  smell,  scintillation,  slight  darkening  or 
blackening,  and  partial  fusion.  The  ignited  residue  may  be  tested 
for  the  presence  of  phosphoric  acid  (see  p.  477). 

5.  Determination  of  Nitrogen  as  Nitrates  and  Nitrites  (Cr urn's 
method). — The  residue  obtained  in  the  preceding  operation  may 
be  used  for  this  determination.  Treat  it  with  about  30  c.c.  of  hot 
distilled  water,  taking  care  to  submit  the  whole  of  the  residue  to 
its  action.  To  ensure  this  it  is  advisable  to  rub  the  dish  gently 
with  the  finger,  so  as  to  detach  the  solid  matter  as  far  as  possible, 
and  facilitate  the  solution  of  the  soluble  matters.  The  finger  may 
be  covered  by  a  caoutchouc  finger-stall.  Then  filter  through  a  very 
small  filter  of  Swedish  paper,  washing  the  dish  several  times  with 
small  quantities  of  hot  distilled  water. 

The  filtrate  must  be  evaporated  in  a  very  small  beaker, 
over  a  steam  bath,  until  reduced  to  about  1  c.c.,  or  even  to 
dryness.  This  concentrated  solution  is  introduced  into  the 
glass  tube  shown  in  fig.  69,  standing  in  the  porcelain 
mercury  trough,  filled  up  to  the  stop-cock  with  mercury. 
(If  the  nitrometer  of  Lunge  is  used  in  place  of  Cr  urn's 
tube,  the  use  of  the  laboratory  tube  and  gas  apparatus  is 
avoided.)  The  tube  is  210  mm.  in  total  length  and  15  mm. 
in  internal  diameter.  By  pouring  the  liquid  into  the  cup 
at  the  top,  and  then  cautiously  opening  the  stop-cock,  it 
may  be  run  into  the  tube  without  admitting  any  air.  The 
beaker  is  rinsed  once  with  a  very  little  hot  distilled  water, 
and  then  two  or  three  times  with  strong  sulphuric  acid 
(C.  i),  the  volume  of  acid  being  to  that  of  the  aqueous 
solution  about  as  3  :  2.  The  total  volume  of  acid  and  water 
should  be  about  6  c.c.  Should  any  air  by  chance  be 
admitted  at  this  stage,  it  may  readily  be  removed  by  suction, 
the  lips  being  applied  to  the  cup.  With  care  there  is  but 
little  danger  of  getting  acid  into  the  mouth. 

In  a  few  cases  carbonic  anhydride  is  given  off  on  addition 
Fi  m*Q9  °^  sulpnuric  acid,  and  must  be  sucked  out  before  proceeding. 
Now  grasp  the  tube  firmly  in  the  hand,  closing  the  open 
end  by  the  thumb,  which  should  be  first  moistened  ;  withdraw  it 
from  the  trough,  incline  it  at  an  angle  of  about  45°,  the  cup  pointing 
from  you,  and  shake  it  briskly  with  a  rapid  motion  in  the  direction 
of  its  length,  so  as  to  throw  the  mercury  up  towards  the  sto  p-cock. 
After  a  very  little  practice  there  is  no  danger  of  the  acid  finding  its 
way  down  to  the  thumb,  the  mixture  of  acid  and  mercury  being 
confined  to  a  comparatively  small  portion  of  the  tube.  In  a  few 
seconds  some  of  the  mercury  becomes  very  finely  divided  ;  and  if 
nitrates  be  present,  in  about  a  minute  or  less  nitric  oxide  is  evolved, 
exerting  a  strong  pressure  on  the  thumb.  Mercury  is  allowed  to 
escape  as  the  reaction  proceeds,  by  partially,  but  not  wholly, 
relaxing  the  pressure  of  the  thumb.  A  slight  excess  of  pressure 


464  WATER  AND    SEWAGE. 

should  be  maintained  within  the  tube  to  prevent  entrance  of  air 
during  the  agitation,  which  must  be  continued  until  no  more  gas 
is  evolved. 

When  the  quantity  of  nitrate  is  very  large,  the  mercury,  on 
shaking,  breaks  up  into  irregular  masses,  which  adhere  to  one 
another  as  if  alloyed  with  lead  or  tin,  and  the  whole  forms  a  stiff 
dark-coloured  paste,  which  it  is  sometimes  very  difficult  to  shake  ; 
but  nitric  oxide  is  not  evolved  for  a  considerable  time,  then  comes 
off  slowly,  and  afterwards  with  very  great  rapidity.  To  have  room 
for  the  gas  evolved,  the  operator  should  endeavour  to  shake  the 
tube  so  as  to  employ  as  little  as  possible  of  the  contained  mercury 
in  the  reaction.  At  the  close  of  the  operation  the  finely  divided 
mercury  will  consist  for  the  most  part  of  minute  spheres,  the  alloyed 
appearance  being  entirely  gone.  An  experiment  with  a  large 
quantity  of  nitrate  may  often  be  saved  from  loss  by  firmly  resisting 
the  escape  of  mercury,  shaking  until  it  is  judged  by  the  appearance 
of  the  contents  of  the  tube  that  the  reaction  is  complete,  and  then 
on  restoring  the  tube  to  the  mercury  trough,  allowing  the  finely- 
divided  mercury  also  to  escape  in  part.  If  the  gas  evolved  be  not 
more  than  the  tube  will  hold,  and  there  be  no  odour  of  nitric 
peroxide  from  the  escaped  finely-divided  mercury,  the  operation 
may  be  considered  successful.  If  the  amount  of  nitrate  be  too 
large,  a  smaller  quantity  of  the  water  must  be  evaporated  and  the 
operation  repeated.  When  no  nitrate  is  present,  the  mercury 
usually  manifests  very  little  tendency  to  become  divided,  that 
which  does  so  remains  bright,  and  the  acid  liquid  does  not  become 
so  turbid  as  in  other  cases. 

The  reaction  completed,  the  tube  is  taken  up  closed  by  the  thumb, 
and  the  gas  is  decanted  into  the  laboratory  vessel,  and  measured  in 
the  usual  way  in  the  gas  apparatus.  The  nitric  acid  tube  is  of  such 
a  length  that  when  the  cup  is  in  contact  with  the  end  of  the  mercury 
trough  the  open  end  is  just  under  the  centre  of  the  laboratory 
vessel.  If  any  acid  has  been  expelled  from  the  tube  at  the  close 
of  the  shaking  operation,  the  end  of  the  tube  and  the  thumb  should 
be  washed  with  water  before  introducing  into  the  mercury  trough 
of  the  gas  apparatus,  so  as  to  remove  any  acid  which  may  be  ad- 
hering, which  would  destroy  the  wood  of  the  trough.  Before 
passing  the  gas  into  the  measuring  tube  of  the  gas  apparatus, 
a  little  mercury  should  be  allowed  to  run  into  the  laboratory 
vessel  to  remove  the  acid  from  the  entrance  to  the  capillary  tube. 

As  nitric  oxide  contains  half  its  volume  of  nitrogen,  if  half  a  litre 
of  water  has  been  employed,  the  volume  of  nitric  oxide  obtained 
will  be  equal  to  the  volume  of  nitrogen  present  as  nitrates  and 
nitrites  in  one  litre  of  the  water,  and  the  weight  of  the  nitrogen 
may  be  calculated  as  directed  in  the  paragraph  on  the  determination 
of  organic  carbon  and  nitrogen  (see  p.  460). 

When  more  than  0'08  part  of  nitrogen  as  ammonia  is  present  in 
100,000  parts  of  liquid,  there  is  danger  of  loss  of  nitrogen  by 
decomposition  of  ammonium  nitrite  on  evaporation  ;  and  therefore 


NITROGEN   AS   NITRATES    AND    NITRITES.  465 

the  residue  from  the  determination  of  total  solid  matter  cannot  be 
used.  In  such  cases  acidify  a  fresh  quantity  of  the  liquid  with 
dilute  sulphuric  acid,  add  solution  of  potassium  permanganate, 
a  little  at  a  time,  until  the  pink  colour  remains  for  about  a  minute, 
and  render  the  liquid  just  alkaline  to  litmus  paper  with  sodium 
carbonate.  The  nitrites  present  will  then  be  converted  into 
nitrates  and  may  be  evaporated  without  fear  of  loss.  Use  as  little 
of  each  reagent  as  possible. 

Allen*  advocated  the  use  of  Lunge's  nitrometer  in  place  of 
Crum'  s  tube  and,  to  obviate  the  difficulty  in  reading  the  volume  of 
gas  which  sometimes  arises  on  account  of  the  mercurial  froth,  he 
used  two  nitrometers  side  by  side.  In  these  he  worked,  under 
identical  conditions,  the  water-residue  and  a  standard  solution  of 
potassium  nitrate  respectively.  A  simple  calculation  then  gives 
the  amount  of  nitrogen  required.  Allen  stated  that  it  is  not 
necessary  to  make  a  test  experiment  each  time,  as  provided  the 
nitrometer  tap  is  tight,  the  standard  measure  of  gas  obtained  from 
the  nitre  solution  may  be  kept  for  an  indefinite  period.  (For  list 
of  Nitrogen  Conversion  Factors,  see  p.  285). 

6.  Determination  of  Nitrogen  as  Nitrates  and  Nitrites  in  Waters 
containing  a  very  large  quantity  of  Soluble  Matter,  with  but  little 
Ammonia  or  Organic  Nitrogen. — When  the  quantity  of  soluble 
matter  is  excessive,  as,  for  example,  in  sea- water,  the  preceding 
method  is  inapplicable,  as  the  solution  to  be  employed  cannot  be 
reduced  to  a  sufficiently  small  bulk  to  go  into  the  shaking  tube. 
If  the  quantity  of  organic  nitrogen  be  less  than  O'l  part  in  100,000 
the  nitrogen  as  nitrates  and  nitrites  may  generally  be  determined 
by  the  following  modification  of  Schulze's  method  devised  by 
E.  T.  Chapman.  To  200  c.c.  of  the  water  add  10  c.c.  of  sodium 
hydrate  solution  (C.  v),  and  boil  briskly  in  an  open  porcelain  dish 
until  it  is  reduced  to  about  70  c.c.  When  cold  pour  the  residue 
into  a  tall  glass  cylinder  of  about  120  c.c.  capacity,  and  rinse  the 
dish  with  water  free  from  ammonia.  Add  a  piece  of  aluminium 
foil  of  about  15  sq.  centim.  area,  loading  it  with  a  piece  of  clean  glass 
rod  to  keep  it  from  floating.  Close  the  mouth  of  the  cylinder 
with  a  cork,  bearing  a  small  tube  filled  with  pumice  (C.  vi),  moistened 
with  hydric  chloride  free  from  ammonia  (C.  vii). 

Hydrogen  will  speedily  be  given  off  from  the  surface  of  the 
aluminium,  and  in  five  or  six  hours  the  whole  of  the  nitrogen  as 
nitrates  and  nitrites  will  be  converted  into  ammonia.  Transfer  to 
a  small  retort  the  contents  of  the  cylinder,  together  with  the  pumice, 
washing  the  whole  apparatus  with  a  little  water  free  from  ammonia. 
Distil,  and  determine  the  ammonia  in  the  usual  way  with  Nessler 
solution.  It  appears  impossible  wholly  to  exclude  ammonia  from 
the  reagents  and  apparatus,  and  therefore  some  blank  experiments 
should  be  made  to  ascertain  the  correction  to  be  applied  for  this. 
This  correction  is  very  small,  and  appears  to  be  nearly  constant. 

*  Analyst,  1880,  181. 

2   H 


466  WATER  AND    SEWAGE. 

7.  Determination  of  Nitrates  as  Ammonia  by  the  Copper-zinc 
Couple. — It  is  well  known  that  when  zinc  is  immersed  in  copper 
sulphate  solution  it  becomes  covered  with  a  spongy  deposit  of 
precipitated  copper.  If  the  solution  of  copper  sulphate  be 
sufficiently  dilute,  this  deposit  of  copper  is  black  in  colour  and 
firmly  adherent  to  the  zinc.  It  is,  however,  not  so  generally  known 
that  the  zinc  upon  which  copper  has  thus  been  deposited  possesses 
the  power  of  decomposing  pure  distilled  water  at  the  ordinary 
temperature,  and  that  it  is  capable  of  effecting  many  other  de- 
compositions which  zinc  alone  cannot.  Among  these  is  the  de- 
composition of  nitrates,  and  the  transformation  of  the  nitric  acid 
into  ammonia.  Gladstone  and  Tribe  have  shown  that  the 
action  of  the  "  copper-zinc  couple  "  upon  a  nitre  solution  consists 
in  the  electrolysis  of  the  nitre,  resulting  in  the  liberation  of 
hydrogen  and  the  formation  of  zinc  oxide.  The  nascent  hydrogen 
liberated  on  the  surface  of  the  copper  reduces  the  nitrate  to  nitrite 
and  this  in  turn  to  ammonia.  M.  W.  Williams*  has  shown  that 
even  in  very  dilute  solutions  of  nitre  the  nitric  acid  can  be  com- 
pletely converted  into  ammonia  in  this  manner  with  considerable 
rapidity  ;  and  further,  that  the  reaction  may  be  greatly  hastened 
by  taking  advantage  of  the  influence  of  temperature,  acids,  and 
certain  neutral  salts,  which  increase  the  electrolytic  action  of  the 
couple.  His  experiments  prove  that  carbonic  acid — feeble  acid  as 
it  is — suffices  to  treble  the  speed  of  the  reaction,  and  that  traces  of 
sodium  chloride  (Ol  per  cent.)  accelerated  it  nearly  as  much  as 
carbonic  acid.  A  rise  of  a  few  degrees  in  temperature  was  also 
found  to  hasten  the  reaction  in  a  very  marked  degree.  The  presence 
of  alkalies,  alkaline  earths,  and  salts  having  an  alkaline  reaction, 
was  found  to  retard  the  speed  of  the  reduction. 

Williams  has,  upon  these  experiments,  founded  a  simple  and 
expeditious  process  for  determining  the  nitric  and  nitrous  acid  in 
water  analysis,  which,  when  used  with  skill,  may  be  applied  to  by 
far  the  greater  number  of  waters  with  which  the  analyst  is  usually 
called  upon  to  deal.  The  requisite  copper-zinc  couple  is  prepared 
in  the  following  manner  : — The  zinc  employed  should  be  clean, 
and  for  the  sake  of  convenience  should  be  in  the  form  of  foil  or 
very  thin  sheet.  It  should  be  introduced  into  a  flask  or  bottle, 
and  covered  with  a  solution  of  copper  sulphate,  containing  about 
3  per  cent,  of  the  crystallized  salt,  which  should  be  allowed  to 
remain  upon  it  until  a  copious,  firmly  adherent  coating  of  black 
copper  has  been  deposited.  This  deposition  should  not  be  pushed 
too  far,  or  the  copper  will  be  so  easily  detached  that  the  couple 
cannot  be  washed  without  impairing  its  activity.  When  sufficient 
copper  has  been  deposited  the  solution  should  be  poured  off,  and 
the  conjoined  metals  washed  with  distilled  water.  The  wet  couple 
is  then  ready  for  use. 

To  use  this  couple  for  the  determination  of  nitrates  it  should  be 
made  in  a  wide-mouthed  stoppered  bottle.  After  washing,  it  is 

*  J.  C.  S.  1881,  100,  and  Analyst,  1881,  36. 


NITROGEN    AS    NITRATES    AND    NITRITES.  467 

soaked  with  distilled  water  ;  to  displace  this,  it  is  first  washed  with 
some  of  the  water  to  be  analyzed,  and  the  bottle  filled  up  with 
a  further  quantity  of  the  water.  The  stopper  is  then  inserted,  and 
the  bottle  kept  in  a  warm  place  for  a  few  hours.  If  the  bottle  be 
well  filled  and  stoppered,  the  temperature  may  be  raised  to  30°  C., 
or  even  higher,  without  any  fear  of  losing  ammonia.  The  reaction 
will  then  proceed  very  rapidly  ;  but  if  it  be  desired  to  hasten  the 
reaction  still  more,  a  little  salt*  should  be  added  (about  Ol  gm.  to 
every  100  c.c.),  or  if  there  be  any  objection  to  this,  the  water  may 
have  carbonic  acid  passed  through  it  for  a  few  minutes  before  it  is 
poured  upon  the  couple.  In  the  case  of  calcareous  waters,  the 
same  hastening  effect  may  be  obtained,  and  the  lime  may  at  the 
same  time  be  removed  by  adding  a  very  little  pure  oxalic  acid  to 
the  water  before  digesting  it  upon  the  couple.  Williams  has 
shown  that  nitrous  acid  always  remained  in  the  solution  until  the 
reaction  was  finished.  By  testing  for  nitrous  acid  the  completeness 
of  the  reaction  may  be  ascertained  with  certainty,  and  perhaps  the 
most  delicate  test  that  can  be  applied  for  this  purpose  is  that  of 
Griess,  in  which  metaphenylene-diamine  is  the  reagent  employed. 
When  a  solution  of  this  substance  is  added  to  a  portion  of  the  fluid, 
and  acidified  with  sulphuric  acid,  a  yellow  colouration  is  produced 
in  about  half  an  hour  if  the  least  trace  of  a  nitrate  be  present. 
The  reaction  easily  detects  one  part  of  nitrous  acid  in  ten  millions 
of  water.  When  no  nitrous  acid  is  found,  the  water  is  poured  off 
the  couple  into  a  stoppered  bottle,  and,  if  turbid,  allowed  to  subside. 
A  portion  of  the  clear  fluid,  more  or  less  according  to  the  concentra- 
tion of  the  nitrates  in  the  water,  is  put  into  a  Nessler  glass, 
diluted  if  necessary,  and  titrated  with  Nessler's  re-agent  in  the 
ordinary  way. 

This  process  may  be  used  for  the  majority  of  ordinary  waters — 
for  those  that  are  coloured,  and  those  that  contain  magnesium  or 
other  substances  sufficient  to  interfere  with  the  Nessler  reagent, 
a  portion  of  the  fluid  poured  off  the  couple  should  be  put  into  a  small 
retort,  and  distilled  with  a  little  pure  lime  or  sodium  carbonate, 
and  the  titration  of  the  ammonia  performed  upon  the  distillates. 

About  one  square  decimetre  of  zinc  should  be  used  for  every 
200  c.c.  of  a  water  containing  five  parts  or  less  of  nitric  acid  in 
100,000.  A  larger  proportion  should  be  used  with  waters  richer 
in  nitrates.  The  couple,  after  washing,  may  be  used  for  two  or 
three  waters  more.  When  either  carbonic  or  oxalic  or  any  other 
acid  has  been  added  to  the  water,  a  larger  proportion  of  Nessler 
reagent  should  be  employed  in  titrating  it  than  it  is  usual  to  add. 
3  c.c.  to  100  of  the  water  are  sufficient  in  almost  all  cases. 

Bluntf  points  out  that  the  above  process  may  be  used  without 
distillation,  and  with  accuracy,  in  the  case  of  any  water,  by  adding 

*  Dr.  Me  Go  wan  (Report  on  Analysis  of  Sewage  and  Sewage  Effluents)  states 
that  "  The  addition  of  sodium  chloride,  to  accelerate  the  action  of  the  couple,  is 
absolutely  necessary  :  if  it  be  omitted,  the  figure  obtained  for  nitrate  in  a  well- 
nitrated  effluent  will  almost  certainly  be  too  low." 

t  Analyst,  vi.  202. 

2  H  2 


468  WATER   AND    SEWAGE. 

oxalic  acid  to  a  double  quantity  of  the  sample,  dividing,  and  using 
one  portion  (clarified  completely  by  subsidence  in  a  closely  stoppered 
bottle)  as  a  comparison  liquid  for  testing  against  the  other,  which 
has  been  treated  with  the  copper-zinc  couple,  When, dilution  is 
used  it  must  be  done  in  both  portions  equally.  This  plan  possesses 
the  advantages  that  an  equal  turbidity  is  produced  by  Nessler 
in  both  portions,  and  any  traces  of  ammonia  contained  in  the 
oxalic  acid  will  have  the  error  due  to  it  corrected. 

A  convenient  method  for  this  process  is  mentioned  by  Keating 
Stock  as  follows  : — A  wide-mouthed  stoppered  bottle  holding  about 
200  c.c.  is  filled  nearly  to  the  neck  with  granulated  zinc.  Water  is 
added,  then  a  few  drops  of  sulphuric  acid  (1  to  3)  and  10  c.c.  of 
3  per  cent,  solution  of  copper  sulphate.  The  stopper  is  inserted, 
and  the  bottle  is  vigorously  shaken  for  one  minute,  during  which 
time  the  stopper  is  held  by  a  finger,  and  the  operation  is  performed 
over  the  sink.  The  stopper  is  now  removed,  and  the  mouth  of  the 
bottle  is  covered  with  a  piece  of  soft  copper  gauze.  The  couple  is 
then  thoroughly  washed  at  the  tap  and  drained.  100  c.c.  of  the 
water  to  be  analyzed  are  placed  in  the  bottle;  the  stopper  is 
securely  inserted,  and  the  arrangement  is  allowed  to  stand  at  rest 
at  a  temperature  of  from  20  to  25°  C.  for  48  hours.  The  test  is 
completed  by  thoroughly  shaking  the  bottle,  drawing  off  50  c.c. 
of  the  water,  adding  this  to  200  c.c.  of  ammonia-free  water  in  a 
retort  or  flask,  running  in  5  c.c.  saturated  sodium  carbonate, 
distilling  and  nesslerizing  as  usual.  This  process  has  been  found 
correct  between  the  limits  of  0'086  and  4-181  grains  of  nitric 
nitrogen  per  gallon,  when  pure  potassium  nitrate  was  used  in 
solution  in  ammonia-free  distilled  water.  The  couple  when  washed 
and  recoppered  is  again  ready  for  use.  These  couples  will  last 
for  many  months,  and  their  convenience  will  be  obvious  to  any 
one  who  has  had  to  clean  and  prepare  a  number  of  zinc  foils  at  one 
operation.  It  will  be  well  to  add  that  all  new  stoppered  bottles 
intended  for  this  purpose  should  have  their  stoppers  carefully 
reground  into  the  necks  with  a  little  fine  emery  and  dilute  sulphuric 
acid. 

In  calculating  the  amount  of  nitric  acid  contained  in  a  water 
from  the  amount  of  ammonia  obtained  in  this  process,  deductions 
must  of  course  be  made  for  any  ammonia  pre-existing  in  the  water, 
as  well  as  for  that  derived  from  any  nitrous  acid  present. 

8.     Colorimetric  Methods. 

Phenol-Sulphonic  Acid  Method  (S  p  r  e  n  g  e  1). — This  method  is 
applicable  chiefly  to  waters  where  only  small  proportions  of  nitric 
acid  or  nitrates  are  to  be  determined,  nitrites  are  not  affected  by 
it.  The  solutions  required  are — 

Standard  potassium  nitrate. — 0'722  gm.  of  KNO3  is  dissolved  in 
a  litre  of  water.  1  c.c.  of  this  solution  =  0'1  mgm.  of  N.  100  c.c. 
of  it  should  be  diluted  to  a  litre  for  use  in  the  actual  analysis,  and 


NITROGEN  AS  NITRATES.  469 

10  c.c.  taken  (  =  TV  mgm.  N),  to  avoid  the  possible  error  resulting 
from  measuring  only  1  c.c. 

Phenol-Sulphonic  Acid. — Mix  together  two  parts  by  measure  of 
phenol*  and  five  parts  of  pure  concentrated  sulphuric  acid,  and 
heat  in  a  porcelain  basin  on  the  water-bath  for  about  six  hours. 
When  cool,  add  1J  volumes  of  water  and  |  volume  strong  hydro- 
chloric acid  to  each  volume  of  the  phenol-sulphonic  acid. 

Convenient  quantities  are  80  c.c.  phenol,  200  c.c.  H2S04 ;  420 
c.c.  water  and  140  c.c.  HC1,  producing  840  c.c.  of  a  light  brown 
solution,  which  is  ready  for  immediate  use. 

According  to  Chamot  and  Pratt  the  phenol-sulphonic  acid 
contains  phenol — 2  :  4 — disulphonic  ^acid,  >  together  with  small 
quantities  of  p — phenol-sulphonic  acid. 

METHOD  OF  PROCEDURE  :  10  c.c.  of  the  water  under  examination  and  10  c.c. 
of  the  standard  potassium  nitrate  are  pipetted  into  two  small  beakers  and  placed 
near  the  edge  of  a  hot  plate.  When  nearly  evaporated  they  are  removed  to  the 
top  of  the  water- oven  and  left  there  till  they  are  evaporated  to  complete  dryness. 
As  this  operation  usually  takes  about  an  hour  and  a  half,  it  is  better,  when  time  is 
an  object,  to  evaporate  to  dryness  in  a  platinum  dish  over  steam.  The  residue  in 
each  case  is  then  treated  with  1  c.c.  of  the  phenol-sulphonic  acid,  and  the  beakers 
are  placed  on  the  top  of  the  water-oven.  If  the  water  under  examination  contain 
a  large  quantity  of  nitrates  the  liquid  speedily  assumes  a  red  colour,  which,. in  a 
good  water,  will  not  appear  for  about  ten  minutes.  After  standing  for  fifteen 
minutes  the  beakers  are  removed,  the  contents  of  each  washed  out  successively 
into  a  100  c.c.  measuring  glass,  a  slight  excess  (about  20  c.c.  of  0'96)  of  ammoniaf 
added,  the  100  c.c.  made  up  by  the  addition  of  water,  and  the  yellow  liquid 
transferred  to  a  Nessler  glass.  The  more  strongly  coloured  liquid  is  then 
partly  transferred  to  the  measuring  glass  again  and  the  tints  compared  a  second 
time.  In  this  way  the  tints  are  adjusted,  and  when,  as  far  as  possible,  matched, 
the  liquid  that  has  been  partially  removed  is  made  up  to  the  100  c.c.  mark  with 
water,  and,  after  well  mixing,  finally  compared.  If  not  exactly  the  same,  a  new 
liquid  can  at  once  be  made  up,  probably  of  exactly  the  same  tint,  as  the  first 
experiment  gives  very  nearly  the  number  of  c.c.,  of  the  one  equivalent  to  the  100 
c.c.  of  the  other.  A.  E.  Johnson  in  his  very  useful  Analyst's  Laboratory 
Companion  (p.  82)  has  given  a  table  for  obtaining  the  nitrogen  in  parts  per  100,000, 
and  also  in  grains  per  gallon,  by  this  method. 

In  the  case  of  very  good  waters,  20,  50,  or  more  c.c.  should  be  evaporated  to  a 
small  bulk,  rinsed  into  a  small  beaker,  and  evaporated  to  dryness  and  treated  as 
above — only  5  c.c.  of  the  standard  potassium  nitrate  (  =0'5  N  in  100,000)  being 
taken.  In  the  case  of  very  bad  waters,  10  c.c.  should  be  pipetted  into  a  100  c.c. 
measuring  flask  and  made  up  to  the  mark  with  distilled  water,  then  10  c.c.  of  the 
well  mixed  liquid  (  =  1  c.c.  original  water)  withdrawn  and  treated  as  above. 

It  has  for  a  long  time  been  thought  that  the  yellow  colour  produced  as  described 
above  was  due  to  the  presence  of  trinitrophenol  or  picric  acid,  but  Chamot  and 
P  r  a  1 1 1  have  recently  isolated  the  yellow  compound  and  found  it  to  consist  of 
tripotassium  6— nitrophenol— 2  :  4— disulphonate  NO2  C6H2  (S03K)2.  OK, 
1£H20  (where  potash  is  added  instead  of  ammonia).  They  found  that  picric  acid 
is  not  formed  except  in  minute  traces. 

According  to  A.  H.  G  i  1 1  \\  this  process  does  not  give  the  nitrogen  present  as  nitrite, 
since  nitrosophenol  C6H4  (NO)  (OH)  is  formed  and  this  is  colourless  in  dilute 
solutions. 

This  method  is  not  affected  by  the  presence  of  chlorides.§ 

*Calvert's  No.  2  Medical  Carbolic  Acid  answers  well. 

t  Caustic  potash  solution  answers  equally  well. 

t  J>  Amer.  Chem.  Soc.  1909,  922,  &  1910,  630  \\J.  S.  C.  I.  1895,  14,  71. 

§  Analyst,  1910,  35,  81. 


470  WATER  AND   SEWAGE. 

9.  Determination  of  Nitrites   by   Griess's    Method. — 100   c.c. 
of  the  water  are  placed  in  a  Nessler  glass,  and  1  c.c.  each  of 
metaphenylene-diamine    C6H4(NH2)2,    and    dilute    acid    (p.     442) 
added.     If  colour  is  rapidly  produced  the  water  must  be  diluted 
with  distilled  water  free  from  N2O3,  and  other  trials  made.     The 
dilution  is  sufficient  when  colour  is  plainly  seen  at  the  end  of  one 
minute.     The  weak  point  of  the  process  is   that   the  colour  is 
progressively  developed ;  however,  this  is  of  little  consequence  if 
the    comparison  with  standard  nitrite  is  made  under  the  same 
conditions  of  temperature,  dilution,  and  duration  of  experiment. 
Twenty  minutes  is  a  sufficient  time  for  allowing  the  colours  to 
develop  before  final  comparison. 

M.  W.  Williams*  obviates  the  uncertainty  of  the  comparison 
tests  by  using  colourless  Nessler  tubes,  30  mm.  wide  and  200  mm. 
long,  graduated  into  millimetres.  They  are  used  as  follows  : — 
A  rough  comparison  of  the  water  to  be  examined  with  the  standard 
nitrite  is  first  made  ;  the  glasses  are  then  filled  to  the  same  height, 
and  the  test  added,  and  allowed  to  stand  a  few  minutes.  Usually 
one  will  be  somewhat  deeper  in  colour  than  the  other.  The  height 
of  the  more  deeply  coloured  liquid  is  read  off  on  the  scale,  and 
a  portion  removed  with  a  pipette,  until  the  colours  correspond. 
The  amount  of  N203  in  the  shortened  column  is  taken  as  equal  to 
the  other,  when  a  simple  calculation  will  show  the  amount  sought. 
Stokes's  colorimeter  is  useful  for  this  purpose. 

10.  Determination  of  Nitrites  by  the  Griess-Ilosvay  Method. 
—This  method,  originally  devised  by  Griess,  has  been  improved 

by  Ilosvay,  who  introduced  the  use  of  acetic  acid  instead  of 
a  mineral  acid  ;  the  colour  so  produced  is  more  intense  and  more 
rapidly  developed.  The  test  depends  on  the  formation  of  a  pink 
azo-dye  by  the  action  of  nitrous  acid  on  a  mixture  of  naphthyamine 
and  sulphanilic  acid.  The  following  solutions  are  required : — 

(1)  1  gram  of  sulphanilic  acid  (C6H4NH2SO3H)  is  dissolved, 
with  the  aid  of  heat,  in  14'7  gm.  of  glacial  acetic  acid  mixed  with  an 
equal  bulk  of  water.     Then  more  water  is  added  gradually  to  the 
warmed  liquid,  with  constant  stirring,  till  285  c.c.  have  been  used 
altogether. 

(2)  0-2  gm.  of  ci-naphthylamine  (C10H7NH2)  is  dissolved,  with 
the  aid  of  heat,  in  14'7  gm.  of  glacial  acetic  acid  mixed  with  twice 
its  bulk  of  water  ;  then  more  water  is  added  until  325  c.c.  have 
been  used  altogether. 

These  two  solutions  are  kept  separately  and  when  mixed  in 
equal  volumes  form  the  Griess-Ilosvay  solution,  which  should  be 
made  only  when  required.  The  latter  on  keeping  tends  to  become 
pink,  owing  to  the  development  of  nitrite  in  the  solution  from 
ammonia  in  the  air  ;  it  is  not  affected  by  light. 

This  test  is  almost  too  delicate  to  be  used  quantitatively,  but  it 
may  be  done  as  follows  : — 

*  Analyst,  1881,  38. 


NITROGEN  AS  NITRITES.  471 

5  c.c.  of  standard  nitrite  solution  (1  c.c.  =0*01  mgm.  N203)  are  mixed  with 
45  c.c.  of  pure  distilled  water  in  a  Nessler  glass,  and  2  c.c,  of  the  Griess- 
Ilosvay  solution  added. 

In  a  similar  Nessler  glass  50  c.c.  of  the  water  to  be  examined  are  placed  and 
2  c.c.  of  the  mixed  solutions  added. 

Both  are  allowed  to  stand  for  15  minutes  before  the  pink  colours  are  compared. 
Then   the    comparison   is   made    as   described   in   the    metaphenyline-diamine 
method  (9). 

For  the  determination  of  nitrous  nitrogen  in  sulphuric  acid  a 
standard  solution  is  prepared  as  follows : — 

0*0493  gm.  of  pure  sodium  nitrite,  which  contains  O'Ol  gm.  of  N,  is  dissolved 
in  100  c.c.  water,  and  10  c.c.  of  this  solution  is  added  drop  by  drop  to  90  c.c.  of 
pure  sulphuric  acid  ;  the  resulting  mixture  contains  y^  mgm.  of  nitrous  nitrogen 
in  a  perfectly  stable  form.  Two  Nessler  glasses  are  used,  and  each  receives 
1  c.c.  of  the  standard  solution,  40  c.c.  of  water,  and  about  5  gm.  of  solid  sodium 
acetate.  To  one  of  these  is  added  1  c.c.  of  the  standard  solution  and  to  the  other 
1  c.c.  of  the  acid  to  be  tested,  then  both  are  well  mixed,  and  after  10  minutes  the 
colours  compared.  If  they  do  not  correspond,  the  more  strongly  coloured  liquid 
is  diluted  up  to  the  point  where  the  colour  corresponds  and  the  percentage  of 
nitrous  nitrogen  is  calculated  from  the  amount  of  dilution. 

11.  Determination  of  Nitrites  by  Potassium  Iodide  and  Starch. — 

Ekin*  has  pointed  out  that  this  well-known  test  will  give  the  blue 
colour  with  nitrous  acid  in  a  few  minutes,  when  the  proportion  is 
one  part  in  ten  millions  ;  in  twelve  hours  when  one  part  in  a  hundred 
millions  ;  and  in  forty-eight  hours  when  one  in  a  thousand  millions. 
Experience  has  proved  that  waters  charged  with  much  organic 
matter  must  be  clarified  by  the  addition  of  a  little  pure  alum,  then 
well  agitated  and  filtered  before  testing. 

Ekin  used  acetic  acid  for  acidifying  the  water  to  be  tested, 
and  blank  experiments  with  pure  water  were  simultaneously  carried 
on.  Sulphuric  or  hydrochloric  acid  will,  no  doubt,  give  a  sharper 
reaction,  but  both  these  acids  are  more  liable  to  contain  impurities 
affecting  the  reaction  than  is  the  case  with  pure  acetic  acid.  Owing 
to  the  instability  of  alkali  iodides,  zinc  iodide,  which  is  not  open 
to  this  objection,  is  now  generally  used. 

12.  Determination   of   Suspended   Matter. — Filters   of   Swedish 
paper,  about  110  mm.  in  diameter,  are  packed  one  inside  another, 
about  15  or  20  together,  so  that  water  will  pass  through  the  whole 
group,  moistened  with  dilute  hydrochloric  acid,  washed  with  hot 
distilled  water  until  the  washings  cease  to  contain  chlorine,  and 
dried.     The  ash  of  the  paper  is  thus  reduced  by  about  60  per  cent., 
and  must  be  determined  for  each  parcel  of  filter-paper  by  inc  nerat- 
ing   10  filters,   and  weighing  the   ash.     For  use  in  determining 
suspended  matter,  these  washed  filters  must  be  dried  for  several 
hours  at  120-130°  C.,  and  each  one  then  weighed  at  intervals  of  an 
hour  until  the  weight  ceases  to  diminish,  or  at  least  until  the  loss 
of  weight  between  two   consecutive  weighings  does  not   exceed 
0*0003  gm.     It  is  most  convenient  to  enclose  the  filter  during  weigh- 
ing in  two  short  tubes,  fitting  closely  one  into  the  other.     The 
closed  ends  of  test  tubes,  50  mm.  long,  cut  off  by  leading  a  crack 

*  Pharm.  Trans.  1881.  286. 


472  WATER  AND   SEWAGE. 

round  with  the  aid  of  a  pastille  or  very  small  gas  jet,  the  sharp 
edges  being  afterwards  fused  at  the  blow-pipe,  answer  perfectly. 
Each  pair  of  tubes  should  have  a  distinctive  number,  which  is 
marked  with  a  diamond  on  both  tubes.  In  the  air  bath  they  should 
rest  in  grooves  formed  by  a  folded  sheet  of  paper,  the  tubes  being 
drawn  apart,  and  the  filter  almost,  but  not  quite,  out  of  the  smaller 
tube.  They  can  then  be  shut  up  whilst  hot  by  gently  pushing  the 
tubes  together,  being  guided  by  the  grooved  paper.  They  require 
to  remain  about  twenty  minutes  in  a  desiccator  to  cool  before 
weighing.  Filtration  will  be  much  accelerated  if  the  filters  be 
ribbed  before  drying.  As  a  general  rule,  it  will  be  sufficient  to 
filter  a  quarter  of  a  litre  of  a  sewage,  half  a  litre  of  a  highly  polluted 
river,  and  a  litre  of  a  less  polluted  water  ;  but  this  must  be  frequently 
varied  to  suit  individual  cases.  Filtration  is  hastened,  and  trouble 
diminished,  by  putting  the  liquid  to  be  filtered  into  a  narrow-necked 
flask,  which  is  inverted  into  the  filter,  being  supported  by  a  funnel- 
stand,  the  ring  of  which  has  a  slot  cut  through  it  to  allow  the  neck 
of  the  flask  to  pass.  With  practice  the  inversion  may  be 
accomplished  without  loss,  and  without  previously  closing  the 
mouth  of  the  flask.  When  all  has  passed  through,  the  flask  should 
be  rinsed  out  with  distilled  water,  and  the  rinsings  added  to  the 
filter.  Thus  any  particles  of  solid  matter  left  in  the  flask  are  secured, 
'and  the  liquid  adhering  to  the  suspended  matter  and  filter  is 
displaced.  The  filtrate  from  the  washings  should  not  be  added  to 
the  previous  filtrate,  which  may  be  employed  for  determination 
of  total  solid  matter,  chlorine,  hardness,  etc. 

Thus  washed,  the  filter  with  the  matter  upon  it  is  dried  at  100°  C., 
then  transferred  from  the  funnel  to  the  same  pair  of  tubes  in  which 
it  was  previously  weighed,  dried  at  120°- 130°  C.  and  weighed 
repeatedly  until  constant.  The  weight  thus  obtained,  minus  the 
weight  of  the  filter  and  tubes,  gives  the  weight  of  the  total 
suspended  matter  dried  at  120°- 130°  C. 

To  ascertain  the  quantity  of  mineral  matter  in  this,  the  filter  with 
its  contents  is  incinerated  in  a  platinum  crucible,  and  the  total  ash 
thus  determined,  minus  the  ash  of  the  filter  alone,  gives  the  weight 
of  the  mineral  suspended  matter. 

13.  Determination  of  combined  Chlorine. — To  50  c.c.  of  the 
water  in  a  porcelain  dish  or  glass  flask  add  two  or  three  drops  of 
solution  of  potassium  chromate  (D.  ii),  so  as  to  give  it  a  faint 
tinge  of  yellow,  and  add  gradually  from  a  burette  standard  solution 
of  silver  nitrate  (D.  i),  until  the  red  silver  chromate  which  forms 
after  each  addition  of  the  nitrate  ceases  to  disappear  on  stirring  or 
shaking.  The  number  of  c.c.  of  silver  solution  employed  will 
express  the  chlorine  present  as  chloride  in  parts  in  100,000.  If 
this  amount  be  much  more  than  10,  it  is  advisable  to  take  a 
-smaller  quantity  of  water. 

If  extreme  accuracy  be  necessary,  after  completing  a  determina- 
tion, destroy  the  slight  red  tint  by  an  excess  of  a  soluble  chloride, 
and  repeat  the  determination  on  a  fresh  quantity  of  the  water  in 


HARDNESS.  473 

a  similar  flask  placed  by  the  side  of  the  former.  By  comparing 
the  contents  of  the  flasks,  the  first  tinge  of  red  in  the  second  flask 
may  be  detected  with  great  accuracy.  It  is  absolutely  necessary 
that  the  liquid  examined  should  not  be  acid,  unless  with  carbonic 
acid,  nor  more  than  very  slightly  alkaline.  It  must  also  be  colour- 
less, or  nearly  so.  These  conditions  are  generally  found  in  waters, 
but,  if  not,  they  may  be  brought  about  in  most  cases  by  rendering 
the  liquid  just  alkaline  with  lime  water  (free  from  chlorine),  passing 
carbonic  anhydride  to  saturation,  boiling,  and  filtering.  The 
calcium  carbonate  has  a  powerful  clarifying  action,  and  the  excess 
of  alkali  is  exactly  neutralized  by  the  carbonic  anhydride.  If  this 
is  not  successful,  the  water  must  be  rendered  alkaline,  evaporated 
to  dryness,  and  the  residue  gently  heated  to  destroy  organic  matter. 
The  chlorides  may  then  be  extracted  with  water,  and  determined 
in  the  ordinary  way  either  gravimetrically  or  volumetrically. 

14.  Determination  of  Hardness.* — The  following  method,  devised 
by  the  late  Dr.  Thomas  Clark,  of  Aberdeen,  is  in  general  use. 
It  serves  to  measure  more  particularly  the  soap-destroying  power 
of  waters  and  its  indications  are  most  useful.  The  test  requires 
to  be  carefully  performed  and  should  never  be  done  in  a  hurry, 
especially  in  the  case  of  magnesian  waters.  Both  the  standard- 
ization of  the  soap  solution  and  the  determination  of  the  hardness 
of  a  water  should  be  carried  out  strictly  according  to  the  following 
directions.  Before  commencing  the  determination,  however,  the 
total  solid  matter  present  in  a  water  should  be  weighed  as  it  gives 
a  useful  approximate  idea  of  how  much  to  use  for  the  soap  test. 
Although  no  rule  can  be  given,  the  total  hardness  in  many  waters 
is  about  half  the  total  solids,  and  the  sample  may  be  so  diluted, 
if  necessary,  as  to  bring  it  within  the  limit  mentioned  below. 
Thus,  if  a  water  contains  50  grains  of  solids  per  gallon  take  25  c.c. 
for  hardness. 

Measure  50  c.c.  of  the  water,  or,  if  necessary,  a  less  quantity 
(usually  25  or  10  c.c.,  together  with  25  or  40  c.c.  of  recently  boiled 
and  cooled  distilled  water)  into  a  well- stoppered  bottle  of  about 
250  c.c.  capacity,  shake  briskly  for  a  few  seconds,  and  suck  the  air 
from  the  bottle  by  means  of  a  glass  tube,  in  order  to  remove  any 
carbon  dioxide  which  may  have  been  liberated  from  the  water. 
Then  run  in  from  a  burette  standard  soap  solution  (E  ii),  one  c.c. 
at  a  time  at  first  and  about  0-5  c.c.  towards  the  end  of  the 
operation,  shaking  vigorously  after  each  addition,  until  a  lather 
is  obtained,  which,  when  the  bottle  is  laid  at  rest  on  its  side, 
remains  persistent  for  5  minutes.  Waters  containing  much  mag- 
nesium salts  give  a  false  lather  after  the  addition  of  only  a  few 
c.c.  of  soap  solution ;  this,  however,  disappears  entirely  on 
allowing  the  bottle  to  remain  for  several  minutes  on  its  side  after 
an  extra  vigorous  shaking.  Such  waters  must  always  be  so  diluted 
that  not  more  than  7  c.c.  of  the  soap  solution  are  required  to 

*For  the  determination  of  hardness  of  waters  by  titration,  see  p.  74. 


474 


WATER   AND    SEWAGE. 

Results  of  Analysis  expressed 


Number 

of 

DESCRIPTION. 

REMARKS. 

Sample. 

Upland  Surface  Waters. 

1. 

The  Dee  above  Balmoral,  March  9th,  1872 

Clear    

II. 

Glasgow  Water  supply  from  Loch  Katrine  —  average  of    ) 
monthly  analyses  during  five  years,  1876  —  81         ) 

Clear  ;  very  pale  brow 

III. 
IV. 

LiverpoolWatersupply  fromRivingtonPike,  June4th,  1869 
Manchester  Water  supply,  May  9th,  1  874 

Clear    
Turbid 

V. 

Cardiff  Water  supply,  Oct.  18th,  1872    

Clear    

Surface  Water  from  Cultivated  Land. 

VI. 

Dundee  Water  supply,  March  12th,  1872 

Turbid;  brownish  yel!< 

VII. 

Norwich  Water  supply,  June  18th,  1872 

Slightly  turbid 

Shallow  Wells. 

VIII. 

Cirencester,  Market  Place,  Nov.  4th,  1870 

Slightly  turbid 

IX. 

Marlborough,  College  Yard,  Aug.  22nd,  1873    .. 

Clear    

X. 

Birmingham,  Hurst  Street,  Sept.  18th,  1873 

Clear;  strong  saline  tast 

XI. 

Sheffield,  Well  near,  Sept.  27th,  1870     

C  Very  turbid  &  offer 
sive.        Swarniin 

(.     with  bacteria,  &< 

XII. 

London,  Aid  gate  Pump,  June  5th,  1872 

Clear    

XIII. 

London,  Wellclose  Square,  June  5th,  1872 

Slightly  turbidjsalineta? 

XIV. 

Leigh,  Essex,  Churchyard  Well,  Nov.  28th,  1871 

Slightly  turbid 

Deep  Wells. 

XV. 

Birmingham,  Short  Heath  Well,  May  16th,  1873 

Clear    

XVI. 

Caterham,  Water  Works  Well,  Feb.  14th,  1873 

Clear    

Ditto   Softened  (Water  supply) 

XVII. 

London,  Albert  Hall,  May,  1872  

Slightly  turbid 

XVIII. 

Gravesend,  Railway  Station,  Jan.  17th,  1873    .. 

Clear    .. 

Springs. 

XIX. 

Dartmouth  Water  supply,  Jan.  8th,  1873 

Turbid  

XX. 

Grantham  Water  supply,  July  llth,  1873 

Clear 

London  Water  supply  —  average  monthly  analyses  during  21 

years,  1869—89. 

XXI. 

From  the  Thames 

XXII. 

From  the  Lea 

XXIII. 

From  Deep  Chalk  Wells  (Kent  Company) 

. 

XXIV. 

•Ditto(ColneValleyCo.)softened  —  thirteen  years,  1877  —  89 

.  . 

XXV. 

Ditto  (Tottenham)—  thirteen  years,  1877—89 



XXVL 

^Birmingham  Water  supply  —  average  monthly  analyses,  18 

75—1880. 

Average  Composition  of  Unpolluted  Water. 

XXVII. 

Rain  \Vater  .  .          .            .  .            .            .     39  samples 

XXVIIL 

Upland  Surface  Water       .  .         .  .         .  .   195      „ 

xxix! 

Deep  Well  Water    157 

XXX. 

Spring  Water           .  .          .  .          .  .          .  .   198       „ 

XXXT 

Sea  Water     .  .         .  .         .  .         .  .                23 

^.v^X^XX* 

Sewage. 

XXXII. 

Average  from  15  "  Midden  "  Towns,  37  analyses 

.  . 

XXXIII. 

Average  from  16  "  Water  Closet  "  Towns,  50  analyses 

.  . 

XXXIV. 

Salford,  Wooden  Street  Sewer,  March  15th,  1869 

• 

XXXV. 

Merthyr  Tydfil,  average  10  a.m.  to  5  p.m.,  Oct.  20th,  } 

1871  (after  treatment  with  lime)      .  .          .  .          ) 

.  . 

XXXVI. 

Ditto,  Effluent  Water 

This  is  the  old  supply,  not  the  Welsh  water  with  which  Birmingham  is  now  supplied. 


TYPICAL   ANALYSES. 


in  parts  per  100,000. 


bal 

>tid 

liter. 

Organic 
Carbon. 

Organic 
Nitro- 
gen. 

4 

|o1* 

Nitro- 
gen as 
Am- 
monia. 

Nitrogen 
as 
Nitrates 
and 
Nitrites. 

Total 
Inorganic 
Nitrogen. 

Total 
Combined 
Nitrogen. 

Chlorine. 

Hardness. 

Tem- 
porary. 

Perma- 
nent. 

Total. 

1-52 

•132 

•014 

9-4 

0 

0 

0 

•014 

•50 

0 

1-5 

1-5 

J-94 

•148 

•016 

9-2 

0 

•005 

•005 

•022 

•64 

— 

— 

•9 

i-OG 

•210 

•029 

7-2 

•002 

0 

•002 

•031 

1-53 

•3 

3-7 

4-0 

700 

•132 

•031 

4-1 

•002 

0 

•002 

•033 

•90 

0 

2-7 

2-7 

J-50 

•212 

•031 

6-8 

0 

•034 

•034 

•065 

1-40 

7-1 

12-9 

20O 

,i  -16 

•418 

•059 

7-1 

•001 

•081 

•082 

•141 

1-75 

0 

6-0 

6-0 

D-92 

•432 

•080 

5-4 

•012 

•048 

•036 

•128 

3-10 

21-3 

5-3 

26-6 

1-00 

•041 

•008 

5-1 

0 

•362 

•362 

•370 

1-60 

18-4 

4-6 

23-0 

2'48 

•049 

•015 

3-3 

0 

•613 

•613 

•628 

1-90 

15-6 

10-1 

25-7 

)-20 

•340 

•105 

3-2 

•511 

14-717 

15-228 

15-333 

36-50 

27-5 

99-6 

127-1 

3-50 

1-200 

•126 

9-5 

•091 

0 

•091 

•217 

2-20 

2-0 

1-4 

3-4 

3-10 

•144 

•141 

1-0 

•181 

6-851 

7-032 

7-173 

12-85 

37-1 

40-0 

77-1 

16-50 

•278 

•087 

3-2 

0 

25-840 

25-840 

25-927 

34-60 

26-7 

164-3 

191-0 

i2-12 

•210 

•065 

3-2 

0 

5-047 

5-047 

5-112 

13-75 

14-3 

45-7 

60-0 

5-08 

•009 

•004 

2-2 

0 

•447 

•447 

•451 

1-30 

4-6 

5-1 

9-7 

7-68 

•028 

•009 

3-1 

0 

•021 

•021 

•030 

1-55 

15-2 

6-0 

21-2 

8-80 

•015 

•003 

5-0 



— 

— 

— 

— 

— 

— 

4-4 

1-68 

•168 

•042 

4-0 

•007 

•066 

•073 

•115 

15-10 

3-4 

2-2 

5-6 

8-00 

•127 

•029 

4-4 

•063 

2-937 

3-000 

3-029 

5-40 

27-9 

14-5 

42-4 

7-36 

•060 

•016 

3-7 

0 

•330 

•330 

•346 

2-45 

1-6 

10-0 

11-6 

0-20 

•048 

•018 

2-7 

0 

•833 

•833 

•851 

2-05 

17-1 

6-5 

23-6 

8-02 

•191 

•033 

5-8 

0 

•210 

•210 

•243 

1-68 

_ 

_ 

20-1 

8-99 

•134 

•025 

5-4 

0 

•226 

•226 

•251 

1-76 

— 

— 

20-9 

1-60 

•049 

•on 

4-5 

0 

•446 

•446 

•458 

2-47 

— 

— 

28-5 

4-40 

•059 

•014 

4-2 

•003 

•367 

•370 

•384 

1-70 

— 

— 

6-0 

1-39 

•068 

•016 

4-2 

•054 

•143 

•196 

•196 

2-85 

— 

— 

23-3 

6-01 

•245 

•054 

4-6 

•002 

•231 

•233 

•287 

1-73 

7-7 

8-8 

16-6 

2-95 

•070 

•015 

4-7 

•024 

•003 

•027 

•042 

•22 

_ 

•3 

9-67 

•322 

•032 

10-1 

•002 

•009 

•Oil 

•043 

M3 

1-5 

4-3 

5-4 

3-78 

•061 

•018 

3-4 

•010 

•495 

•505 

•523 

5-11 

15-8 

9-2 

25-0 

8-20 

•056 

•013 

4-3 

•001 

•383 

•384 

•397 

2-49 

11-0 

7-5 

18-5 

'98-7 

•278 

•165 

1-7 

•005 

•033 

•038 

•203 

1975-6 

48-9 

748-0 

796-9 

Suspended  Matter. 

Mineral.     Organic.     Total. 

82-4 

4-181 

1-975 

2-1 

4-476 

0 

4-476 

6-451 

11-54 

17-81 

21-30 

39-11 

72-2 

4-696 

2-205 

2-1 

5-520 

•003 

5-523 

7-728 

10-66 

24-18 

20-51 

24-69 

19-6 

11-012 

7-634 

1-4 

5-468 

0 

5-468 

13-102 

20-50 

18-88 

26-44 

45-32 

9-20 

1-282 

•952 

1-3 

1-054 

•052 

1-106 

2-058 

5-25 

7-88 

6-56 

14-44 

i-48 

•123 

•031 

4-0 

•048 

•300 

•348 

•379 

2-60 

Trace. 

476  WATER   AND    SEWAGE. 

produce  a  permanent  lather.  In  other  cases  the  addition  of  soap 
solution  may  be  continued  until  not  more  than  16  c.c.  are  required 
to  produce  a  permanent  lather,  which  in  all  cases  is  attained  when, 
on  rolling  the  bottle  half-way  round  after  5  minutes  standing,  the 
lather  still  covers  the  whole  surface  without  breaking.  The 
burette  is  then  read  and  the  hardness  ascertained  from  Table  7, 
the  results  being  multiplied  by  2  or  5  when  25  or  10  c.c.  of  the 
water,  diluted  to  50  c.c.,  have  been  used. 

It  is  very  important  to  note  that  even  when  the  hardness  of  a 
water  is  approximately  known,  or  when  making  a  duplicate 
determination,  the  soap  solution  must  always  be  added  in  c.c.'s 
(or  less)  at  a  time,  with  shaking  after  each  addition,  and  never  in 
large  quantities. 

When  water  containing  calcium  and  magnesium  carbonates,  held 
in  solution  by  carbonic  acid,  is  boiled,  carbonic  anhydride  is  expelled, 
and  the  carbonates  precipitated.  The  hardness  due  to  these  is 
said  to  be  temporary,  whilst  that  due  to  calcium  and  magnesium 
sulphates,  chlorides,  etc.,  and  to  the  amount  of  their  carbonates 
soluble  in  pure  water  (the  last-named  being  about  three  parts  per 
100,000)  is  called  permanent. 

To  determine  permanent  hardness,  a  known  volume  of  the 
water  (say  100  c.c.)  is  boiled  gently  for  half  an  hour  in  a  flask, 
the  mouth  of  which  is  freely  open.  The  level  of  the  water  should 
be  marked  by  an  ink  or  blue  pencil  mark  on  the  flask,  and  hot 
water  added  from  time  to  time  to  make  up  the  loss  by  evaporation. 
At  the  end  of  half  an  hour,  close  the  flask  with  a  glass  marble 
and  cool  to  ordinary  temperature,  then  make  up  to  the  original 
volume  by  addition  of  recently  boiled  and  cooled  distilled  water, 
filter  through  a  dry  filter,  and  determine  the  hardness  in  the 
filtrate.  The  hardness  thus  found,  deducted  from  that  of  the 
unboiled  water,  will  give  the  temporary  hardness. 

According  to  Prof.  H.  Jackson*:  "  Every  gallon  of  pure 
water  requires  about  10  grains  of  ordinary  soap  before  a  lather  can 
be  produced,  and  each  degree  of  hardness  will  necessitate  the 
addition  of  another  quantity  of  10  grains  of  soap." 

15.  Mineral  Constituents  and  Metals. — The  quantities  of  the 
following  substances  which  may  be  present  in  a  sample  of  water 
are  subject  to  such  great  variations  that  no  definite  directions  can 
be  given  as  to  the  volume  of  water  to  be  used.  The  analyst  must 
judge  in  each  case  from  a  preliminary  experiment  what  will  be 
a  convenient  quantity  to  take. 

Sulphuric  Acid. — Acidify  a  litre  or  less  of  the  water  with 
hydrochloric  acid,  concentrated  on  the  water-bath  to  about  100  c.c. 
and  while  still  hot  add  a  slight  excess  of  barium  chloride.  Filter, 
wash,  ignite,  and  weigh  as  barium  sulphate,  or  determine  volu- 
me trically,  as  on  p.  349. 

*  Cantor  Lectures  on  Detergents  and  Bleaching  Agents  used  in  Laundry  Work, 


MINERAL   CONSTITUENTS.  477 

Sulphuretted  Hydrogen. — Titrate  with  a  standard  solution  of 
iodine,  as  on  p.  348. 

Phosphoric  Acid. — This  substance  may  be  determined  in  the 
solid  residue  obtained  by  evaporation,  by  moistening  it  with  nitric 
acid,  and  again  drying  to  render  silica  insoluble  ;  the  residue  is 
again  treated  with  dilute  nitric  acid,  filtered,  molybdic  solution 
added,  and  set  aside  for  twelve  hours  in  a  warm  place  ;  filter, 
dissolve  the  precipitate  in  2J  %  ammonia,  precipitate  with  magnesia 
mixture,  and  weigh  as  magnesium  pyrophosphate,  or  determine 
volumetrically  as  on  p.  307  et  seq. 

Another  method  is  to  add  to  500  c.c.  of  the  sample  about  10  c.c. 
of  solution  of  alum,  then  a  few  drops  of  ammonia,  lastly  acidify 
slightly  with  acetic  acid,  and  set  aside  to  allow  the  precipitated 
A1P04  to  settle.  The  clear  Liquid  may  then  be  poured  off,  the 
precipitate  dissolved  in  nitric  acid  and  determined  with  molybdic 
solution. 

These  determinations  are  only  possible  in  cases  where  the  P205 
is  very  large.  In  most  waters  it  is  simply  necessary  to  record 
whether  the  molybdic  precipitate  is  in  heavy  or  minute  traces. 

Silicic  Acid. — Acidify  a  litre  or  more  of  the  water  with  hydro- 
chloric acid,  evaporate,  and  dry  the  residue  thoroughly.  Then 
moisten  with  hydrochloric  acid,  dilute  with  hot  water,  and  filter 
off,  wash,  ignite,  and  weigh  the  separated  silica. 

Iron.  To  the  filtrate  from  the  determination  of  silicic  acid  add 
a  few  drops  of  nitric  acid,  dilute  to  about  100  c.c.,  and  determine 
by  colour  titration,  as  on  p.  238  ;  or  where  the  amount  is  large, 
add  a  slight  excess  of  ammonia,  and  heat  gently  for  a  short  time. 
Filter  off  the  precipitate  and  determine  the  iron  in  the  washed 
precipitate  colorimetrically. 

Calcium. — To  the  filtrate  from  the  iron  determination  add  excess 
of  ammonium  oxalate,  filter  off  the  calcium  oxalate,  ignite  and 
weigh  as  calcium  carbonate  or  as  lime,  or  determine  volumetrically 
with  permanganate  as  on  p.  172. 

Magnesium. — To  the  concentrated  filtrate  from  the  calcium 
determination  add  sodium  phosphate  (or,  if  alkalies  are  to  be 
determined  in  the  filtrate,  ammonium  phosphate),  and  allow  to 
stand  for  twelve  hours  in  a  warm  place.  Filter,  ignite  the 
precipitate,  and  weigh  as  magnesium  pyrophosphate,  or,  without 
ignition,  titrate  with  uranium. 

Barium.  Is  best  detected  in  a  water  by  acidifying  with  hydro- 
chloric acid,  filtering  perfectly  clear  if  necessary,  then  add  a  clear 
solution  of  calcium  sulphate,  and  set  aside  in  a  warm  place.  Any 
white  precipitate  which  forms  is  due  to  barium. 

Potassium  and  Sodium. — These  are  generally  determined  jointly, 
and  for  this  purpose  the  filtrate  from  the  magnesium  determination 
may  be  used.  Evaporate  to  dryness,  and  heat  gently  to  expel 
ammonium  salts,  remove  phosphoric  acid  with  lead  acetate,  and 


478  WATER   AND    SEWAGE. 

the  excess  of  lead  in  the  hot  solution  by  ammonia  and  ammonium 
carbonate.  Filter,  evaporate  to  dryness,  heat  to  expel  ammonium 
salts,  and  weigh  the  alkalies  as  chlorides. 

It  is,  however,  generally  less  trouble  to  employ  a  separate  portion 
of  water.  Add  to  a  litre  or  less  of  the  water  enough  pure  barium 
chloride  to  precipitate  the  sulphuric  acid,  boil  with  pure  milk  of 
lime,  filter,  concentrate,  and  remove  the  excess  of  lime  with 
ammonium  carbonate  and  a  little  oxalate.  Filter,  evaporate,  and 
weigh  the  alkali  chlorides  in  the  filtrate.  If  the  water  contains  but 
little  sulphate,  the  barium  chloride  may  be  omitted,  and  a  little 
ammonium  chloride  added  to  the  solution  of  alkali  chlorides. 

If  potassium  and  sodium  must  each  be  determined,  separate  the 
potassium  by  means  of  platinic  chloride  ;  or,  after  weighing  the 
mixed  chlorides,  determine  the  chlorine,  present  in  them,  and 
calculate  the  amounts  of  potassium  and  sodium  by  the  following 
formula  : — Calculate  all  the  chlorine  present  as  potassium  chloride  ; 
deduct  this  from  the  weight  of  the  mixed  chlorides,  and  call  the 
difference  d.  Then  as  16*1  :  58*46  :  :  d  :  NaCl  present.  (See  also 
p.  144.)  Or  the  sodium  chloride  may  be  determined  byFenton's 
method,  p.  65. 

Lead. — May  be  determined  by  the  method  proposed  by  Miller. 
Acidulate  the  water  with  two  or  three  drops  of  acetic  acid,  and 
add  ~  of  its  bulk  of  saturated  aqueous  solution  of  sulphuretted 
hydrogen.  Compare  the  colour  thus  produced  in  the  colorimeter, 
or  a  convenient  cylinder,  with  that  obtained  with  a  known  quantity 
of  a  standard  solution  of  a  lead  salt,  in  a  manner  similar  to  that 
described  for  the  determination  of  iron  (p.  238).  The  lead  solution 
should  contain  0*1831  gm.  of  normal  crystallized  lead  acetate  in 
a  litre  of  distilled  water,  and  therefore  each  c.c.  contains  0*0001  gm. 
of  metallic  lead. 

It  is  obvious  that  in  the  presence  of  copper  or  other  heavy  metals 
the  colour  produced  by  the  above  method  will  all.be  ascribed  to 
lead  ;  it  is  preferable,  therefore,  to  adopt  the  method  of  Harvey,* 
in  which  the  lead  is  precipitated  as  chromate.  The  results,  how- 
ever, are  not  absolute  as  to  quantity,  except  so  far  as  the  eye  may 
be  able  to  measure  the  amount  of  precipitate. 

The  standard  lead  solution  is  the  same  as  in  the  previous  method. 
The  precipitating  agent  is  pure  potassium  dichromate,  in  fine 
crystals  or  powder. 

250  c.c.  or  so  of  the  water  are  placed  in  a  Phillips's  jar  with 
a  drop  or  two  of  acetic  acid,  and  a  few  grains  of  the  reagent  added, 
and  agitated  by  shaking.  One  part  of  lead  in  a  million  parts  of 
water  will  show  a  distinct  turbidity  in  five  minutes  or  less.  In  six 
or  eight  hours  the  precipitate  will  have  completely  settled,  and  the 
yellow  clear  liquid  may  be  poured  off  without  disturbing  the 
sediment,  which  may  then  be  shaken  up  with  a  little  distilled 
water,  and  its  quantity  judged  by  comparison  with  a  similar 
experiment  made  with  the  standard  lead  solution. 

*  Analyst,  6,  146. 


METALS.  479 

Copper. — Determine  by  colour  titration,  as  on  p.  204. 

Arsenic. — Add  to  half  a  litre  or  more  of  the  water  enough  sodium 
hydrate,  free  from  arsenic,  to  render  it  slightly  alkaline,  evaporate 
to  dryness,  and  extract  with  a  little  concentrated  hydrochloric 
acid.  Introduce  this  solution  into  the  generating  flask  of  a  small 
Marsh's  apparatus,  and  pass  the  evolved  hydrogen,  first  through 
a  U-tube  filled  with  pumice,  moistened  with  lead  acetate,  and  then 
through  a  piece  of  hard  glass  tube  about  150  mm.  in  length,  and 
3  mm.  in  diameter  (made  by  drawing  out  combustion  tube).  At 
about  its  middle,  this  tube  is  heated  to  redness  for  a  length  of  about 
20  mm.  by  the  flame  of  a  small  Bun  sen  burner,  and  here  the 
arseniuretted  hydrogen  is  decomposed,  arsenic  being  deposited  as 
a  mirror  on  the  cold  part  of  the  tube.  The  mirror  obtained  after 
the  gas  has  passed  slowly  for  an  hour  is  compared  with  a  series  of 
standard  mirrors  obtained  in  a  similar  way  from  known  quantities 
of  arsenic.  Care  must  be  taken  to  ascertain  in  each  experiment 
that  the  hydrochloric  acid,  zinc,  and  whole  apparatus  are  free  from 
arsenic,  by  passing  the  hydrogen  slowly  through  the  heated  tube 
before  introducing  the  solution  to  be  tested.  The  best  form  of 
apparatus  (Marsh-Berzelius)  is  that  which  is  now  used  for  detect- 
ing and  determining  small  quantities  of  arsenic  in  beer,  malt,  etc. 
An  electrolytic  form  of  apparatus  is  also  largely  used. 

Zinc.— This  metal  usually  exists  in  waters  as  bicarbonate,  and  on 
exposure  of  such  waters  in  open  vessels  a  film  of  zinc  carbonate 
forms  on  the  surface  ;  this  is  collected  on  a  platinum  knife  or  foil 
and  ignited.  The  residue  is  of  a  yellow  colour  when  hot,  and  turns 
white  on  cooling.  The  reaction  is  exceedingly  delicate.  Potassium 
ferrocyanide  produces  a  turbidity  in  such  waters  owing  to  the 
insolubility  of  zinc  ferrocyanide.  The  reagent  will  detect  1  part 
of  zinc  in  2,000,000  of  water. 


DETERMINING    THE    HARDNESS    OF    WATERS. 

The  method  as  arranged  by  Hehner  is  described  on  p.  74,  but 
a  paper  read  before  the  Yorkshire  section  of  the  Society  of  Chemical 
Industry,  by  H.  R.  Procter,  proposes  a  somewhat  modified 
method  leading  in  many  cases  to  greater  accuracy.  The  following 
is  a  part  of  that  paper  as  written  by  him.*  The  greater  part  has 
relation  to  the  technical  plans  of  water  softening  for  steam  boilers 
and  other  purposes. 

Hehner  titrates  the  temporary  or  bicarbonate  hardness  with  N/io  HC1,  using 
methyl  orange  as  an  indicator,  which  is  practically  insensitive  to  carbonic  acid, 
The  method  gives  very  exact  results  if  certain  precautions  are  taken.  Methyl 
orange  is  the  sodium  salt  of  a  colour  acid  of  moderate  strength,  and  the  change 
from  the  yellow  salt  condition  to  the  red  colour  of  the  free  acid  marks  the  end 
point,  which  is  sharp  and  exact  when  working  with  strong  mineral  acids,  and 
with  normal  solutions.  Even  in  this  case  it  is  desirable  to  use  the  smallest 

*  J.  S.  C.  /.  23,  1904,  8. 


480  WATER   AND    SEWAGE. 

possible  amount  of  the  indicator,  but,  in  working  with  N/1O  solutions,  the 
amount  of  acid  required  to  completely  decompose  the  colour  salt  becomes  very 
perceptible,  and  the  change  from  yellow  to  red  is  not  instantaneous,  but  passes 
through  orange  to  pink  with  the  consumption  of  an  appreciable  amount  of  acid. 
Thus  it  was  found  that,  using  a  10  gm.  per  litre  solution  of  the  indicator  in 
25  c.c.  of  water  freed  from  carbonic  acid  by  previous  boiling,  the  following 
quantities  of  N/iO  HC1  were  required  to  produce  a  clear  pink: — 8  drops  of 
methyl  orange  solution  =1  '5  c.c.,  4  drops  =0*5  c.c.,  2  drops  =0'5  c.c.  As  even 
0'5  c.c.  in  titrating  100  c.c.  of  water  would  correspond  to  2*5  parts  of  hardness 
per  100,000,  and  there  is  always  a  question  as  to  what  particular  colour 
corresponds  to  the  neutral  point,  the  following  procedure  may  be  recommended. 
To  100  c.c.  of  distilled  water,  one  drop,  or  some  other  definite  quantity  of  the 
indicator  is  added,  and  titrated  to  orange,  or  to  the  tint  to  the  change  of  which 
the  eye  of  the  individual  operator  is  most  sensitive.  The  water  of  which  the 
hardness  is  to  be  determined  is  similarly  titrated  with  the  same  quantity  of 
indicator,  and  in  a  similar  beaker,  until  it  exactly  matches  the  distilled  water, 
and  from  the  amount  of  acid  so  used  the  quantity  is  deducted  as  a  correction 
which  was  required  to  produce  the  same  colour  change  with  distilled  water  only. 
The  results  so  obtained  accurately  correspond  with  those  got  by  using  alizarin  as 
an  indicator  in  boiling  solution,  though  in  the  latter  method  the  end  reaction  is 
sharper.  It  may  be  noted  that  methyl  orange  is  not  absolutely  unaffected  by 
carbonic  acid,  a  somewhat  crocus-yellow  being  attained  instead  of  the  lemon 
yellow  reached  with  pure  boiled  water,  but  the  difference  is  insufficient  to  interfere 
with  its  satisfactory  use  as  an  indicator. 

Hehner's  method  for  the  determination  of  permanent  hardness  is  less 
satisfactory  than  the  foregoing.  It  consists  in  evaporating  100  c.c.  of  the  water 
to  dryness  with  a  known  excess,  say  20  c.c.,  of  N/io  sodium  carbonate  solution, 
taking  up  the  soluble  matter  with  cold  distilled  water,  filtering  off  the  precipitated 
calcium  carbonate  and  magnesia  on  a  small  filter,  washing  the  precipitate  with 
cold  water  and  titrating  back  the  excess  of  sodium  carbonate  in  the  filtrate  with 
methyl  orange  or  rosolic  acid  as  indicator.  With  lime-hardness  only,  and  with 
the  precautions  above  described,  the  method  may  be  pronounced  fairly  satisfactory  ; 
with  magnesia,  it  is  well  not  merely  to  evaporate  to  dryness  but  to  slightly  heat 
the  residue  to  thoroughly  decompose  any  magnesium  carbonate  present,  and  even 
then  the  washing  should  not  be  excessive,  as  calcium  carbonate  is  soluble  to  the 
extent  of  3  parts  per  100,000,  and  magnesia  to  about  2 '5  parts.  A  more  accurate, 
as  well  as  a  more  rapid,  method  is  to  employ  a  fair  excess  of  sodium  carbonate,  and 
to  make  up  the  solution  to  a  known  volume,  say  100  c.c.,  and  pipette  off  an  aliquot 
part  for  titration,  as  the  presence  of  excess  of  sodium  carbonate  materially  reduces 
the  solubility  both  of  calcium  and  magnesium  carbonates.  Both  these  methods, 
however,  should  be  superseded,  where  really  accurate  work  is  required,  by  those 
introduced  by  P  f  e  i  f  e  r  and  W  a r  t h  a  .*  That  for  the  determination  of  temporary 
hardness  is  identical  with  that  of  Hehner,  except  that,  in  place  of  methyl  orange, 
a  drop  of  a  mixture  of  about  1  gm.  of  the  purest  alizarin  paste  in  200  c.c.  of  distilled 
water  is  employed.  This  indicator  is  surprisingly  sensitive  ;  even  more  so  I  think 
than  phenolphthalein,  but  as  it  is  unfortunately  affected  by  carbon  dioxide,  it  is 
necessary  to  complete  the  titration  at  a  boiling  temperature.  The  change  is 
from  violet  in  alkaline  solution  (perhaps  slightly  varying  in  shade  with  the 
nature  of  the  particular  base  present)  to  a  perfectly  clear  pale  lemon-yellow 
when  neutral  or  acid.  The  titration  of  the  water  should  be  done  with  N/io  HC1 
or  H2SO4  in  a  silver,  platinum,  or  hard  porcelain  basin.  The  acid  should  be 
added  in  the  cold  till  the  violet  shade  gives  place  to  a  clean  yellow,  and  the  liquid 
then  brought  to  a  boil,  when,  with  the  escape  of  carbonic  acid,  the  violet  colour 
will  return,  and  should  at  once  be  destroyed  by  the  addition  of  another  drop  of 
the  acid,  and  so  on,  until  no  further  change  of  colour  takes  place.  It  is 
undesirable  to  boil  the  indicator  long,  especially  in  an  alkaline  condition, 
as  a  violet  deposit  is  formed  on  the  sides  of  the  basin,  presumably  of  calcium  and 
magnesium  alizarates,  which  can  only  be  dissolved  by  excess  of  acid,  and  is  thus 
apt  to  cause  perceptible  errors.  In  place  of  titrating  to  exact  neutrality,  the  acid 
may  be  added  in  very  small  excess,  and  the  whole  of  the  liberated  carbon  dioxide 

*  Z.  a.  C.t  1902,  198. 


HARDNESS.  481 

boiled  off  at  once,  and  the  solution  then  brought  back  to  neutrality  by  N/io 
NaOH,  the  solution  boiled  for  a  moment,  and  the  titration  completed.  The 
results  in  either  case  are  exact,  a  fraction  of  a  drop  of  alkali  changing  the  clear 
lemon  colour  to  a  dirty  yellow.  If  100  c.c.  of  water  are  used,  multiplication  of 
the  c.c.  of  acid  by  5  gives  the  temporary  hardness  in  parts  of  CaC03  per  100,000. 
The  boiling  must  in  no  case  take  place  in  an  ordinary  glass  beaker  or  flask,  as  an 
amount  ol  alkali  is  dissolved  which  may  lead  to  serious  inaccuracy.  Even  hard 
Jena  glass  is  not  free  from  this  effect,  though  the  amount  dissolved  is  so  small 
that  for  most  practical  purposes  it  may  be  neglected.  The  following  experiment 
will  illustrate  the  point.  100  c.c.  of  distilled  water  boiled  for  an  hour  (with 
additions  to  maintain  the  volume)  in  a  Berlin  porcelain  basin  showed  an  alkalinity 
or  colour-change  with  alizarin  ;  in  a  Jena  flask  a  perceptible  change  of  colour  was 
visible,  but  pure  yellow  was  restored  with  one  drop  of  N/io  acid,  while  when 
boiled  in  an  ordinary  Bohemian  flask,  0'4  c.c.  of  acid  was  consumed,  and  if  the 
neutralized  liquid  were  boiled  further  it  again  became  alkaline,  and  further 
additions  of  acid  were  required,  so  that  no  coincident  results  could  be  obtained. 
With  the  precautions  named,  the  results  with  a  known  solution  of  hydric  calcic 
carbonate  containing  only  5 '5  parts  of  temporary  hardness,  and  whether  titrated 
alone  or  with  additions  of  magnesium  sulphate,  were  accurate  within  one  part  in 
100,000,  and  experiments  with  other  quantities  were  equally  satisfactory. 

In  the  determination  of  permanent  hardness,  Pfeifer  and  Wartha,  in 
addition  to  the  use  of  alizarin  as  indicator,  have  introduced  the  important 
improvement  of  replacing  the  sodium  carbonate  of  Hehner's  method  by 
a  mixture  of  equal  parts  of  N/io  sodium  carbonate  and  hydroxide  solutions. 
While,  as  has  been  already  explained,  sodium  carbonate  perfectly  precipitates 
calcium  salts  as  carbonates  on  merely  boiling,  it  becomes  necessary  to  evaporate 
to  dryness  and  to  heat  whenever  any  magnesium  salt  is  present,  in  order  to 
convert  magnesium  carbonate  into  oxide,  since  magnesium  carbonate  is  not 
sufficiently  insoluble.  In  presence  of  sodium  hydroxide,  however,  the  magnesium 
carbonate  is  at  once  converted  into  magnesium  hydroxide,  and  perfectly  efficient 
precipitation  is  obtained  by  merely  boiling  for  some  time  with  sufficient  excess 
of  the  reagent.  A  good  excess,  say  50  per  cent,  or  more,  is  essential,  not  only 
because  it  is  impossible  to  say  before  analysis  what  proportion  of  sodium 
carbonate  and  what  of  caustic  will  be  required,  but  because  the  presence  of  the 
C03  ions  of  the  sodium  carbonate  in  the  solution  greatly  lessens  the  solubility  of 
the  calcium  carbonate,  and  similarly  the  OH  ions  of  the  sodium  hydroxide  lessen 
that  of  the  magnesium  hydroxide.  Unless  the  water  is  extremely  hard,  50  c.c. 
of  the  N/io  mixed  solution  to  200  c.c.  of  water  is  a  convenient  and  sufficient 
quantity.  The  mixture  may  be  boiled  until  reduced  within  200  c.c.  in  a 
platinum  or  porcelain  basin,  or,  more  conveniently,  and  with  no  material  loss  of 
accuracy,  in  a  300  c.c.  Jena  flask,  but  on  no  account  in  ordinary  Bohemian 
glass.  Even  the  Jena  flask  will  become  perceptibly  etched  at  the  water  line  if 
used  repeatedly.  The  solution,  after  cooling,  is  made  up  to  200  c.c.  with  distilled 
water  in  a  gauged  flask,  and  allowed  to  stand  till  the  precipitated  bases  have 
settled,  and  100  c.c.  is  pipetted  or  siphoned  off  and  titrated.  As  the  quantity 
named  corresponds  to  100  c.c.  of  the  original  water,  and  25  c.c.  of  N/io  alkali, 
the  difference  between  the  acid  actually  used  and  25  c.c.  will  correspond  to  the 
amount  of  alkali  neutralized  by  the  acids  of  the  permanent  hardness,  and 
multiplied  by  5  will  give  the  latter  in  terms  of  mgms.  per  100,000  calculated  as 
calcium  carbonate.  The  temporary  hardness  will  also  be  precipitated,  but,  con- 
taining no  fixed  acids,  will  not  interfere.  Pfeifer  employs  the  water  which  has 
been  neutralized  in  the  titration  of  temporary  hardness,  in  place  of  the  original 
water.  In  this  case  the  result  obtained  will  represent  total  hardness,  from  which 
the  permanent  hardness  is  obtained  by  deducting  the  temporary.  In  place  of 
allowing  the  precipitate  to  settle,  the  solution  may  be  filtered  through  a  small 
filter,  which  is  carefully  washed  with  the  solution,  of  which  the  first  50  c.c.  or  so 
is  rejected,  as  filters  are  rarely  absolutely  free  from  acidity  or  alkalinity,  and, 
even  if  at  first  perfectly  neutral,  easily  absorb  acids  or  ammonia  from  the 
laboratory  air,  unless  very  carefully  protected.  Many  irregularities  occurred  in 
the  determinations  until  this  source  of  error  was  detected.  15  cm.  filters  of 
three  different  makes  were  macerated  with  hot  distilled  water,  and  proved  in  all 

2  i 


482  WATER   AND    SEWAGE. 

cases  alkaline  to  methyl  orange  and  acid  to  phenolphthalein,  the  difference 
between  the  two  indicators,  +  or  — ,  amounting  in  each  case  to  about  0*75  c.c. 
of  N/io  solution.*  A  case  must  now  be  considered  which  is  not  very  infrequent 
in  waters  of  this  district.  It  occasionally  happens  that  in  the  determination  of 
permanent  hardness,  a  larger  quantity  of  acid  is  required  to  neutralize  the 
mixture  than  corresponds  to  the  volume  of  N/io  alkali  which  has  been  added, 
and  that  therefore  the  permanent  hardness  would  appear  as  a  minus  quantity. 
This  somewhat  puzzling  result  is  due  to  the  presence  of  sodium  carbonate  in  the 
original  water,  which  in  this  case  can  have  no  permanent  hardness  other  than 
that  due  to  the  solubility  of  calcium  carbonate,  which  cannot  be  removed  by 
softening,  but  which  is  not  reckoned  in  the  above  methods  of  analysis,  though  it 
is  counted  in  the  soap  test.  Where  sodium  carbonate  is  thus  found,  a  pro- 
portionate amount  must  be  deducted  from  the  temporary  hardness.  If  the  total 
hardness  after  neutralization  is  determined  by  Pfeifer's  method,  the  presence 
of  sodium  carbonate  will  be  indicated  by  the  total  hardness  coming  out  as  less 
than  the  temporary,  the  difference  being  obviously  the  alkalinity  due  to  the  soda  ; 
each  part  of  hardness  corresponding  to  1'06  part  of  sodium  carbonate.  Since  in 
the  ordinary  methods  of  water  softening  lime  is  precipitated  as  carbonate,  but 
magnesia  as  oxide,  with  the  consumption  of  a  double  quantity  of  caustic  lime  or 
caustic  alkali,  it  is  impossible  from  hardness-determinations  alone  to  calculate 
the  materials  required  for  softening,  or  the  actual  weights  of  the  bases  titrated, 
so  long  as  it  is  uncertain  whether  or  in  what  proportion  magnesia  is  present. 
Pfeifer  determines  this  in  the  following  manner: — 100  c.c.  of  the  water  is 
neutralized  with  N/iO  acid  in  presence  of  alizarin,  in  boiling  solution,  exactly  as 
in  the  determination  of  temporary  hardness,  which  may  be  combined  with  that  of 
magnesia.  A  known  quantity  of  clear  limewater  (25  or  50  c.c.),  which  should  be 
at  least  50  per  cent,  in  excess  of  that  required  for  precipitating  the  magnesia 
present,  is  measured  into  a  200  c.c.  flask,  the  hot  neutralized  solution  is  rinsed  in 
with  boiling  distilled  water  free  from  carbonic  acid,  and  made  up  with  the  latter 
to  5  c.c.  above  the  mark  to  allow  of  contraction  in  cooling  ;  the  flask  is  tightly 
corked  or  stoppered,  and  well  shaken  to  mix,  for  which  purpose  the  neck  above 
the  mark  must  be  a  long  one,  and  set  aside  to  cool  and  settle.  Though  not 
essential,  it  probably  increases  the  completeness  of  the  precipitation  if  the  corked 
flask  is  heated  for  half  an  hour  or  so  on  the  water-bath.  I  prefer  to  allow 
sufficient  time  for  the  liquid  to  completely  clear,  and  to  pipette  off  100  c.c.  to 
titrate  back  with  N/io  acid,  which  may  be  done,  cold  with  phenolphthalein,  or 
hot  with  alizarin  with  equal  accuracy.  Pfeifer  filters,  but  in  this  case  the 
strength  of  the  limewater  must  be  determined  by  a  blank  experiment  conducted 
in  exactly  the  same  way  with  distilled  water  ;  and  it  is  better  to  reject  the  first 
50  c.c.  in  each  case  to  avoid  error  from  want  of  neutrality  of  the  filter-paper,  and 
great  care  must  be  taken  to  filter  rapidly,  to  avoid  possibilities  of  carbonation 
by  the  atmosphere,  for  which  purpose  a  suction  filter  with  a  perforated  porcelain 
disc  covered  with  a  neatly-fitted  disc  of  filter-paper  answers  well.  If,  on  the 
other  hand,  the  liquid  is  settled  and  pipetted,  the  risk  of  carbonation  is  so  small 
that  an  equal  quantity  of  the  same  limewater  may  be  measured  direct,  and 
titrated,  using  the  same  indicator  as  has  been  employed  for  the  water, 
phenolphthalein  in  the  cold  being  on  the  whole  preferable.  Deducting  the  N/io 
acid  required  for  the  mixture  of  limewater  and  water  from  that  employed  for  the 
limewater  alone,  and  multiplying  the  difference  by  five,  gives  the  hardness  due  to 
magnesia  in  terms  of  mgms.  of  calcium  carbonate  per  100,000,  from  which 
actual  Mg  may  be  reckoned  by  multiplying  by  0'24  ;  or  MgO  multiplying  by 
0-4.  Carefully  conducted,  the  method  is  extremely  exact,  its  accuracy  being 
quite  equal  to  that  of  the  determination  of  hardness,  and  probably  superior  to 
that  of  any  gravimetric  method  for  such  minute  quantities.  The  theory  of  the 
process  is  that,  while  calcium  hydrate  will  precipitate  magnesia,  it  has  no  action 
on  lime  salts  ;  and  a  good  excess  of  lime  serves  not  only  to  quicken  the  reaction 
but  to  diminish  the  solubility  of  the  magnesia.  If  iron  is  present  it  will  of 

*  In  confirmation  of  this  defect  in  filter  papers  Lenormand  has  given  the 
history  of  some  experiments  with  various  papers  (C.  N.  Ixxxix.  229),  and  comes  to 
the  conclusion  that  in  the  analysis  of  either  fresh  or  salt  waters  they  should  never 
be  filtered  but  allowed  time  to  settle  clear,  and  in  that  state  used  in  the  analysis  of 
waters  for  dissolved  organic  matters. 


HARDNESS.  483 

course  be  reckoned  with  the  magnesia,  and  should  be  determined  colorimetrically 
with  thiocyanate  (also  a  process  of  great  accuracy  for  small  quantities),  and 
deducted.  It  may  be  assumed  that  it  is  present  in  the  ferric  state,  and  that 
therefore  0'24  of  Mg.  corresponds  to  0'3733  of  Fe.  Aluminium,  if  present, 
would  behave  like  iron,  any  traces  of  alumina  dissolved  by  lime  having  no  effect 
(on  phenolphthalein  at  least),  but  it  is  rare  that  more  than  traces  of  alumina 
exist  in  natural  waters,  though  it  would  have  to  be  reckoned  with  in  river- waters 
receiving  manufacturing  effluents,  and  its  determination  would  not  be  particularly 
easy.  Possibly  a  colorimetric  method  with  alcoholic  extract  of  logwood  or  some 
other  mordant  dyestuff  might  be  devised  where  the  water  was  required  for 
dyeing,  but  it  is  not  likely  that  it  would  introduce  any  material  error  into  water- 
softening  calculations,  and  it  would  be  removed  with  the  other  impurities. 
Having  determined  the  magnesia,  or,  more  strictly,  the  acid  with  which  it  and 
any  other  bases  are  combined  which  are  precipitated  by  lime,  it  becomes  possible 
to  calculate  the  calcium  present  in  the  water,  by  deducting  the  magnesia-hardness 
from  the  total  hardness,  and  calculating  the  remainder  into  Ca  by  multiplication 
by  0'4.  The  carbon  dioxide  present  as  hydric  carbonate  is  given  in  parts  per 
100,000  by  multiplication  of  the  temporary  hardness  by  0'88  for  C02  or  1*2  for 
C03.  When  the  proportion  of  hardness  due  to  magnesia  is  known,  it  is  possible 
to  calculate  the  quantities  of  lime  and  sodium  carbonate  required  for  softening, 
since  magnesium  salts,  as  has  been  stated,  cannot  be  satisfactorily  removed  as 
carbonates,  but  must  be  converted  into  hydroxides  by  lime  or  some  other  caustic 
alkali  ;  and  this  applies  to  the  permanent  hardness  which  is  converted  into 
carbonate  by  sodium  carbonate,  as  well  as  to  the  bicarbonate  reduced  to  carbonate 
by  lime.  Thus  each  equivalent  of  magnesia  present  requires  an  additional 
equivalent  of  lime  beyond  that  required  by  the  corresponding  calcium  salt. 
Pfeifer  gives  a  formula  for  this  purpose,  calculated  for  German  degrees  of 
hardness,  which  are  reckoned  in  parts  per  100,000  of  CaO  instead  of  parts  of 
CaC03  as  is  customary  in  France  and  England.  I  have,  therefore,  taken  the 
liberty  of  transposing  it  into  terms  of  parts  of  CaC03  per  100,000.  Ht  in  the 
formula  signifies  temporary,  and  Hp  permanent  hardness,  and  Hra  hardness  due 
to  magnesia,  whether  temporary  or  permanent.  The  quantities  given  are  in 
mgms.  per  litre,  gms.  per  cubic  metre,  or  Ib.  per  100,000  gallons  of  the  water  to 
be  treated.  5'6  (H£+Hm)=lime  (CaO)  required;  10'6  Hp  =dry  sodium 
carbonate;  or  28'6  Hp=soda  crystals  (Na2C03.  10H20).  If  only  temporary 
hardness  is  to  be  softened  by  liming  only,  the  quantity  required  is  5 '6 
(H£+Hra  —  Up)  if  Hm  is  larger  than  Hp,  but  if  not,  only  the  temporary  hard- 
ness need  be  taken  into  account.  Finally,  for  softening  with  sodium  hydroxide 
and  sodium  carbonate  only,  which  is  sometimes  convenient  for  small  boiler 
installations,  we  have  8  (H«+Hw)=NaOH  required;  10'6  Hp-(m+Hra) 
=Na2CO3  required.  Consequently,  if  the  water  has  less  permanent  hardness 
than  the  sum  of  the  temporary  and  magnesia  hardness,  it  cannot  be  softened 
completely  in  this  way  without  leaving  excess  of  sodium  carbonate  in  the  water. 

Some  waters  contain  large  quantities  of  dissolved  free  carbon  dioxide  in 
addition  to  the  "  half-combined  "  present  as  temporary  hardness,  and  though 
this  is  not  included  in  any  hardness  determination,  it,  of  course,  combines  with 
and  renders  useless  an  equivalent  quantity  of  the  lime  added  for  softening,  and 
must,  therefore,  be  taken  into  account  in  reckoning  the  lime  required.  The 
free  C02  is  easily  determined  by  a  method  described  on  p.  100.  100  c.c.  of  the 
water  is  titrated  slowly  with  N/IQ  solution  of  Na2C03  and  phenolphthalein, 
till  a  tinge  of  permanent  pink  is  produced,  when  the  number  of  c.c.  used, 
multiplied  by  2 -2,  will  give  the  parts  of  CO2  per  100,000,  or  with  multiplication 
by  2 '8  will  give  the  weight  of  lime  required  to  remove  it.  Of  course,  such 
a  determination  is  of  no  use  unless  there  is  some  security  that  the  sample  of  water 
really  represents  the  average,  and  has  not  lost  carbonic  acid  by  exposure.  The 
reaction  depends  on  the  fact  that  sodium  bicarbonate  is  neutral  to  phenolphthalein, 
while  the  normal  carbonate  is  alkaline.  It  must  be  remembered  that  the 
theoretical  quantity  of  precipitants  does  not  always  give  the  best  practical  results, 
owing  to  difficulties  of  settling  and  filtration,  and  in  some  cases  it  is  necessary  to 
be  content  with  less  than  the  theoretical  softening  (see  Archbutt  and  Dee  ley  , 
J.S.C.L,  1891,  511). 

2   I   2 


484  WATER   AND    SEWAGE. 

METHODS    OF    DETERMINING    THE    ORGANIC 

IMPURITIES    IN 
WATER    WITHOUT    GAS    APPARATUS. 

THE  foregoing  methods  of  estimating  the  organic  impurities  in 
potable  waters,  though  very  comprehensive  and  trustworthy,  yet 
possess  the  disadvantage  of  occupying  a  good  deal  of  time,  and 
necessitate  the  use  of  a  complicated  and  expensive  set  of  apparatus, 
which  may  not  always  be  within  the  reach  of  the  operator. 

No  information  of  a  strictly  reliable  character  as  to  the  nature 
of  the  organic  matter  or  its  quantity  can  be  gained  from  the 
use  of  standard  permanganate  solution  as  originally  devised  by 
Forchammer,  and  the  same  remark  applies  to  the  loss  on 
ignition  of  the  residue,  both  of  which  have  been  in  past  time  largely 
used. 

The  Forchammer  or  oxygen  process,  however,  as  improved  by 
Letheby,  and  further  elaborated  by  Tidy,  may  be  considered  as 
worthy  of  considerable  confidence  in  determining  the  amount  of 
organic  substances  contained  in  a  water. 

The  Oxygen  Absorption  Process. 
For  the  Preparation  of  the  Reagents  required  see  p.  444. 

This  process  depends  upon  the  determination  of  the  amount  of 
oxygen  required  to  oxidize  the  organic  and  other  oxidizable  matters 
in  a  known  volume  of  water,  slightly  acidified  with  pure  sulphuric   | 
acid.     For  this  purpose,   a  standard  solution  of  potassium  per- 
manganate  is   employed   in   excess.     The   amount   of   unchanged 
permanganate,  after  a  given  time,  is  ascertained  by  means  of  a  j 
solution  of  sodium  thiosulphate,  by  the  help  of  the  iodine  and 
starch  reaction. 

Tidy  and  Frankland  in  all  cases  made  a  blank  experiment 
with  pure  distilled  water  side  by  side  with  the  sample,  and  this 
procedure  has  been  adopted  generally. 

Two  tests  are  usually  made,  viz.,  the  amount  of  oxygen  absorbed 
in  three  minutes  and  in  four  hours  respectively.  The  former, 
which  practically  gives  the  amount  absorbed  instantaneously, 
affords  a  means  of  differentiating  between  one  class  of  oxidizable 
substance  and  another,  and,  in  the  case  of  sewages  and  effluents, 
affords  an  excellent  method,  in  conjunction  with  the  incubator,  of  | 
determining  the  amount  of  putrefaction  which  is  taking  place.  In 
the  case  of  waters,  an  immediate  reduction  of  permanganate  may 
be  caused  by  such  reducing  agents  as  nitrites,  ferrous  salts,  or 
sulphuretted  hydrogen,  and  Tidy  was  disposed  to  attribute  this 
reduction,  in  the  known  absence  of  iron  and  sulphuretted  hydrogen, 
to  nitrites. 

The  process  is  carried  out  as  follows  : — 

The  vessels  used  for  this  determination  should  be  rinsed  with 
sulphuric  acid  and  then  with  water.  12-oz.  stoppered  flasks  or 


OXYGEN    ABSORBED.  485 

^on.«j 

Cf         'fl 

500  c.c.  W.M.  bottles  answer  well.  First  measure  out  into  each 
flask  10  c.c.  of  the  dilute  sulphuric  acid,  then  10  c.c.  of  the 
permanganate,  and  finally  250  c.c.  of  distilled  water  for  the  blank 
and  the  same  quantity  of  each  of  the  samples  to  be  tested.  The 
whole  of  the  vessels  are  then  placed  in  water  at  80°  F.,  or  in  an 
incubator,  and  maintained  at  that  temperature  for  four  hours. 
The  flasks  should  be  examined  at  intervals,  and  if  the  pink  colour 
becomes  much  diminished  a  further  10  c.c.  of  permanganate 
should  be  added.  In  all  cases  a  good  excess  of  permanganate 
should  be  maintained  during  the  whole  of  the  four  hours  ;  the  flasks 
should  also  be  shaken  occasionally.  At  the  end  of  four  hours  add 
to  each  a  few  drops  of  the  potassium  iodide  solution  till  the  clear 
yellowish-brown  colour  of  iodine  replaces  the  pink  of  the  per- 
manganate. Now  run  in  from  a  burette  the  standard  solution  of 
thiosulphate  with  occasional  shaking  till  the  colour  of  the  solution 
is  reduced  to  a  pale  yellow,  then  add  a  few  drops  of  starch  solution 
and  more  thiosulphate  till  the  blue  colour  disappears. 

Ex.  1.  250  c.c.  of  water  treated  as  above  required  22*4  c.c. 
thiosulphate  ;  the  blank  took  32-5  c.c. 

•4  /32-5-22-4\  =0*124  parts  of  oxygen  absorbed  per  100,000 
\       32-5       /          of  water. 

Ex.  2.  Suppose  that  a  sewage  effluent  required  the  addition  of 
30  c.c.  of  permanganate  to  maintain  a  good  pink  colour,  and  that 
28-1  c.c.  of  thiosulphate  were  needed  (blank  as  before),,  then 

/  32'5  x3  —  28*1  \  =0'854    parts    of    oxygen    absorbed    per 
\         32^5""  100,000  of  effluent. 

To  calculate  "  grains  per  gallon  "  instead  of  "  parts  per  100,000," 
use  the  factor  0'28  instead  of  0-4  in  the  above  formulae. 

The  three  minutes'  test  is  carried  out  as  follows  :— 

Ten  c.c.  of  the  permanganate  solution  and  the  same  volume  of 
the  dilute  sulphuric  acid  are  measured  into  a  small  stoppered 
bottle  of  about  100  c.c.  capacity,  and  the  latter  is  carefully  warmed 
in  a  water-bath  to  the  temperature  of  80°  F.  A  portion  of  the 
sample  (sewage,  effluent,  etc.)  is  warmed  separately  to  the  same 
temperature,  and  25  c.c.  of  it  (or  10  c.c.,  diluted  to  25  c.c.  with 
distilled  water,  in  the  case  of  a  very  strong  sewage)  are  added  to 
the  acid  permanganate.  The  contents  of  the  bottle  are  mixed  by 
gentle  rotation,  and  after  three  minutes'  standing  potassium  iodide 
is  added  and  the  titration  finished  as  described  above.  In  the  case 
of  waters,  250  c.c.  are  measured  into  a  flask  or  bottle,  such  as  is 
used  for  the  four  hours'  test,  warmed  in  a  water-bath  to  80°  F., 
the  acid  and  permanganate  added,  etc.,  as  above. 

Dupre*  carried  out  a  number  of  experiments  with  this  process 
and  arrived  at  the  following  conclusions  : — 

(1)  That,  practically  no  decomposition  of  permanganate  takes 
place  during  four  hours  when  digested  in  a  closed  vessel  at  80°  with 
perfectly  pure  water  and  the  usual  proportion  of  pure  sulphuric 

*  Analyst,  1882,  7,  1. 


486  WATER   AND    SEWAGE. 

acid.     By    adopting    the    closed    vessel,    all    dust    or    reducing 
atmospheric  influence  is  avoided. 

(2)  The  standardizing  of  the  thiosulphate  and  permanganate, 
originally  and  from  time  to  time,  must  be  made  in  a  closed  vessel  in 
the  same  manner  as  the  analysis  of  a  water,  since  it  has  been  found 
that  when  the  titration  is  made  slowly  in  an  open  beaker  less 
thiosulphate  is  required  than  in  a  stoppered  bottle.     This  is  probably 
due  to  a  trifling  loss  of  iodine  by  evaporation. 

(3)  That  with  very  pure  waters  no  practical  difference  is  pro- 
duced by  a  rise  or  fall  of    temperature,  the   same  results  being 
obtained  at  32°  F.  as  at  80°  F.     On  the  other  hand,  with  polluted 
waters,  the  greater  the  organic  pollution,  the  greater  the  difference 
in  the  amount  of  oxygen  absorbed  according  to  temperature. 

(4)  As  to  time,  it  appears  that  very  little  difference  occurs  in 
good  waters  between  three  and  four  hours'  digestion  ;  but  with 
bad  waters  there  is  often  a  very  considerable  increase  in  the  extra 
hour  ;  and  thus  Dupre  doubts  whether  even  four  hours'  digestion 
suffices  for  very  impure  waters. 

Dupre  in  further  comment  on  the  temperature  at  which  it  is  advisable  to 
carry  out  this  method  (Analyst  x.  118),  and  also  as  to  the  reactions  involved, 
points  out  one  feature  which  has  in  all  probability  impressed  itself  upon  other 
operators,  that  is  to  say,  the  effect  of  chlorides  when  present  in  any  quantity.  It 
is  evident  that  if  in  this  case  the  permanganate  is  used  at  a  high  temperature 
and  in  open  vessels,  chlorine  will  be  liberated ;  part  escaping  into  the  air,  and  the 
rest  nullifying  the  reducing  effect  of  any  organic  matter  present  on  the  per- 
manganate. If,  however,  the  experiment  be  conducted  at  high  temperature  in 
a  closed  vessel,  the  probable  error  is  eliminated,  because  the  chlorine  is  retained, 
and  subsequently,  when  cool  and  the  potassium  iodide  added,  the  free  Gl  liberates 
exactly  the  same  amount  of  iodine  as  would  have  been  set  free  by  the  per- 
manganate from  which  it  was  produced.  It  thus  becomes  possible  to  determine 
the  amount  of  oxidizable  organic  matter,  even  in  sea  water.  In  order,  however, 
to  reduce  the  probable  error  from  the  presence  of  chlorides/Dupre  prefers  to 
carry  on  the  experiment  at  a  very  low  temperature,  in  fact,  as  near  0°  C.  or  32° 
F.  as  possible,  and  uses  phosphoric  acid  in  place  of  sulphuric  (250  gm.  glacial 
acid  to  the  litre ;  10  c.c.  of  which  is  used  for  each  quarter  or  half  litre  of  water). 
The  sample  is  cooled,  the  reagent  added  in  a  stoppered  bottle,  and  kept  in  an 
ordinary  refrigerator  for  twenty-four  hours.  The  same  operator  very  rightly 
condemns  the  practice  adopted  by  some  chemists,  especially  those  of  Germany, 
of  boiling  a  water  with  permanganate  and  sulphuric  acid.  The  presence  of 
chlorides  in  varying  proportions  must  in  such  case  totally  vitiate  the  results. 

Comparison  of  the  Results  of  this  Process  with  the  Combustion 
Method. — I  cannot  do  better  than  quote  Fraiikland's  remarks 
on  this  subject,  as  contained  in  his  treatise  on  Water  Analysis. 

"  The  objections  to  the  oxygen  process  are  first,  that  its  indications  are 
only  comparative,  and  not  absolute ;  and,  second,  that  its  comparisons  are 
only  true  when  the  organic  matter  compared  is  substantially  identical  in 
composition. 

"  For  many  years,  indeed,  after  this  process  was  first  introduced,  the  action  of 
the  permanganate  was  tacitly  assumed  to  extend  to  the  complete  oxidation  of  the 
organic  matter  in  the  water,  and,  therefore,  the  result  of  the  experiment  was 
generally  stated  as  '  the  amount  of  oxygen  required  to  oxidize  the  organic 
matter  ;  '  whilst  some  chemists  even  employed  the  number  so  obtained  to  calculate 
the  actual  weight  of  organic  matter  in  the  water  on  the  assumption  that  equal 
weights  of  all  kinds  of  organic  matter  required  the  same  weight  of  oxygen  for 
their  complete  oxidation. 


OXYGEN    ABSORBED. 


487 


"  Both  those  assumptions  have  been  conclusively  proved  to  be  entirely 
fallacious,  for  it  has  been  experimentally  demonstrated,  by  operating  upon  known 
quantities  of  organic  substances  dissolved  in  water,  that  there  is  no  relation 
either  between  the  absolute  or  relative  weight  of  different  organic  matters  and 
the  oxygen  which  such  matters  abstract  from  permanganate. 

"  Nevertheless,  in  the  periodical  examination  of  waters  from  the  same  source, 
I  have  noticed  a  remarkable  parallelism  between  the  proportions  of  organic  carbon 
and  of  oxygen  abstracted  from  permanganate.  Thus,  for  many  years  past, 
I  have  seen  in  the  monthly  examination  of  the  waters  of  the  Thames  and  Lea 
supplied  to  London  such  a  parallelism  between  the  numbers  given  by  Tidy 
expressing  '  oxygen  consumed,'  and  those  obtained  by  myself  in  the  determination 
of  '  organic  carbon.' 

"  This  remarkable  agreement  of  the  two  processes,  extending  as  it  did  to  1,418 
out  of  1,686  samples,  encouraged  me  to  hope  that  a  constant  multiplier  might 
be  found,  by  which  the  'oxygen  consumed'  of  the  Forchammer  process  could 
be  translated  into  the  '  organic  carbon  '  of  the  combustion  method  of  analysis. 
To  test  the  possibility  of  such  a  conversion,  my  pupil,  Woodland  Toms,  made, 
at  my  suggestion,  the  comparative  experiments  recorded  in  the  following  tables  : 


I. — River  Water. 


Source  of  Sample. 

Oxygen         C 
consumed         O 

Organic 
carbon  by 
combustion. 

Chelsea  Company's  Supply 

0-098    x   2-6      = 

0-256J 

West  Middlesex  Go's  ,, 

0-116    x   2-5      = 

0-291 

Lambeth  Co.'s             ,. 

0-119    x  243    = 

0-282 

South  wark  Co.'s          ,, 

0-121    x   2-22    = 

0-269 

New  River  Co.'s         „ 

0-076    x   2-4      = 

0-183 

Chelsea  Co.'s  second  sample 

0-070    x   2-69    = 

0-188 

Lambeth  Co.'s             ,, 

0-119    x    1-99    = 

0-234 

New  River  Co.'s         ,,         ,-.    - 

0-107    x   2-25    - 

0-221     ; 

"As  the  result  of  these  experiments  the  average  multiplier  is  2 -38,  and  the 
maximum  errors  incurred  by  its  use  would  be — 0'021  part  of  organic  carbon 
in  the  case  of  the  second  example  of  the  Chelsea  Company's  water,  and  +0*049 
part  in  that  of  the  second  sample  of  the  Lambeth  Company's  water.  These 
errors  would  have  practically  little  or  no  influence  upon  the  analyst's  opinion 
of  the  quality  of  the  water.  It  is  desirable  that  this  comparison  should  be 
extended  to  the  water  of  other  moderately  polluted  rivers. 


II.— Deep  Well  Water. 


Source  of  Sample. 

Oxygen       C 
consumed,        C) 

Organic 
carbon  by 
combustion. 

lent  Company's  Supply      ..          ,.-: 
olne  Valley  Co.'s     „          .. 
[odgson's  Brewery  well      .  .          ,  .    ; 

0-015      x   5-1    = 
0-0133    x   6-9    = 
0-03        x   5-3    = 

0-077 
0-094 
0-158 

"  The  relation  between  '  oxygen  consumed  '  and  '  organic  carbon  '  in  the  case 
of  deep  well  waters  is  thus  very  different  from  that  which  obtains  in  the  case  of 
river  waters,  and  the  average  multiplier  deduced  from  the  foregoing  examples 
is  5-8,  with  maximum  errors  of  +0'01  of  organic  carbon  in  the  case  of  the  Kent 
Company's  water,  and — 0'015  in  that  of  the  Colne  Valley  water.  Such  slight 
errors  are  quite  unimportant. 


488  WATER   AND    SEWAGE. 

"  Similar  comparative  experiments  made  with  shallow  well  and  upland  surface 
waters  showed  amongst  themselves  a  wider  divergence,  but  pointed  to  an  average 
multiplier  of  2 '28  for  shallow  well  water,  approximately  the  same  as  that  found 
for  moderately  polluted  river  water,  and  1*8  for  upland  surface  water. 

"  In  the  interpretation  of  the  results  obtained,  either  by  the  Forchammer 
or  combustion  process,  the  adoption  of  a  scale  of  organic  purity  is  often  useful 
to  the  analyst,  although  a  classification  according  to  such  a  scale  may  require  to 
be  modified  by  considerations  derived  from  the  other  analytical  data.  It  is 
indeed  necessary  to  have  a  separate  and  more  liberal  scale  for  upland  surface 
water,  the  organic  matter  of  which  is  usually  of  a  very  innocent  nature,  and 
derived  from  sources  precluding  its  infection  by  zymotic  poisons. 

"  Subject  to  modification  by  the  other  analytical  data,  the  following  scale  of 
classification  has  been  suggested  by  Tidy  and  myself : — 

Section  I. — Upland  Surface  Water. 

"  Class  I.  Water  of  great  organic  purity,  absorbing  from  permanganate  not 
more  than  O'l  part  of  oxygen  per  100,000  parts  of  water,  or  0'07  grain  per  gallon. 

"Class  n.  Water  of  medium  purity,  absorbing  from  O'l  to  0'3  part  of  oxygen 
per  100,000  parts  of  water,  or  0'07  to  0'21  grain  per  gallon. 

"  Class  HI.  Water  of  doubtful  purity,  absorbing  from  0'3  to  0'4  part  per  100,000, 
or  0'21  to  0'28  grain  per  gallon. 

*'  Class  IV.  Impure  water,  absorbing  more  than  0'4  part  per  100,000  or  0'28 
grain  per  gallon. 

Section  II. — Water  other  than  Upland  Surface. 

"Class  I.  Water  of  great  organic  purity,  absorbing  from  permanganate  not 
more  than  0'05  part  of  oxygen  psr  100,000  parts  of  water,  or  0'035  grain  per  gallon. 

"  Class  n.  Water  of  medium  purity,  absorbing  from  0'05  to  0'15  part  of  oxygen 
per  100,000,  or  0'035  to  O'l  grain  per  gallon. 

"  Class  HI.  Water  of  doubtful  purity,  absorbing  from  0'15  to  0'2  part  of  oxygen 
per  100,000,  or  O'l  to  0'15  grain  per  gallon. 

"  Class  IV.  Impure  water,  absorbing  more  than  0"2  part  of  oxygen  per  100,000, 
of  0-15  grain  per  gallon. 

Determination  of  Free  and  Saline  Ammonia  and  of 
Albuminoid  Ammonia  (W  a  n  k  1  y  n). 

(For  the  preparation  of  Reagents  required,  see  p.  437). 

Measure  500  c.c.  of  the  water  to  be  examined  into  a  stoppered 
retort  or  a  32  oz.  round-bottomed  Bohemian  flask  with  a  rubber 
stopper,  the  vessel  used  being  sufficiently  large  to  prevent  any 
of  the  sample  being  spirted  over  into  the  condenser.*  Add  a  small 
quantity  of  recently-ignited  sodium  carbonate  and  connect  by  an 
india-rubber  joint  with  a  Liebig's  or  other  condenser,  which 
should  be  thoroughly  rinsed  out  with  good  tap-water  immediately 
before  use.  The  distillation  should  be  conducted  as  rapidly  as  is 
compatible  with  a  certainty  that  no  spirting  takes  place,  and  a 
stream  of  water  should  be  passed  through  the  condenser  during 
the  whole  of  the  distillation.  When  100  c.c.  have  distilled  over, 
the  receiver  (a  stoppered  100  c.c.  flask)  is  changed  and  the  distillation 
continued  till  another  50  c.c.  have  been  collected  in  a  Nessler  glass. 

*  Thorpe's  well-known  "  Revenue  Still  "  is  very  compact  and  answers  well 
for  water  and  sewage  distillation. 


AMMONIA.  489 

The  latter  is  then  tested  at  once  with  2  c.c.  of  Nessler's  solution, 
and  if  no  colour  is  produced,  which  is  most  usually  the  case,  the 
distillation  is  stopped.  If  a  colour  is  produced,  further  portions 
are  collected  and  tested  till  ammonia  ceases  to  come  over.  In  this 
way  the  free  and  saline  ammonia  are  obtained. 

Whilst  the  distillation  has  been  going  on,  50  c.c.  of  alkaline 
permanganate  and  about  25  c.c.  of  distilled  water  are  together 
boiled  in  a  nickel  dish  or  a  flask  for  a  few  minutes  to  expel  any 
ammonia.  This  is  then  poured  into  the  flask  and  the  distillation 
continued,  the  distillate  being  collected  in  a  100  c.c.  flask  followed 
by  successive  portions  in  Nessler  glasses  till  the  evolution  of 
ammonia  has  ceased.  The  ammonia  thus  collected  forms  the 
albuminoid  ammonia. 

The  respective  distillates  are  now  "  Nesslerized  "  as  follows  : — 
Transfer  each  100  c.c.  distillate  to  a  Nessler  glass,  add  4  c.c.  of 
Nessler's  solution,  and  mix  well  with  a  long  glass  tube  having 
a  bulbed  end.  The  colour  that  develops  enables  an  approximate 
estimate  to  be  made  of  the  amount  of  ammonia  present,  and  one  or 
more  standards  are  made  up  with  ammonium  chloride  and  ammonia- 
free  water  as  quickly  as  possible,  4  c.c.  of  Nessler's  solution  added 
to  each,  and  after  mixing  all  are  allowed  to  stand  at  least  ten 
minutes,  so  that  the  colour  may  fully  develop  and  all  may  acquire 
the  room  temperature,  this  latter  being  a  matter  of  importance. 
The  final  adjustment  is  made  by  pouring  some  of  the  more  strongly- 
coloured  liquid  into  a  100  c.c.  measuring  cylinder  until  a  perfect 
match  of  tints  is  obtained,  or  a  Stokes's  Colorimeter  may  be 
used.  The  mode  of  calculation  is  best  shown  by  an  example. 

Ex.     500  c.c.  of  water  distilled. 

Free  and  saline  ammonia.  1st  100  c.c.  85  c.c.  gave  the  same 
colour  as  a  standard  made  up  of  0'5  c.c.  standard  NH4C1  solution 
(1  c.c.  =0-00005  gm.  NH3).  2nd  50  c.c.  None. 

ion 
Result,  ^r  xO'005  =  M8  x  -005 

=0-0059  parts  free  and  saline  ammonia  per 
100,000. 

Albuminoid  ammonia.  1st  100  c.c.  was  such  that  90  c.c.  of 
a  standard  containing  0'8  c.c.  NH4C1  solution  in  the  100  c.c.  was 
equal  to  it  in  colour.  2nd  50  c.c.  none. 

90  x -008  =0-0072   parts   of   albuminoid   ammonia   per 
100  100,000. 

(With  500  c.c.  water  distilled,  1  c.c.  of  the  standard  NH4C1 
solution =0-01  parts  NH3  per  100,000  of  water.) 

In  the  case  of  sewages  and  tank  effluents  take  50  or  100  c.c. 
according  to  strength,  add  500  c.c.  of  pure  water,  or  of  good  tap- 
water  of  which  the  free  and  albuminoid  ammonia  are  known,  then 
a  little  Na2C03  and  distil  till  200  c.c.  have  "been  collected  in 


490  WATER   AND    SEWAGE. 

a  stoppered  flask.  Then  add  50  c.c.  of  alkaline  permanganate  and 
100  c.c.  of  tap-water,  also  3  pieces  of  ignited  pumice  to  prevent 
bumping  and  continue  the  distillation  until  another  200  c.c.  have 
been  collected.  Dr.  Me  Go  wan*  states  that:  "In  actual 
practice  this  distillation  is  never  carried  further  than  the  200  c.c., 
no  matter  at  what  rate  the  albuminoid  ammonia  may  be  coming  off 
when  this  limit  is  reached.  The  estimation  is  thus  only  approximate, 
at  least  in  the  case  of  a  sewage  or  an  ordinary  effluent.  If  the 
distillation  of  this  was  carried  further  (with  the  addition  of  successive 
quantities  of  ammonia-free  water  to  the  retort),  albuminoid  ammonia 
would  be  obtained  for  an  indefinite  period  in  gradually  decreasing 
amounts  in  the  successive  fractions  of  the  distillate."  Suitable 
fractions  of  the  distillates  are  then  diluted  with  ammonia-free  water 
to  100  c.c.  and  Nesslerized.  For  good  filter  and  land  effluents 
take  250  c.c.  and  250  c.c.  of  tap-water,  but  if  bad  100  c.c.  or  less 
and  500  c.c.  of  tap-water  should  be  used. 

Determination  of  Organic  Nitrogen  in  Sewages  or  Effluents 
by  the  Kjeldahl  Process. 

Dr.  Me  Gowanf  proceeds  as  follows  : — 

From  10  c.c.  to  100  c.c.  of  the  sample  (according  to  its  strength, 
are  boiled  down  with  4  c.c.  of  sulphuric  acid  (free  from  nitrogen) 
in  a  round-bottomed  Jena  flask,  the  heating  being  continued) 
after  all  the  water  has  been  evaporated  off,  until  the  acid  becomes 
colourless — the  mouth  of  the  flask  being  closed  by  a  balloon  stopper 
as  soon  as  the  acid  begins  to  fume.  When  cold,  the  flask  is  rinsed 
out  with  distilled  water  into  a  stoppered  measuring  flask,  a  few 
c.c.  of  a  solution  of  oxalate  of  potash  (ammonia-free)  being  some- 
times added  to  throw  down  the  lime  present.  The  solution  is  then 
rendered  just  alkaline  with  purified  potash,  and  filled  up  to  the 
mark  with  water.  The  flocculent  precipitate  which  invariably 
forms  upon  the  addition  of  the  potash,  both  in  the  presence  and 
absence  of  oxalate,  is  allowed  to  subside  for  at  least  twenty-four 
hours,  and  a  suitable  fraction  of  the  clear  liquid  is  then 
Nesslerized. 

A  "  blank "  is  made  in  exactly  the  same  way.  substituting 
distilled  water  for  sewage  or  effluent.  The  nitrogen  found  in  the 
"  blank  "  is  then  corrected  for  the  minute  quantity  of  nitrogen 
in  the  distilled  water  added  before  boiling  down.  The  corrected 
"blank"  thus  obtained,  representing  the  unoxidized  nitrogen 
in  the  4  c.c.  of  sulphuric  acid  used,  is  then  subtracted 
from  the  corresponding  one  in  the  actual  determination,  the 
difference  giving  the  organic  nitrogen  plus  the  nitrogen  from  the 
free  and  saline  ammonia  present.  Finally,  by  deducting  the 

*  Royal  Commission  on  Sewage  Disposal.  Vol.  IV.  Part  V.  Dr.  M  c  .  G  o  w  a  n  '  s 
Report  on  methods  of  chemical  analysis  as  applied  to  Sewage  and  Sewage  Effluents, 
1904  (p.  14). 

t  Report  on  Methods  of  Chemical  Analysis  of  Sewage  and  Effluents.  Vol.  IV., 
Part  V.  of  the  Fourth  Report  of  the  Royal  Commission  on  Sewage  Disposal,  1904. 


ORGANIC   NITROGEN.  491 

latter — determined,  if  possible,  by  direct  Nesslerization  of  the 
sample — the  organic  nitrogen  present  is  obtained.  As  a  rule 
direct  Nesslerization  for  free  and  saline  ammonia  is  impracticable, 
owing  to  the  turbidity  produced  on  adding  Nessler's  solution. 
(Dr.  Me  Go  wan  also  describes  a  more  elaborate  process  "  with 
reduction,"  for  details  of  which  the  reader  is  referred  to  the  blue- 
book  already  mentioned.) 

Dr.  G.  J.  Fowler*  gives  the  following  process  :— 
30  c.c.  of  the  sample  are  placed  in  an  8  oz.  flask,  a  little  sodium 
carbonate  added,  and  it  is  then  distilled  with  steam  till  the 
distillate  shows  no  indication  of  free  ammonia.  The  steam  is 
generated  in  a  32-oz.  flask  and  passed  into  the  flask  containing 
the  sample.  This  flask  should  be  heated  by  a  Buns  en  burner 
to  prevent  the  steam  condensing  in  it.  In  this  way  the  sample 
is  concentrated  to  about  5  c.c.  and  is  then  transferred  to  an  8-oz. 
Jena  flask  with  a  long  neck  and  20  c.c.  of  pure  H2S04  (free  from 
Nitrogen)  added.  The  mixture  is  carefully  heated  over  a  naked 
Bunsen  flame  in  the  draught  chamber  for  about  half  an  hour, 
a  little  phosphoric  anhydride  (P205)  is  then  added,  and  the  heating 
continued  till  the  mixture  is  quite  clear.  After  allowing  to  cool 
somewhat  the  mixture  is  poured  into  about  200  c.c.  of  water,  and 
after  complete  cooling  made  up  exactly  to  250  c.c.  ;  50  c.c.  of  this 
solution  are  then  taken,  made  alkaline  with  a  saturated  solution  of 
caustic  soda,  500  c.c.  of  tap-water  added,  and  the  ammonia  distilled 
off  and  Nesslerized  in  lots  of  100  c.c.  A  "  blank "  should  be 
made  with  35  c.c.  of  distilled  water  and  all  reagents  as  above,  and 
allowed  for.  The  organic  ammonia  thus  found  is  always  higher 
than  the  albuminoid  ammonia,  and  the  relation  between  the  two 
has  been  found  to  vary  within  fairly  wide  limits  with  different 
samples.  In  the  Fifth  Report  of  the  Royal  Commission  on  Sewage 
Disposal!  it  is  stated  that  "  The  ratio  of  albuminoid  to  total 
nitrogen  in  an  effluent  is  usually  from  1  :  2  to  1  :  3,  though  it  may 
in  certain  cases  be  either  below  or  above  those  figures.  When  an 
effluent  contains  a  large  quantity  of  suspended  solids,  the  ratio 
tends  to  be  high." 

Method  of  Procedure  for  Mossy  or  Peaty  Waters. 

R.  R.  Tatlock  and  R.  T.  Thomsonf  have  contributed  a  paper 
on  this  subject  and  the  following  is  a  portion  of  it  : — 

CHLORIDES. — The  determination  of  these  seldom  presents  any  difficulties, 
the  titration  with  standard  silver  nitrate,  and  the  employment  of  potassium 
chromate  as  indicator,  being  usually  sufficient.  Difficulties  arise,  however, 
when  we  have  to  deal  with  mossy  or  peaty  waters  and  with  waters  containing 
acids  or  iron  salts.  With  mossy  waters,  which  are  also  sometimes  acid,  we  have 
found  the  best  mode  of  dealing  to  consist  in  adding  some  calcined  magnesia 
(magnesium  oxide)  to  a  portion  (not  necessarily  measured)  of  the  water,  and 

*  Sewage  Works  Analyses,  p.  58.  t  Cd.  4278.  1908,  p.  223. 

t  J.  8.  C.  J.  23,  428, 


492  WATER   AND    SEWAGE. 

agitating  thoroughly  for  a  few  minutes.  In  this  way  acid,  if  present,  is 
neutralized,  and  the  mossy  or  peaty  matter  is  precipitated  and  removed  from 
solution,  while  the  magnesia  remains  practically  insoluble.  All  that  is  then 
necessary  is  to  filter  through  a  dry  filter,  and  titrate  a  portion  of  the  decolorized 
water  with  standard  silver  nitrate. 

Acid  waters  are  treated  with  magnesia  as  just  described,  and  so  are  waters 
containing  iron,  but  in  the  latter  case  a  few  drops  of  hydrogen  peroxide  should 
be  added  in  order  to  convert  any  ferrous  compounds  into  the  ferric  condition. 
The  magnesia  then  removes  the  iron  wholly,  and  it  only  remains  to  filter  the 
mixture  and  determine  the  chlorides  in  the  solution  as  before. 

NITRATES  AND,  NITRITES. — The  determination  of  nitrates  in  a  water  is  in 
our  opinion  one  of  the  greatest  importance,  at  least  in  the  case  of  water 
intended  for  dietetic  purposes,  and  therefore  an  accurate  and  speedy  method 
is  of  great  value.  We  have  come  to  the  conclusion  that,  when  properly 
adapted,  the  phenol-sulphonic  acid  method  (p.  468)  is  decidedly  the  most  handy 
and  reliable.  In  natural  waters,  however,  there  are  two  ingredients  which  are 
fatal  to  a  correct  determination  of  nitrates,  namely,  organic  matters,  especially 
the  brown  mossy  or  peaty  organic  matter,  which  is  so  often  present  in  the 
waters  we  are  familiar  with  in  Scotland,  and  the  chlorides  of  magnesium  and 
sodium.  For  the  removal  of  organic  colouring  matters,  such  as  is  found  in 
mossy  waters,  there  is  nothing  superior  to  agitation  with  calcined  magnesia  as 
already  described  under  the  determination  of  chlorine.  We  have  found  that 
when  chlorine  is  present  to  the  extent  of  3 '5  grs.  per  gallon  (5  parts  per  100,000) 
of  chloride  of  sodium,  only  about  60  per  cent,  of  the  whole  is  obtained,  and  when 
21  grs.  per  gallon  (30  parts  per  100,000)  are  present,  only  about  34  per 
cent,  is  obtained.  These  proportions  are,  however,  only  roughly  approximate, 
as  we  have  found  that  the  results  in  presence  of  the  same  proportion  of  chlorides 
are  somewhat  erratic.  In  order  to  obviate  this  adverse  influence  of  the 
chlorides,  Mason  recommends  the  use  in  the  standard  of  the  same  proportion 
of  chlorides  as  is  present  in  the  water  being  tested,  so  as  to  counterbalance  their 
influence  ;  but  owing  to  the  somewhat  erratic  effect  of  the  chlorides  we  came  to 
the  conclusion  that  it  would  be  more  satisfactory  to  remove  them  entirely.  For 
this  purpose  we  have  applied  and  adapted  the  method  which  is  employed  for  the 
removal  of  chlorides  from  crude  glycerine  in  testing  the  strength  of  that  article 
by  the  dichromate  method.  This  consists  in  agitating  the  sample  with  excess 
of  silver  oxide,  which  removes  the  chlorine  in  the  form  of  silver  chloride. 
When  treated  in  this  way,  however,  a  considerable  proportion  of  silver  remained 
in  solution,  apparently  as  oxide,  and  this  was  deposited  on  evaporation  of  the 
filtrate  to  dryness  for  treatment  with  phenol-sulphonic  acid,  which  soon  converted 
the  brown  silver  oxide  into  sulphate.  The  silver  compound  seemed  to  have  the 
rather  unexpected  effect  of  lowering  the  result  for  nitrates,  although  not  nearly 
to  the  same  extent  as,  say,  3  grs.  of  sodium  chloride  per  gallon.  We  were  thus 
compelled  to  work  with  a  limited  supply  of  silver  oxide,  adding  just  enough  to 
convert  all  the  chlorides  into  silver  chloride,  this  being  determined  by  the  usual 
volumetric  method.  When  this  was  carried  out  the  exact  proportion  of  nitrates 
present  was  obtained,  but  considerable  difficulty  was  experienced  in  obtaining 
a  clear  filtrate,  as  traces  of  silver  chloride  passed  through  the  filter.  This 
difficulty  was  also  overcome  by  the  use  of  a  little  aluminium  sulphate  followed 
by  calcined  magnesia.  The  final  method  adopted  was  therefore  as  follows  : — 
100  to  200  c.c.  of  the  water  are  treated  with  enough  silver  oxide,  in  a  fine  state 
of  division,  to  decompose  the  chlorides,  the  proportion  of  which  had  been 
previously  ascertained.  After  agitating  thoroughly,  a  little  aluminium  sulphate 
(say  about  O'l  gr.)  is  dissolved  in  the  mixture,  then  excess  of  calcined  magnesia 
is  added,  and  the  agitation  continued  for  a  minute  or  two.  In  this  way  the 
chlorides  and  organic  matter  are  entirely  precipitated,  and  are  then  filtered  off 
through  a  dry  filter,  when  the  filtered  solution  will  be  found  as  free  from  colour 
as  distilled  water.  A  portion  of  the  filtrate  (50  to  100  c.c.)  is  now  evaporated 
to  dryness  over  an  open  water-bath,  and  the  residue  tested  for  nitrates  by  the 
well-known  phenol-sulphonic  acid  method.  A  number  of  test  experiments  showed 
that  in  every  case  the  whole  of  the  nitrates,  added  to  a  water  containing  com- 


METHOD    OF   RECORDING   RESULTS.  493 

paratively  large  proportions  of  chlorides  and  organic  matter,  was  obtained  by 
this  method  of  determination. 

The  next  point  which  arose  for  consideration  was  the  effect  of  nitrites  on  this 
determination,  but  it  was  clearly  shown  that  this  was  almost  nil,  or  that  their 
presence  only  slightly  raised  the  proportion  of  nitrates.  This  fact  suggested  to 
us  the  idea  of  applying  the  phenol-sulphonic  acid  method  to  the  determination 
of  nitrites  also,  provided  these  could  be  readily  converted  into  nitrates.  The 
ideal  reagent  was  soon  found  in  hydrogen  peroxide,  which  suits  admirably  for 
the  purpose  in  view.  To  determine  the  nitrites,  therefore,  it  is  only  necessary 
to  remove  the  chlorine  from,  and  render  colourless,  a  quantity  of  the  water  to 
be  tested,  exactly  in  the  manner  just  described.  In  such  dilute  solutions  as 
generally  occur  in  waters  there  is  no  danger  of  any  nitrite  being  precipitated 
by  the  silver  compound.  A  portion  of  the  treated  water  is  tested  for  nitrates, 
and  to  an  equal  portion  there  is  added  a  little  hydrogen  peroxide,  and  the 
mixture  evaporated  to  dryness.  The  nitrites  which  existed  in  the  water  are 
now  present  in  the  residue  in  the  form  of  nitrate,  and  it  only  remains  to  apply 
the  phenol-sul phonic  test,  then  subtract  the  nitrates  actually  present  in  the  water 
as  such  from  the  total  nitrates  now  obtained,  and  calculate  the  remainder  to 
nitrites,  or  to  bring  these  compounds  to  nitric  and  nitrous  nitrogen  respectively. 
Of  course  it  would  be  advisable  to  make  certain  of  the  presence  of  nitrites  by  one 
of  the  well-known  colour  tests  for  these  compounds. 

Microscopical  Examination  of  Deposit. — The  most  convenient  plan  of  collecting 
the  deposit  is  to  place  a  circular  microscopical  covering  glass  at  the  bottom  of 
a  large  conical  glass  holding  about  20  oz.  The  glass  should  have  no  spout,  and 
should  be  ground  smooth  on  the  top.  After  shaking  up  the  sample,  this  vessel 
is  rilled  with  the  water,  covered  with  a  plate  of  ground  glass,  and  set  aside  to 
settle.  After  settling,  the  supernatant  water  is  drawn  off  by  a  fine  siphon,  and 
the  glass  bearing  the  deposit  lifted  out,  either  by  means  of  a  platinum  wire  (which 
should  have  been  previously  passed  under  it),  or  in  some  other  convenient  way, 
and  inverted  on  to  an  ordinary  microscopical  slide  for  examination.  It  is  desirable 
to  examine  the  deposit  first  by  a  |th  and  then  by  a  |th  objective.  The  examination 
should  be  made  as  soon  as  the  water  has  stood  overnight.  If  the  water  be 
allowed  to  stand  longer,  organisms  peculiar  to  stagnant  water  may  be  developed 
and  mislead  the  observer.  Particular  notice  should  be  taken  of  bacteria,  infusoria, 
ciliata  or  flagellata,  disintegrated  fibres  of  cotton,  or  linen,  or  epithelial  debris. 

It  is  particularly  desirable  to  report  clearly  on  this  microscopical  examination  ; 
not  merely  giving  the  general  fact  that  organisms  were  present,  but  stating  as 
specifically  as  possible  the  names  or  classes  of  the  organisms,  so  that  more  data 
may  be  obtained  for  the  application  of  the  examination  of  this  deposit  to  the 
characters  of  potable  waters. 

It  is  also  desirable  to  examine  the  residue  left  on  a  glass  slide  by  the  evapora- 
tion of  a  single  drop  of  the  water.  This  residue  is  generally  most  conveniently 
examined  without  a  covering  glass.  The  special  appearances  to  be  noticed  are 
the  presence  or  absence  of  particles  of  organic  matter,  or  organized  structure, 
contained  in  the  crystallized  forms  which  may  be  seen ;  and  also  whether  any 
part  of  the  residue  left,  especially  at  the  edges,  is  tinted  more  or  less  with  green, 
brown,  or  yellow. 


Method  of  Recording  Water  and  Sewage  Examination 
Results. 

The  report*  of  the  committee  appointed  to  establish  a  Uniform 
System  of  recording  the  Results  of  the  Chemical  and  Bacterial 
Examination  of  Water  and  Sewage  is  as  follows  : — That  it  is 
desirable  that  results  of  analysis  should  be  expressed  in  parts  per 
100.000  except  in  the  case  of  dissolved  gases,  when  these  should  be 

*  British  Association  Report,  1899. 


494  WATER   AND    SEWAGE. 

stated  as  cubic  centimetres  of  gas  at  0°  C.,  and  760  millimetres  in 
1  litre  of  water.  This  method  of  recording  results  is  in  accordance 
with  that  suggested  by  the  committee  appointed  in  1887  to  confer 
with  the  committee  of  the  American  Association  for  the  Advance- 
ment of  Science,  with  a  view  to  forming  a  uniform  system  of  record- 
ing the  results  of  water  analysis. 

The  committee  suggest  that  in  the  case  of  all  nitrogen  compounds 
the  results  be  expressed  as  parts  of  nitrogen  over  100,000,  including 
the  ammonia  expelled  on  boiling  with  alkaline  permanganate, 
which  should  be  termed  albuminoid  nitrogen.  The  nitrogen  will 
therefore  be  returned  as  : 

(1)  Ammoniacal  nitrogen  from  free  and  saline  ammonia. 

(2)  Nitrous  nitrogen  from  nitrites. 

(3)  Nitric  nitrogen  from  nitrates. 

(4)  Organic  nitrogen  (either  by  Kj  eld  a  hi  or   by  combustion, 
but  the  process  used  should  be  stated). 

(5)  Albuminoid  nitrogen. 

The  total  nitrogen  of  all  kinds  will  be  the  sum  of  the  first  four 
determinations. 

The  committee  are  of  opinion  that  the  percentage  of  nitrogen 
oxidized — that  is,  the  ratio  of  (2)  and  (3)  to  (1)  and  (4) — gives 
sometimes  a  useful  measure  of  the  stage  of  purification  of  a  particular 
sample.  The  purification  effected  by  a  process  will  be  measured  by 
the  amount  of  oxidized  nitrogen  as  compared  with  the  total  amount 
of  nitrogen  existing  in  the  crude  sewage. 

In  raw  sewage  and  in  effluents  containing  suspended  matter,  it  is 
also  desirable  to  determine  how  much  of  the  organic  nitrogen  is 
present  in  the  suspended  matter. 

In  sampling,  the  committee  suggest  that  the  bottles  should  be 
filled  nearly  completely  with  the  liquid,  only  a  small  air-bubble 
being  allowed  to  remain  in  the  neck  of  the  bottle.  The  time  at 
which  a  sample  is  drawn,  as  well  as  the  time  at  which  its  analysis 
is  begun,  should  be  noted.  An  effluent  should  be  drawn  to  corre- 
spond as  nearly  as  possible  with  the  original  sewage,  and  both  it 
and  the  sewage  should  be  taken  in  quantities  proportional  to  the 
rate  of  flow  when  that  varies  (e.g.,  in  the  emptying  of  a  filter- 
bed). 

In  order  to  avoid  the  multiplication  of  analyses,  the  attendant  at 
a  sewage  works  (or  any  other  person  who  draws  the  samples) 
might  be  provided  with  sets  of  twelve  or  twenty-four  stoppered 
quarter- Winchester  bottles,  one  of  which  should  be  filled  every 
hour  or  every  two  hours,  and  on  the  label  of  each  bottle  the  rate  of 
flow  at  the  time  should  be  written.  When  the  bottles  reach  the 
laboratory,  quantities  would  be  taken  from  each  proportional  to 
these  rates  of  flow  and  mixed  together,  by  which  means  a  fair 
average  sample  for  the  twenty-four  hours  would  be  obtained. 

The  committee  at  present  are  unable  to  suggest  a  method  of 
reporting  bacterial  results,  including  incubator  tests,  which  is 
likely  to  be  acceptable  to  all  workers. 


STANDARDS.  495 

Standards  for  Sewage  Effluents. 

The  following  standards  of  purity  or  limits  of  impurity,  all  of 
which  are  of  a  somewhat  arbitrary  character,  have  from  time  to 
time  been  put  forward  as  applying  to  effluents  that  it  was  desired 
to  pass  into  streams. 

Effluents  are  classed  as  good  when  they  show  no  more  than  : — 

(Mersey  and  Irwell  Joint  Committee) 

1  grain  per  gallon  of  oxygen  absorbed  in  4  hours. 
0-1  grain  per  gallon  of  albuminoid  ammonia. 

(Kibble  Joint  Committee's  Inspector) 
Ol  part  of  albuminoid  ammonia  per  100,000. 
No  suspended  matter 
Nitrates  present. 

(Derbyshire  County  Council) 

0-1  part  of  albuminoid  ammonia  per  100,000,  more  than  0'5  part 
of  nitric  nitrogen  per  100,000,  and  an  effluent  should  be  so  thoroughly 
oxidized  that  it  does  not  absorb  more  oxygen  after  incubation  for 
one  week  than  it  does  at  the  time  of  collection. 

All  the  above,  however,  have  now  been  superseded  by  the 
recommendations  given  in  the  Fifth  Report  of  the  Royal  Commission 
on  Sewage  Disposal.*  In  this  the  Commissioners  report  that  : — 
;<  The  experiments  which  we  have  already  made  show  that  the 
mere  estimation  of  the  amount  of  organic  matter  in  an  effluent 
does  not,  by  itself,  afford  a  sufficiently  reliable  index  as  to  the  effect 
which  that  effluent  will  have  on  any  stream  into  which  it  may  be 
discharged  "  (par.  320).  Further  on  we  read  :  "  According  to 
our  present  knowledge,  an  effluent  can  best  be  judged  by  ascertain- 
ing, first,  the  amount  of  suspended  matter  which  it  contains,  and 
second,  the  rate  at  which  the  effluent,  after  the  removal  of  the 
suspended  solids,  takes  up  oxygen  from  water. 

In  applying  this  test  it  is  important  that  the  suspended  solids 
should  be  removed,  and  estimated  separately. 

Small  variations  in  the  amount  of  suspended  solids  in  effluents 
may  seriously  affect  the  rate  at  which  the  effluents  take  up  oxygen, 
and  unless  these  solids  are  first  removed,  the  dissolved  oxygen 
absorption  test  might  give  a  misleading  figure  as  to  the  character 
of  the  effluent  "  (par.  321). 

Consequently,  no  arbitrary  standards  based  on  the  amounts  of 
oxygen  absorbed  or  albuminoid  ammonia  are  suggested  by  the 
Commissioners,  and  instead  the  Report  (par.  322)  proceeds  : — 

"  For  the  guidance  of  local  authorities,  we  may  provisionally 
state  that  an  effluent  would  generally  be  satisfactory  if  it  complied 
with  the  following  conditions  : — 

(1)  That  it  should  not  contain  more  than  three  parts  per  100,000 
of  suspended  matter  ;  and 

*  Cd.  4278.     Issued  in  1908. 


496 


WATER   AND    SEWAGE. 


(2)     That,  after  being  filtered  through  paper,  it  should  not  absorb 
more  than 

(a)  0*5  part  by  weight  per  100,000  of  dissolved  or  atmospheric 
oxygen  in  twenty-four  hours. 

(b)  1*0  part  by  weight  per  100,000  of  dissolved  or  atmospheric 
oxygen  in  forty-eight  hours  ;  or 

(c)  1*5  part  by  weight  per  100,000  of  dissolved  or  atmospheric 
oxygen  in  five  days." 

The  following  recent  analyses*  of  effluents  may  be  found  useful : — 


(Parts  per  100,000,). 

Effluent 
from 
Land. 

Effluent 
from 
Land. 

Effluent 
from 
Bacterial 
Treatment. 

Effluent 
from 
Bacterial 
Treatment. 

Total  solids 

64-1 

86-4 

70-3 

59-4 

Suspended  matter,  total 

Less  than  3 

Less  than  3 

Less  than  3 

4-40      • 

„             „           volatile 

.  . 

. 

2-98 

„             „           non-  volatile 

.  . 

.  , 

1-42 

Ammonia,  free  .  . 

0-58 

0-53 

0-95 

0-22 

„          albuminoid 

0-26 

0-075 

0-13 

0-17 

Oxygen    absorbed     from     per- 

manganate : 

In  3  minutes  at  80°  F. 

0-89 

In  4  hours  at  80°  F. 

0-65 

0-65 

1-38 

2-23 

Nitrogen  as  nitrates 

1-09 

1-61 

2-73 

1-95 

„         „  nitrites 

0-015 

0-04 

0-02 

0-03 

Chlorine 

6-1 

7-1 

6-7                5-5 

Dissolved  oxygen  absorbed  : 

(a)  in  24  hours 

0-3 

o-o 

o-o 

0-02 

(b)  in  48  hours 

0-8 

0-04 

0-30 

0-06 

(c)  in  5  days 

1-4 

0-28 

0-72 

0-22 

Characteristics  of  Waters  derived  from  various  Geological 

Formations. 
Dr.  Ri dealf  gives  the  following  useful  summary  of  the  above  : — 

Hard  Waters,  as  a  rule,  are  furnished  by  the  following  formations  : 
Calcareous  strata  of  Silurian,  Devonian  or  Old  Red  Sandstone,  and 
Coal  Measures,  Mountain  Limestone,  Lias,  Oolite,  Upper  Greensand, 
Chalk.  Soft  Waters,  by  Igneous,  Metamorphic,  non-calcareous 
Cambrian,  Silurian,  Devonian,  and  Coal  Measures,  Lower  Green- 
sand,  London  and  Oxford  Clay,  Bagshot  Beds  (hardness  1-9, 
average  4),  and  non-calcareous  gravel.  Water  from  Gault  Clay 
varies  very  much  :  some  of  it  is  soft  and  pure,  some  of  "  fair 
quality,"  hardness  9-11  degrees  ;  in  Bedfordshire  it  often  contains 
much  lime  and  iron,  derived  from  pyrites  and  coprolites.  Lower 
Greensand  and  shale  waters  are  frequently  very  ochreous.  Water 
from  Oxford  and  Kimmeridge  Clays  contains  much  vegetable 
matter,  and  is  sometimes  bituminous  ;  other  clays  often  include 

*  From  Rideal  &  Burgess's  paper  on  "  The  New  Standards  for  Sewage 
Effluents,"  Analyst,  34,  1909,  201. 

t   "Water  and  its  Purification,"  pp.  259-260.     Detailed  information  on  this 
topic  is  given  in  an  appendix  to  the  book. 


INTERPRETATION  OF  RESULTS.  497 

much  sulphate  of  lime  and  give  waters  of  high  permanent  hardness. 
The  new  Red  Sandstone  waters  are  generally  briny  and  quite  unfit 
for  drinking,  besides  containing  much  sulphate  of  lime  and 
magnesium  salts.  Magnesian  limestone  (Dolomite)  also  yields 
usually  a  bad  supply.  The  water  in  porous  strata  below  the 
central  portions  of  clay  basins  is  usually  bad,  containing  much 
alkali  chloride  and  sulphate,  and  also  sodium  carbonate,  due  to  the 
rain  having  percolated  laterally  through  a  large  body  of  soil  before 
reaching  the  spot,  and  having  dissolved  and  accumulated  the 
soluble  constituents  :  from  the  presence  of  alkali  carbonate  the 
lime  is  generally  low,  and  there  is  often  little  organic  matter. 

Rules  for  Converting  Parts  per  100,000  into  Grains  per  Gallon, 

and  Vice  Versa. 
To  convert — 

Parts  per  100,000  into  grains  per  gallon,  multiply  by  0*7. 
Grains  per  gallon  into  parts  per  100,000,  divide  by  0'7. 
Grams  per  litre  into  grains  per  gallon,  multiply  by  70. 


THE    INTERPRETATION    OF    THE    RESULTS    OF 
ANALYSIS. 

All  figures  refer  to  parts  per  100,000. 

THE  primary  form  of  natural  water  is  rain,  the  chief  impurities  in  which  are 
traces  of  organic  matter,  ammonia,  and  ammonium  nitrate  derived  from  the 
atmosphere.  On  reaching  the  ground  it  becomes  more  or  less  charged  with  the 
soluble  constituents  of  the  soil,  such  as  calcium  and  magnesium  carbonates, 
potassium  and  sodium  chlorides,  and  other  salts,  which  are  dissolved,  some  by 
a  simple  solvent  action,  others  by  the  agency  of  carbonic  acid  in  solution. 
Draining  off  from  the  land,  it  will  speedily  find  its  way  to  a  stream  which,  in  the 
earlier  part  of  its  course,  will  probably  be  free  from  pollution  by  animal  matter, 
except  that  derived  from  any  manure  which  may  have  been  applied  to  the  land 
on  which  the  rain  fell.  Thus  comparatively  pure,  it  will  furnish  to  the  inhabi- 
tants on  its  banks  a  supply  of  water  which,  after  use,  will  be  returned  to  the 
stream  in  the  form  of  sewage  charged  with  impurity  derived  from  animal 
excreta,  soap,  household  refuse,  etc.,  the  pollution  being  perhaps  lessened  by 
submitting  the  sewage  to  some  purifying  process,  such  as  irrigation  of  land, 
filtiation,  or  clarification.  The  stream  in  its  subsequent  course  to  the  sea  will 
be  in  some  measure  purified  by  slow  oxidation  of  the  organic  matter,  and  by 
the  absorbent  action  of  vegetation.  Some,  of  the  rain  will  not,  however, 
go  directly  to  a  stream,  but  sink  through  the  soil  to  a  well.  If  this  be  shallow 
it  may  be  considered  as  merely  a  pit  for  the  accumulation  *of  drainage  from  the 
immediately  surrounding  soil,  which,  as  the  well  is  in  most  cases  close  to  a  dwelling, 
will  be  almost  inevitably  charged  with  excretal  and  other  refuse  ;  so  that  the 
water  when  it  reaches  the  well  will  be  contaminated  with  soluble  impurities  thence 
derived,  and  with  nitrites  and  nitrates  resulting  from  their  oxidation.  After 
use  the  water  from  the  well  will,  like  the  river  water,  form  sewage,  and  find  its 
way  to  a  river,  or  again  to  the  soil;  according  to  circumstances. 

In  the  case  of  a  deep  well,  from  which  the  surface  water  is  excluded,  the 
conditions  are  different.  The  shaft  will  usually  pass  through  an  impervious 
stratum,  so  that  the  water  entering  it  will  not  be  derived  from  the  rain 
which  falls  on  the  area  immediately  surrounding  its  mouth,  but  from  that 
which  falls  on  the  outcrop  of  the  pervious  stratum  below  the  impervious  one 

2   K 


498  WATER   AND    SEWAGE. 

just  mentioned  ;  and  if  this  outcrop  be  in  a  district  which  is  uninhabited  and 
uncultivated,  the  water  of  the  well  will  probably  be  entirely  free  from  organic 
impurity  or  products  of  decomposition.  But  even  if  the  water  be  polluted  at 
its  source,  still  it  must  pass  through  a  very  extensive  filter  before  it  reaches  the 
well,  and  its  organic  matter  will  probably  be  in  great  measure  converted  by 
oxidation  into  bodies  in  themselves  innocuous. 

This  is  very  briefly  the  general  history  of  natural  waters,  and  the  problem 
presented  to  the  analyst  is  to  ascertain,  as  far  as  possible,  from  the  nature  and 
quantity  of  the  impurities  present,  the  previous  history  of  the  water,  and  its 
present  condition  and  fitness  for  the  purpose  for  which  it  is  to  be  used. 

It  is  impossible  to  give  any  fixed  rule  by  which  the  results  obtained  by  the 
foregoing  method  of  analysis  should  be  interpreted.  The  analyst  must  form  an 
independent  opinion  for  each  sample  from  a  consideration  of  all  the  results  he  has 
obtained.  Nevertheless,  the  following  remarks,  illustrated  by  reference  to  the 
examples  given  in  the  accompanying  table,  which  may  be  considered  as  fairly 
typical,  will  probably  be  of  service.  (See  Table,  pp.  474,  475). 

Total  Solid  Matter. 

Waters  which  leave  a  large  residue  on  evaporation  are,  as  a  rule,  less  suited  for 
general  domestic  purposes  than  those  which  contain  less  matter  in  solution,  and 
are  unfit  for  many  manufacturing  purposes.  The  amount  of  residue  is  also  of 
primary  importance  as  regards  the  use  of  the  water  for  steam  boilers,  as  the 
quantity  of  incrustation  produced  will  chiefly  depend  upon  it.  It  may  vary 
considerably,  apart  from  any  unnatural  pollution  of  the  water,  as  it  depends 
principally  on  the  nature  of  the  soil  through  or  over  which  the  water  passes. 
River  water,  when  but  slightly  polluted,  contains  generally  from  10  to  40  parts. 
Shallow  well  water  varies  greatly,  containing  from  30  to  150  paits,  or  even  more, 
as  in  examples  X.  and  XIII. ,  the  proportion  here  depending  less  on  the  nature  of 
the  soil  than  on  the  original  pollution  of  the  water.  Deep  well  water  also  varies 
considerably  ;  it  usually  contains  from  20  to  70  parts,  but  this  range  is  frequently 
overstepped,  the  quantity  depending  largely  upon  the  nature  of  the  strata  from 
which  the  water  is  obtained.  Example  XV.  being  in  the  New  Red  Sandstone, 
has  a  small  proportion,  but  XVII.  and  XVIII.  in  the  Chalk  have  a  much  larger 
quantity.  Spring  waters  closely  resemble  those  from  deep  wells.  Sewage  contains 
generally  from  50  to  100  parts,  but  occasionally  less,  and  frequently  much  more 
as  in  example  XXXIV.  The  total  solid  matter,  as  a  rule,  exceeds  the  sum  of  the 
constituents  determined  ;  the  nitrogen,  as  nitrates  and  nitrites,  being  calculated 
as  potassium  nitrate,  and  the  chlorine  as  sodium  chloride  ;  but  occasionally  this 
is  not  the  case,  owing,  it  is  likely,  to  the  presence  of  some  of  the  calcium  as  nitrate 
or  chloride. 

Organic  Carbon  or  Nitrogen. 

The  existing  condition  of  the  sample,  so  far  as  organic  contamination  is 
concerned,  must  be  inferred  from  the  amount  of  these  two  constituents.  In  a 
good  water,  suitable  for  domestic  supply,  the  former  should  not,  under  ordinary 
circumstances,  exceed  0'2  and  the  latter  0'02  part. 

Waters  from  districts  containing  much  peat  are  often  coloured  more  or  less 
brown,  and  contain  an  unusual  quantity  of  organic  carbon,  but  this  peaty 
matter  is  probably  innocuous  unless  the  quantity  be  extreme.  The  large 
proportion  of  organic  carbon  and  nitrogen  given  in  the  average  for  unpolluted 
upland  surface  water  in  Table  (XXVIII.)  is  chiefly  due  to  the  fact  that  upland 
gathering  grounds  are  very  frequently  peaty.  The  examples  given  (I.  to  V.) 
may  be  taken  as  fairly  representative  of  the  character  of  upland  surface  waters 
free  from  any  large  amount  of  peaty  matter.  In  surface  waters  from  cultivated 
areas  the  quantity  of  organic  carbon  and  nitrogen  is  greater,  owing  to  increased 
density  of  population,  the  use  of  organic  manures,  etc.,  the  proportion  being 
about  0'25  to  0*3  part  of  organic  carbon,  and  0'04  to  0-05  part  of  organic 
nitrogen.  The  water  from  shallow  wells  varies  so  widely  in  its  character  that  it 
is  impossible  to  give  any  useful  average.  In  many  cases,  as  for  example  in  XIII. 


INTERPRETATION  OF  RESULTS.  499 

and  XIV,  the  amount  is  comparatively  small,  although  the  original  pollution, 
as  shown  by  the  total  inorganic  nitrogen  and  the  chlorides,  was  very  large  ;  the 
organic  matter  in  these  cases  having  been  almost  entirely  destroyed  by  powerful 
oxidation.  In  VIII.  and  IX.  the  original  pollution  was  slight ;  and  oxidation 
being  active,  the  organic  carbon  and  nitrogen  have  been  reduced  to  extremely 
small  quantities.  On  the  other  hand,  in  XI.  the  proportion  of  organic  matter  is 
enormous,  the  oxidizing  action  of  the  surrounding  soil  being  utterly  insufficient 
to  deal  with  the  pollution.  The  danger  attending  the  use  of  shallow  well  waters, 
which  contain  when  analyzed  very  small  quantities  of  organic  matter,  arises 
chiefly  from  the  liability  of  the  conditions  to  variation.  Change  of  weather  and 
many  other  circumstances  may  at  any  time  prevent  the  purification  of  the  water, 
which  at  the  time  of  the  analysis  appeared  to  be  efficient.  Moreover,  it  is  by 
no  means  certain  that  an  oxidizing  action  which  would  be  sufficient  to  reduce 
the  organic  matter  in  a  water  to  a  very  small  proportion  would  be  equally  com- 
petent to  remove  the  specific  poison  of  disease.  Hence  the  greater  the  impurity 
of  the  source  of  a  water  the  greater  the  risk  attending  its  use. 

In  deep  well  waters  the  quantity  of  organic  carbon  and  nitrogen  also  extends 
through  a  wide  range,  but  is  generally  low,  the  average  being  about  0*06  part 
carbon  and  0*02  part  nitrogen  (XXIX.).  Here  the  conditions  are  usually  very 
constant,  and  if  surface  drainage  be  excluded,  the  source  of  the  water  is  of  less 
importance.  Springs  in  this,  as  in  most  other  respects,  resemble  deep  wells  ; 
the  water  from  them  being  generally,  however,  somewhat  purer.  In  sewage 
great  variations  are  met  with.  On  the  average  it  contains  about  four  parts  of 
organic  carbon  and  two  parts  of  organic  nitrogen  (XXXII.  and  XXXIII.),  but 
the  range  is  very  great.  In  the  table,  XXXIV.  is  a  very  strong  sample,  and 
XXXV.  a  weak  one.  The  effluent  water  from  land  irrigated  with  sewage  is 
usually  analogous  to  waters  from  shallow  wells,  and  its  quality  varies  greatly 
according  to  the  character  of  the  sewage  and  the  conditions  of  the  irrigation. 

Ratio  of  Organic  Carbon  to  Organic  Nitrogen. 

The  ratio  of  the  organic  carbon  to  the  organic  nitrogen  given  in  the  seventh 
column  of  the  table  (which  shows  the  fourth  term  of  the  proportion — organic 
nitrogen  :  organic  carbon  :  :  1  :  x},  is  of  great  importance  as  furnishing  a 
valuable  indication  of  the  nature  of  the  organic  matter  present.  When  this  is  of 
vegetable  origin,  the  ratio  is  very  high,  and  when  of  animal  origin  very  low. 
This  statement  must,  however,  be  qualified,  on  account  of  the  different  effect  of 
oxidation  on  animal  and  vegetable  substances.  It  is  found  that  when  organic 
matter  of  vegetable  origin,  with  a  high  ratio  of  carbon  to  nitrogen,  is  oxidized, 
it  loses  carbon  more  rapidly  than  nitrogen,  so  that  the  ratio  is  reduced.  Thus 
unoxidized  peaty  waters  exhibit  a  ratio  varying  from  about  8  to  20  or  even 
more,  the  average  being  about  12  ;  whereas,  the  ratio  in  spring  water  originally 
containing  peaty  matter,  varies  from  about  2  to  5,  the  average  being  about  3 '2. 
When  the  organic  matter  is  of  animal  origin  the  action  is  reversed,  the  ratio 
being  increased  by  oxidation.  In  unpolluted  upland  surface  waters  the  ratio 
varies  from  about  6  to  12,  but  in  peaty  waters  it  may  amount  to  20  or  more.  In 
surface  water  from  cultivated  land  it  ranges  from  about  4  to  10,  averaging  about 
6.  In  water  from  shallow  wells  it  varies  from  about  2  to  8,  with  an  average 
of  about  4,  but  instances  beyond  this  range  in  both  directions  are  very  frequent. 
In  water  from  deep  wells  and  springs,  the  ratio  varies  from  about  2  to  6  with 
an  average  of  4,  being  low  on  account,  probably,  of  the  prolonged  oxidation  to 
which  it  has  been  subjected,  which,  as  has  been  stated  above,  removes  carbon 
more  rapidly  than  nitrogen.  In  sea  water  this  action  reaches  a  maximum,  the 
time  being  indefinitely  prolonged,  and  the  ratio  is  on  the  average  about  1*7. 
This  is  probably  complicated  by  the  presence,  in  some  cases,  of  multitudes  of 
minute  living  organisms.  In  sewage  the  ratio  ranges  from  about  1  to  3,  with 
an  average  of  about  2. 

When,  in  the  case  of  a  water  containing  much  nitrogen  as  nitrates  and 
nitrites,  this  ratio  is  unusually  low,  incomplete  destruction  of  nitrates  during 
the  evaporation  may  be  suspected,  and  the  determination  should  be  repeated. 
To'provide  for  this  contingency,  if  a  water  contain  any  considerable  quantity  of 

2   K    2 


500  WATER   AND    SEWAGE. 

ammonia,  it  is  well,  when  commencing  the  evaporation  in  the  first  instance,  to 
set  aside  a  quantity  sufficient  for  this  repetition,  adding  to  it  the  usual  proportion 
of  sulphurous  acid. 

Nitrogen  as  Ammonia. 

The  ammonia  in  natural  waters  is  derived  almost  exclusively  from  animal 
contamination,  and  its  quantity  varies  between  very  wide  limits.  In  upland 
surface  waters  it  seldom  exceeds  0'008  part,  the  average  being  about  0'002  part. 
In  water  from  cultivated  land  the  average  is  about  O'OOS,  and  the  range  is  greater, 
being  from  nil  to  0'025  part,  or  even  more.  In  water  from  shallow  wells  the 
variation  is  so  great  that  it  would  be  useless  to  attempt  to  state  an  average,  all 
proportions  from  nil  to  as  much  as  2 -5  parts  having  been  observed.  In  waters 
from  deep  wells  a  very  considerable  proportion  is  often  found,  amounting  to  0*1 
'part  or  even  more,  the  average  being  O'Ol  part,  and  the  variations  considerable. 
In  spring  water  it  is  seldom  that  more  than  O'Ol  part  of  nitrogen  as  ammonia 
occurs,  the  average  being  only  0*001  part.  Sewage  usually  contains  from  2  to 
6  parts,  but  occasionally  as  much  as  9  or  10  parts,  the  average  being  about  five. 
Ammonia  is  readily  oxidized  to  nitrites  and  nitrates,  and  hence  its  presence,  in 
considerable  quantity,  usually  indicates  the  absence  of  oxidation,  and  is  generally 
coincident  with  the  presence  of  organic  matter.  That  sometimes  found  in  waters 
from  very  deep  wells  is,  however,  probably  due  to  subsequent  decomposition 
of  nitrates. 

Albuminoid  Ammonia. 

Wanklyn's  standards  for  albuminoid  ammonia  are 

High  purity,          O'O  to  0-0041  parts  per  100,000. 

Satisfactory,  0-0041  to  0'0082 

Impure,  over  0-0082 

In  the  absence  of  free  ammonia,  he  does  not  condemn  a  water  unless  the 
albuminoid  exceeds  -0082,  but  a  water  yielding  -0123  he  condemns  in  any  circum- 
stances. When  the  albuminoid  ammonia  process  was  introduced  it  was  well 
known  that  there  was  a  varying  relation  between  the  quantities  of  albuminoid 
ammonia  and  the  amounts  of  different  kinds  of  nitrogenous  organic  matter. 
The  researches  of  numerous  chemists  have  confirmed  the  inference  that,  although 
a  useful  indication,  too  much  importance  must  not  be  attached  to  this  figure. 

Nitrogen  as  Nitrates  and  Nitrites. 

Nitrates  and  nitrites  are  produced  by  the  oxidation  of  nitrogenous  organic 
matter,  and  almost  always  from  animal  matter.  In  upland  surface  waters  the 
proportion  varies  from  nil  to  0'05  part  or  very  rarely  more,  but  the  majority  of 
samples  contain  none  or  mere  traces  (I.  to  V.),  the  average  being  about  0-009 
part.  In  surface  waters  from  cultivated  land  the  quantity  is  much  greater, 
varying  from  nil,  which  seldom  occurs,  to  1  part,  the  average  being  about  0'25 
part.  The  proportion  in  shallow  wells  is  usually  much  greater  still,  ranging 
from  nil,  which  very  rarely  occurs,  to  as  much  as  25  parts,  It  would  probably 
be  useless  to  attempt  to  state  an  average,  but  quantities  of  from  2  to  5  parts 
occur  most  frequently.  In  water  from  deep  wells  the  range  is  from  nil  to  about 
3  parts,  and  occasionally  more,  the  average  being  about  0'5  part.  In  spring 
water  the  range  is  about  the  same  as  in  deep  well  water,  but  the  average  is  some- 
what lower. 

It  sometimes  happens  that,  when  the  supply  of  atmospheric  oxygen  is  deficient, 
the  organic  matter  in  water  is  oxidized  at  the  expense  of  the  nitrates  present ; 
and  occasionally,  if  the  quantities  happen  to  be  suitably  proportioned,  they 
are  mutually  destroyed,  leaving  no  evidence  of  pollution.  This  reduction  of 
nitrates  often  occurs  in  deep  well  water,  as  for  example,  in  that  from  wells  in 
the  Chalk  beneath  London  Clay,  where  the  nitrates  are  often  totally  destroyed. 
In  sewages,  putrefaction  speedily  sets  in,  and  during  this  condition  the  nitrates 


INTERPRETATION  OF  RESULTS.  501 

are  rapidly  destroyed,  and  so  completely  and  uniformly  that  it  is  probably 
needless  to  attempt  their  determination,  except  in  sewages  which  are  very  weak,  or 
for  other  special  reasons  abnormal.  Out  of  a  large  number  of  samples,  only  a 
very  few  have  been  found  which  contained  any  nitrates,  and  those  only  very 
small  quantities. 

Nitrites  occurring  in  deep  springs  or  wells  no  doubt  arise  from  the  deoxidation 
of  nitrates  by  ferrous  oxide,  or  certain  forms  of  organic  matter  of  a  harmless 
nature  ;  but  whenever  they  occur  in  shallow  wells  or  river  water,  they  may  be 
of  much  greater  significance.  Their  presence  in  such  cases  is  most  probably 
due  to  recent  sewage  contamination,  and  such  waters  must  be  looked  upon  with 
great  suspicion. 

Total  Inorganic  Nitrogen. 

When  organic  matter  is  oxidized  it  is  ultimately  resolved  into  inorganic, 
substances.  Its  carbon  appears  as  carbonic  acid,  its  hydrogen  as  water,  and  its 
nitrogen  as  ammonia,  nitrous  acid,  or  nitric  acid  ;  the  last  two  combining  with 
the  bases  always  present  in  water  to  form  nitrites  and  nitrates.  The  carbon 
and  hydrogen  are  thus  clearly  beyond  the  reach  of  the  analyst ;  but  the  nitrogen 
compounds,  as  has  been  shown,  can  be  accurately  determined,  and  furnish  us 
with  a  means  of  estimating  the  amount  of  organic  matter  which  was  formerly 
present  in  the  water,  but  which  has  already  undergone  decomposition. 

The  sum  of  the  amounts  of  nitrogen  found  in  these  three  forms  constitutes 
then  a  distinct  and  valuable  term  in  the  analysis,  the  organic  nitrogen  relating  to 
the  present,  and  the  total  inorganic  nitrogen  to  the  past  conditions  of  the  water. 
Since  ammonia,  nitrites,  and  nitrates  are  quite  innocuous,  the  total  inorganic 
nitrogen  does  not  indicate  actual  evil  like  the  organic  nitrogen,  but  potential  evil, 
as  it  is  evident  that  the  innocuous  character  of  a  water  which  contains  much 
nitrogen  in  these  forms  depends  wholly  on  the  permanence  of  the  conditions 
of  temperature,  aeration,  nitration  through  soil,  etc.,  which  have  broken  up  the 
original  organic  matter ;  it  these  should  at  any  time  fail,  the  past  contamination 
would  become  present,  the  nitrogen  appearing  in  the  organic  form,  the  water 
being  loaded  in  all  likelihood  with  putrescent  and  contagious  matter. 

In  upland  surface  waters  which  have  not  been  contaminated  to  any  extent  by 
animal  pollution  the  total  inorganic  nitrogen  rarely  exceeds  0'03  part.  In  water 
from  cultivated  districts  the  amount  is  greater,  ranging  as  high  as  1  part,  the 
average  of  a  large  number  of  samples  being  about  0'22  part.  It  is  useless  to 
attempt  any  generalization  for  shallow  wells,  as  the  proportion  depends  upon  local 
circumstances.  The  amount  is  usually  large  and  may  reach,  as  seen  in  Examples 
XIII.,  the  enormous  quantity  of  twenty-five  parts  per  100,000.  Waters  con- 
taining one  to  fivejparts  are  very  commonly  met  with.  In  water  from  deep 
wells  and  springs,  quantities  ranging  up  to  3 '5  parts  have  been  observed,  the 
average  on  a  large  series  of  analyses  being  0'5  part  for  deep  wells,  and  about 
0'4  part  for  springs.  It  must  be  remembered  that  the  conditions  attending 
deep  wells  and  springs  are  remarkably  permanent,  and  the  amount  of  filtration 
which  the  water  undergoes  before  reaching  the  well  itself,  or  issuing  from  the 
spring  is  enormous.  Meteorological  changes  here  have  either  no  effect,  or  one 
so  small  and  slow  as  not  to  interfere  with  any  purifying  actions  which  may  be 
taking  place.  All  other  sources  of  water,  and  especially  shallow  wells,  are  on 
the  other  hand  subject  to  considerable  changes.  A  sudden  storm  after  drought 
will  wash  large  quantities  of  polluting  matter  into  the  water-course  ;  or  dissolve 
the  filth  which  has  been  concentrating  in  the  pores  of  the  soil  during  the  dry 
season,  and  carry  it  into  the  well.  Small  indications  therefore  of  a  polluted 
origin  are  very  serious  in  surface  waters  and  shallow  well  waters,  but  are  of  less 
moment  in  water  from  deep  wells  and  springs  ;  the  present  character'  of  these 
being  of  chief  importance,  since  whatever  degree  of  purification  may  be  observed, 
may  usually  be  treated  as  permanent.  The  term  "  total  inorganic  nitrogen  " 
has  been  chosen  chiefly  because  it  is  based  on  actual  results  of  analysis  without 
the  introduction  of  any  theory  whatever.  It  will  be  seen  that  it  corresponds 
very  nearly  with  the  term  "  previous  sewage  or  animal  contamination,"  which 
was  introduced  by  Dr.  Frankland,  and  which  was  employed  in  the  second 
edition  of  this  work.  Perhaps  few  terms  have  been  more  woefully  mis- 


502  WATER   AND    SEWAGE. 

understood  and  misrepresented  than  that  phrase,  and  it  is  hoped  that  the  new 
term  will  be  less  liable  to  misconception.  It  will  be  remembered  the  "  previous 
sewage  contamination  "  of  a  water  was  calculated  by  multiplying  the  sum  of 
the  quantities  of  nitrogen  present  as  ammonia,  nitrates,  and  nitrites,  by  10,000 
and  deducting  320  from  the  product,  the  number  thus  obtained  representing 
the  previous  animal  contamination  of  the  water  in  terms  of  average  filtered 
London  sewage.  In  was  purely  conventional,  for  the  proportion  of  organic 
nitrogen  present  is  such  sewage  was  assumed  to  be  10  parts  per  100,000, 
whereas  in  the  year  1857  it  was  actually  8*4  parts,  and  in  1869  only  7  parts. 
The  deduction  of  320  was  made  to  correct  for  the  average  amount  of  inorganic 
nitrogen  in  rain  water,  and  this  is  omitted  in  calculating  "total  inorganic 
nitrogen  "  for  the  following  reasons : — The  quantity  is  small,  and  the  variations 
in  composition  of  rain  water  at  different  times  and  under  different  circumstances 
very  considerable,  and  it  appears  to  obscure  the  significance  of  the  results  of 
analysis  of  very  pure  waters  to  deduct  from  all  the  same  fixed  amount.  As, 
too,  the  average  amount  of  total  inorganic  nitrogen  in  unpolluted  surface 
waters  is  only  O'Oll  part  (XXVIII.),  it  cannot  be  desirable  to  apply  a  correction 
amounting  to  nearly  three  times  that  average,  and  so  place  a  water  which  contains 
0-032  part  of  total  inorganic  nitrogen  on  the  same  level  as  one  which  contains 
no  trace  of  any  previous  pollution. 

Chlorine. 

This  is  usually  present  as  sodium  chloride,  but  occasionally,  as  has  been 
mentioned  before,  it  is  most  likely  as  a  calcium  salt.  It  is  derived,  in  some 
cases,  from  the  soil,  but  more  usually  from  animal  excreta  (human  urine 
contains  about  500  parts  per  100,000),  and  is  therefore  of  considerable  importance 
in  forming  a  judgment  as  to  the  character  of  a  water.  Unpolluted  river  and 
spring  waters  usually  contain  less  than  one  part ;  average  town  sewage  about 
eleven  parts.  Shallow  well  water  may  contain  any  quantity  from  a  mere  trace 
up  to  fifty  parts  or  even  more.  Its  amount  is  scarcely  affected  by  any  degree 
of  filtration  through  soil :  thus  the  effluent  water  from  land  irrigated  with  sewage 
contains  the  same  proportion  of  chlorine  as  the  sewage,  unless  it  has  been  diluted 
by  subsoil  water  or  concentrated  by  evaporation.  Of  course,  attention  should 
be  given  to  the  geological  nature  of  the  district  from  which  the  water  comes,  the 
distance  from  the  sea  or  other  source  of  chlorine,  etc.,  in  order  to  decide  on  the 
origin  of  the  chlorine.  Under  ordinary  circumstance,  a  water  containing  more 
than  three  or  four  parts  of  chlorine  should  be  regarded  with  suspicion. 

Hardness. 

This  is  chiefly  of  importance  as  regards  the  use  of  the  water  for  cleansing 
and  manufacturing  purposes,  and  for  steam  boilers.  It  is  still  a  moot  point 
as  to  whether  hard  or  soft  water  is  better  as  an  article  of  food.  The 
temporary  hardness  is  often  said  to  be  that  due  to  carbonates  held  in  solution 
by  carbonic  acid,  but  this  is  not  quite  correct ;  for  even  after  prolonged 
boiling,  water  will  still  retain  about  three  parts  of  carbonate  in  solution,  and 
therefore  when  the  total  hardness  exceeds  three  parts,  that  amount  should 
be  deducted  from  the  permanent  hardness  and  added  to  the  temporary,  in  order 
to  get  the  quantity  of  carbonate  in  solution.  But  the  term  "  temporary  " 
hardness  properly  applies  to  the  amount  of  hardness  which  may  be  removed 
by  boiling,  and  hence,  if  the  total  hardness  be  less  than  three  parts,  there  is 
usually  no  temporary.  As  the  hardness  depends  chiefly  on  the  nature  of  the 
soil  through  and  over  which  the  water  passes,  the  variations  in  it  are  very 
great ;  that  from  igneous  strata  has  least  hardness,  followed  in  approximate 
order  by  that  from  Metamorphic,  Cambrian,  Silurian  and  Devonian  rocks, 
Millstone  Grit,  London  Clay,  Bagshot  Beds,  New  Red  Sandstone,  Coal  Measures, 
Mountain  Limestone,  Oolite,  Chalk,  Lias,  and  Dolomite,  the  average  in  the  case 
of  the  first  being  2*4  parts,  and  of  the  last  41  parts.  As  animal  excreta  contain 
a  considerable  quantity  of  lime,  highly  polluted  waters  are  usually  extremely  hard. 
Water  from  shallow  wells  contains  varying  proportions  up  to  nearly  200  parts 


INTERPRETATION   OF  RESULTS.  503 

of  total  hardness  (XIII.).     No  generalization  can  be  made  as  to  the  proportion 
of  permanent  to  temporary  hardness. 

Suspended  Matter. 

This  is  of  a  less  degree  of  importance  than  the  matters  hitherto  considered. 
From  a  sanitary  point  of  view  it  is  of  minor  interest,  because  it  may  be  in  most 
cases  readily  and  completely  removed  by  nitration.  Mineral  suspended  matter 
is,  however*  of  considerable  mechanical  importance  as  regards  the  formation  of 
impediments  in  the  river  bed  by  its  gradual  deposition,  and  as  regards  the  choking 
of  the  sand  filters  in  water- works ;  and  organic  suspended  matter  is  at  times 
positively  injurious,  and  always  favours  the  growth  of  minute  organisms. 

From  the  determinations  which  have  been  described,  it  is  believed  that  a  sound 
judgment  as  to  the  character  of  a  water  may  be  made,  and  the  analyst  should 
hardly  be  content  with  a  less  complete  examination.  If,  however,  from  lack 
of  time  or  other  cause,  so  much  cannot  be  done,  a  tolerably  safe  opinion  may 
be  formed,  omitting  the  determination  of  total  solid  matter,  and  organic  carbon 
and  nitrogen.  But  it  must  not  be  forgotten  that  by  so  doing  the  inquiry  is 
limited,  as  regards  organic  impurity,  to  the  determination  of  that  which  was 
formerly  present,  but  has  already  been  converted  into  inorganic  substances.  If 
still  less  must  suffice,  the  determination  of  nitrogen  as  nitrates  and  nitrites  may  be 
omitted,  its  place  being  to  a  certain  extent  supplied  by  that  of  chlorine,  but 
especial  care  must  then  be  taken  to  ascertain  the  source  of  the  latter  by  examination 
of  the  district.  If  it  be  in  any  degree  of  mineral  origin,  no  opinion  can  be  formed 
from  it  as  to  the  likelihood  of  organic  pollution. 

General  Considerations. 

In  judging  of  the  character  of  a  sample  of  water,  due  attention  must  of 
course  be  paid  to  the  purpose  for  which  it  is  proposed  to  be  used.  The  analyst 
frequently  has  only  to  decide  broadly  whether  the  water  is  good  or  bad  ;  as,  for 
example,  in  cases  of  the  domestic  supply  to  isolated  houses  or  of  existing  town 
supplies.  Water  which  would  be  fairly  well  suited  for  the  former  might  be 
very  objectionable  for  the  latter,  where  it  would  be  required  to  a  certain  extent 
for  manufacturing  purposes.  Water  which  would  be  dangerous  for  drinking  or 
cooking  may  be  used  for  certain  kinds  of  cleansing  operations ;  but  it  must  not 
be  forgotten,  that  unless  great  care  and  watchfulness  are  exercised  there  is  con- 
siderable danger  of  this  restriction  being  neglected,  and  especially  if  the  objection- 
able water  is  nearer  at  hand,  than  the  purer  supply.  There  would  for  this  reason, 
probably,  be  some  danger  attending  a  double  supply  on  a  large  scale  in  a  town, 
even  if  the  cost  of  a  double  service  of  mains,  etc.,  were  not  prohibitive. 

It  is  often  required  to  decide  between  several  proposed  sources  of  supply,  and 
here  great  care  is  necessary,  especially  if  the  differences  between  the  samples 
are  not  great.  If  possible,  samples  should  be  examined  at  various  seasons  of 
the  year ;  and  care  should  be  taken  that  the  samples  of  the  several  waters  are 
collected  as  nearly  as  possible  simultaneously  and  in  a  normal  condition.  The 
general  character  of  a  water  is  most  satisfactorily  shown  by  the  average  of  a 
systematic  series  of  analyses ;  and  for  this  reason  the  average  analysis  of 
the  water  supplies  of  London,  taken  from  the  Reports  of  Dr.  Frankland  to  the 
Registrar  General,  of  Glasgow  by  Dr.  Mills,  and  of  Birmingham  by  Dr.  Hill, 
are  included  in  the  table.  River  waters  should,  as  a  rule,  not  be  examined 
immediately  after  a  heavy  rain  when  they  are  in  flood.  A  sudden  rainfall  after 
a  dry  season  will  often  foul  a  river  more  than  a  much  heavier  and  more 
prolonged  downfall  after  average  weather.  Similarly  the  sewage  discharged 
from  a  town  at  the  beginning  of  a  heavy  rainstorm  is  usually  extremely  foul, 
the  solid  matter  which  has  been  accumulating  on  the  sides  of  the  sewers,  and  in 
.  corners  and  recesses,  being  rapidly  washed  out  by  the  increased  stream. 

The  possibility  of  improvement  in  quality  must  also  be  considered.  A  turbid 
water  may  generally  be  rendered  clear  by  filtration,  and  this  will  often  also 
effect  some  slight  reduction  in  the  quantity  of  organic  matter ;  but  while 
somewhat  rapid  filtration  through  sand  or  similar  material  will  usually  remove 


504  WATER   AND    SEWAGE. 

all  solid  suspended  matter,  it  is  generally  necessary  to  pass  the  water  very 
slowly  through  a  more  efficient  material  to  destroy  any  large  proportion  of  the 
organic  matter  in  solution.  Very  fine  sand,  animal  charcoal,  and  spongy  iron 
are  all  in  use  for  this  purpose.  The  quantity  of  available  oxygen  must  not  be 
neglected  in  considering  the  question  of  filtration.  If  the  water  contains  only 
a  small  quantity  of  organic  matter  and  is  well  aerated,  the  quantity  of  oxygen 
in  solution  may  be  sufficient,  and  the  filtration  may  then  be  continuous ;  but 
in  many  instances  this  is  not  the  case,  and  it  is  then  necessary  that  the  filtration 
should  be  intermittent,  the  water  being  allowed  at  intervals  to  drain  off  from 
the  filtering  material  in  order  that  the  latter  may  be  well  aerated,  after  which 
it  is  again  fit  for  work. 

Softening  water  by  Clark's  process  generally  removes  a  large  quantity  of 
organic  matter  (see  Table,  XVI.)  from  solution,  it  being  carried  down  with 
the  calcium  carbonate  precipitate. 

It  is  evident  that  no  very  definite  distinction  can  be  drawn  between  deep 
and  shallow  wells.  In  the  foregoing  pages,  deep  wells  generally  mean  such  as 
are  more  than  100  feet  deep,  but  there  are  many  considerations  which  qualify 
this  definition.  A  deep  well  may  be  considered  essentially  as  one  the  water 
in  which  has  filtered  through  a  considerable  thickness  of  porous  material,  and 
whether  the  shaft  of  such  a  well  is  deep  or  shallow  will  depend  on  circumstances. 
If  the  shaft  passes  through  a  bed  of  clay  or  other  impervious  stratum,  and  the 
surface  water  above  that  is  rigidly  excluded,  the  well  should  be  classed  as  "  deep," 
even  if  the  shaft  is  only  a  few  feet  in  depth,  because  the  water  in  it  must  have 
passed  for  a  considerable  distance  below  the  clay.  On  the  other  hand,  however 
deep  the  shaft  of  a  well,  it  must  be  considered  as  "  shallow  "  if  water  can  enter 
the  shaft  near  the  surface,  or  if  large  cracks  or  fissures  give  free  passage  for 
surface  water  through  the  soil  in  which  the  well  is  sunk.  With  these  principles 
iri  view,  the  water  from  wells  may  often  be  improved.  Every  care  should  be 
taken  to  exclude  surface  water  from  deep  wells ;  that  is  to  say,  all  water  from 
strata  within  about  100  feet  from  the  surface  or  above  the  first  impervious  bed. 
In  very  deep  wells  which  pass  through  several  such  beds,  it  is  desirable  to 
examine  the  water  from  each  group  of  pervious  strata,  as  this  often  varies  in 
quality,  and  if  the  supply  is  sufficient,  exclude  all  but  the  best. 

In  shallow  wells  much  may  occasionally  be  accomplished  in  a  similar  manner 
by  making  the  upper  part  of  the  shaft  water-tight.  It  is  also  desirable  that 
the  surface  for  some  distance  round  the  well  should  be  puddled  with  clay, 
concreted,  or  otherwise  rendered  impervious,  so  as  to  increase  the  thickness  of 
the  soil  through  which  the  water  has  to  pass.  Drains  passing  near  the  well 
should  be,  if  possible,  diverted  ;  and  of  course  cesspools  should  be  either  abolished, 
or,  if  that  is  impracticable,  removed  to  as  great  a  distance  from  the  well  as  is 
possible,  and  in  addition  made  perfectly  water-tight.  Changes  such  as  these  tend 
to  diminish  the  uncertainty  of  the  conditions  attending  a  shallow  well,  but  in 
most  cases  such  a  source  of  supply  should,  if  possible,  be  abandoned  as  dangerous 
at  best. 


GAS   ANALYSIS. 


505 


PART    VII. 
VOLUMETRIC    ANALYSIS    OF-LGASES. 

Description  of  the  necessary  Apparatus,  with  Instructions  for 
Preparing,  Etching,  Graduating,  etc. 

THIS  branch  of  chemical  analysis,  on  account  of  it's  extreme 
accuracy,  and  in  consequence  of  the  possibility  of  its  application  to 
the  analysis  of  carbonates,  and  of  many  other  bodies  from  which 
gases  may  be  obtained,  deserves  more  attention  than  it  has  generally 
received,  in  this  country  at  least.     It  will  therefore  be  advisable  to 
devote  some  considerable  space  to  the  consideration  of  the  subject. 
For  an  historical  sketch  of  the  progress  of  gas  analysis,  the  reader 
is  referred  to  Dr.  Frankland's  article  in  the  Handworterbuch 
der  Chemie,  and  more  complete  details  of  the  process  than  it 
/~N     will  be  necessary  to  give  here  will  be  found  in  that  article  ; 
also  in  Bunsen's  Oasometry  and  in  Dr.  Russell's  contri- 
butions to  Watts's  Dictionary  of  Chemistry. 

The  apparatus  employed  by  Bunsen,  who  was  the  first 
successfully  to  work  out  the  processes  of  gas  analysis,  is  very 
simple.  Two  tubes,  the  absorption  tube  and  the  eudiometer, 
are  used,  in  which  the  measurement  and  analysis  of  the  gases 
are  performed.  The  first  of  these  tubes  is  about  250  mm. 
long  and  20  mm.  in  diameter,  closed  at  one  end,  and  with 
a  lip  at  one  side  of  the  open  extremity,  to  facilitate  the 
transference  of  the  gas  from  the  absorption  tube  (fig.  70)  to 
the  eudiometer  (fig.  71).  The  eudiometer  has  a  length  of 
from  700  to  800  mm.,  and  a  diameter  of  20  mm.  Into  the 
closed  end  two  platinum  wires  are  sealed,  so  as  to  enable  the 
operator  to  pass  an  electric  spark  through  any  gas  which  the 
tube  may  contain.  The  mode  of  sealing  in  the  platinum 
wires  is  as  follows  : — When  the  end  of  the  tube  is  closed,  and 
while  still  hot,  a  finely  pointed  blowpipe  flame  is  directed 
against  the  side  of  the  tube  at  the  base  of  the  hemispherical 
Fig-  70.  end.  When  the  glass  is  soft,  a  piece  of  white-hot  platinum 
wire  is  pressed  against  it  and  rapidly  drawn  away.  By  this 
means  a  small  conical  tube  is  produced.  This  operation  is  then 
repeated  on  the  opposite  side  (fig.  72).  One  of  the  conical  tubes  is 
next  cut  off  near  to  the  eudiometer,  so  as  to  leave  a  small  orifice 
(fig.  73),  through  which  a  piece  of  the  moderately  thin  platinum 
wire,  reaching  about  two-thirds  across  the  tube,  is  passed.  The  fine 
blowpipe  flame  is  now  brought  to  play  on  the  wire  at  the  point  where 
it  enters  the  tube  ;  the  glass  rapidly  fuses  round  the  wire,  making 
a  perfectly  gas-tight  joint.  If  it  should  be  observed  that  the  tube 
has  any  tendency  to  collapse  during  the  heating,  it  will  be  necessary 


506  GAS   ANALYSIS. 

to  blow  gently  into  the  open  end  of  the  tube.  This  may  be  con- 
veniently done  by  means  of  a  long  piece  of  caoutchouc  connector, 
attached  to  the  eudiometer,  which  enables  the  operator  to  watch  the 
effect  of  the  blowing  more  easily  than  if  the  mouth  were  applied 
directly  to  the  tube.  When  a  perfect  fusion  of  the  glass  round  the 
wire  has  been  effected,  the  point  on  the  opposite  side  is  cut  off, 
and  a  second  wire  sealed  in  in  the  same  manner  (fig.  74).  The 
end  of  the  tube  must  be  allowed  to  cool  very  slowly  ;  if 
proper  attention  is  not  paid  to  this,  fracture  is  very  liable  to 
ensue.  When  perfectly  cold,  a  piece  of  wood  with  a  rounded 
end  is  passed  up  the  eudiometer,  and  the  two  wires  carefully 
pressed  against  the  end  of  the  tube,  so  as  to  He  in  contact  ^  ith 
the  glass,  with  a  space  of  1  or  2  mm.  between  their  points  • 
(fig.  75).  It  is  for  this  purpose  that  the  wires,  when  sealed  in, 
are  made  to  reach  so  far  across  the  tube.  The  ends  of  the 
wires  projecting  outside  the  tube  are  then  bent  into  loops. 
These  loops  must  be  carefully  treated,  for  if  frequently  bent 
they  are  very  apt  to  break  off  close  to  the  glass  ;  besides  this, 
the  bending  of  the  wire  sometimes  causes  a  minute  crack  in 
the  glass,  which  may  spread  and  endanger  the  safety  of  the 
tube.  These  difficulties  may  be  overcome  by  cutting  off  the 
wire  close  to  the  glass;  and  carefully  smoothing  the  ends  by 
rubbing  them  with  a  piece  of  ground  glass  until  they  are 
level  with  the  surface  of  the  tube  (fig.  76).  In  order  to  make 
contact  with  the  induction  coil,  a  wooden  American  paper- 
clip, lined  with  platinum  foil,  is  made  to  grasp  the  tube  ;  the 
foil  is  connected  with  two  strong  loops  of  platinum  wires, 
and  to  these  the  wires  from  the  coil  are  attached  (fig.  77). 
In.  this  way  no  strain  is  put  on  the  eudiometer  wires  by  the 
weight  of  the  wires  from  the  coil,  and  perfect  contact  is 
ensured  between  the  foil  and  platinum  wires.  It  is  also  easy 
to  clean  the  outside  of  the  eudiometer  without  fear  of  injuring 
the  instrument. 

It  will  now  be  necessary  to  examine  if  the  glass  is  perfectly 
fused  to  the  wires.  For  this  purpose  the  eudiometer  is  filled 
with  mercury,  and  inverted  in  the  trough.  If  the  tube  has 
800  mm.  divisions,  a  vacuous  space  will  be  formed  in  the 
upper  end.  Note  the  height  of  the  mercury,  and  if  this 
remains  constant  for  a  while  the  wires  are  properly  sealed. 
Should  the  eudiometer  be  short,  hold  it  in  the  hands,  and 
bring  it  down  with  a  quick  movement  upon  the  edge  of  the 
india-rubber  cushion  at  the  bottom  of  the  trough,  taking  care 
that  the  force  of  impact  is  slight,  else  the  mercury  may 
fracture  the  sealed  end  of  the  tube.  By  jerking  the  eudi- 
ometer thus,  a  momentary  vacuum  is  formed,  and  if  there  is  F.  ^ 
any  leakage,  small  bubbles  of  air  will  arise  from  the  junction 
of  the  wires  with  the  glass. 

The  tubes  are  graduated  by  the  following  processes  : — A  cork  is 
fitted  into  the  end  of  the  tube,  and  a  piece  of  stick,  a  file,  or  any- 


GRADUATION. 


507 


thing  that  will  make  a  convenient  handle,  is  thrust  into  the  cork. 
The  tube  is  heated  over  a  charcoal  fire  or  combustion  furnace,  and 
coated  with  melted  wax  by  means  of  a  camel' s-hair  brush.  Some- 
times a  few  drops  of  turpentine  are  mixed  with  the  wax  to  render 
it  less  brittle,  but  this  is  not  always  necessary.  If  on  cooling  it 
should  be  found  that  the  layer  of  wax  is  not  uniform,  the  tube  may 
be  placed  in  a  perpendicular  position  before  a  fire  and  slowiy 
rotated  so  as  to  heat  it  evenly.  The  wax  will  then  be  evenly 
distributed  on  the  surface  of  the  glass,  the  excess  flowing  off.  The 
Fig.  72.  Fig.  73. 


Fig.  75. 


Fig.  76.  Fig.  77. 

tube  must  not  be  raised  to  too  high  a  temperature,  or  the  wax  may 
become  too  thin  ;  but  all  thick  masses  should  be  avoided,  as  they 
may  prove  troublesome  in  the  subsequent  operation. 

The  best  and  most  accurate  mode  of  marking  the  millimetre 
divisions  on  the  wax  is  by  a  graduating  machine  ;  but  the  more 
usual  process  is  to  copy  the  graduations  from  another  tube  in  the 
following  manner.  A  hard  glass  tube,  on  which  millimetre  divisions 
have  already  been  deeply  etched,  is  fixed  in  a  groove  in  the  graduat- 
ing table,  a  straight-edge  of  brass  being  screwed  down  on  the  tube 


508 


GAS    ANALYSIS. 


and  covering  the  ends  of  the  lines.     The  standard  tube  is  shown  in 

the  figure  at  the  right-hand  end  of  the  apparatus  (fig.  78).     The 

waxed  tube  is  secured  at  the  other  end  of  the  same  groove,  and  above 

it  are  fixed  two  brass  plates,  one  with  a  straight-edge,  and  the  other 

with  notches  at  intervals  of  5  mm. 

the  alternate  notches  being  longer 

than  the  intermediate  ones  (fig. 

79).     A  stout  rod  of  wood  pro- 
vided with   a  sharp   steel  point 

near    one    end,    and    a    penknife 

blade   at   the   other   (fig.    80),   is 

held  so  that  the  steel  point  rests 

in   one    of   the   divisions   of   the 

graduated     tube,     being     gently 

pressed  at  the  same  time  against 

the  edge  of  the  brass  plate  ;  the 

point  of  the  knife-blade  is  then 

moved    by    the    operator's   right 

hand  across   the  portion   of   the 

waxed   tube  which  lies   exposed 
between    the    two    brass    plates. 
When  the  line  has  been  scratched 
on  the  wax,  the  point  is  moved 
along  the  tube  until  it  falls  into 
the  next  division  ;  another  line  is 
now  scratched  on  the  wax,  and     c 
so   on.     At   every   fifth   division 
the    knife-blade,   will    enter    the     £ 
notches  in  the  brass  plate,  making 
a  longer  line  on  the  tube.     After 
a  little  practice  it  will  be  found 
easy  to  do  fifty  or  sixty  divisions 
in   a   minute,    and   with   perfect 
regularity.     Before    the    tube    is 
removed  from  the  apparatus,  it 
must   be   carefully   examined   to 
see  if  any  mistake  has  been  made. 
It  may  have  happened  that  during 
the   graduation    the    steel   point 
slipped  out  of  one  of  the  divisions 
in  the  standard  tube  ;  if  this  has 
taken  place,  it  will  be  found  that 
the    distance    between    the    line 
made  at  that  time  and  those  on 
each  side  of  it  will  not  be  equal, 
or  a  crooked  or  double  line  may 
have    been    produced.     This    is 
easily    obliterated    by    touching 
the  wax  with  a  piece  of  heated 


GRADUATION. 


509 


platinum  wire,  after  which  another  line  is  marked.  The  tube  is 
now  taken  out  of  the  table,  and  once  more  examined.  If  any 
portions  of  wax  have  been  scraped  off  by  the  edges  of  the  apparatus, 
or  by  the  screws,  the  coating  must  be  repaired  with  the  hot  platinum 
wire.  Numbers  have  next  to  be  marked  opposite  each  tenth 
division,  beginning  from  the  closed  end  of  the  tube,  the  first 
division,  which  should  be  about  10  mm.  from  the  end,  being  marked 
10  (see  fig.  75).  The  figures  may  be  well  made  with  a  steel  pen. 


Fig.  81. 

This  has  the  advantage  of  producing  a  double  line  when  the  nib 
is  pressed  against  the  tube  in  making  a  down  stroke.  The  date, 
the  name  of  the  maker  of  the  tube,  or  its  number,  may  now  be 
written  on  the  tube. 

The  etching  by  gaseous  hydrofluoric  acid  is  performed  by  support- 
ing the  tube  by  two  pieces  of  wire  over  a  long  narrow  leaden  trough 
containing  sulphuric  acid  and  powdered  fluor-spar  (fig.  81),  and  the 
whole  covered  with  a  cloth  or  sheet  of  paper.  Of  course  it  is 
necessary  to  leave  the  cork  in  the  end  of  the  tube  to  prevent  the 


Fig.  82. 


Fig.  83. 


access  of  hydrofluoric  acid  to  the  interior,  which  might  cause  the 
tube  to  lose  its  transparency  to  a  considerable  extent.     The  time 


510  GAS   ANALYSIS. 

required  for  the  action  of  the  gas  varies  with  the  kind  of  glass 
employed.  With  ordinary  flint  glass  from  ten  minutes  to  half  an 
hour  is  quite  sufficient ;  if  the  leaden  trough  is  heated,  the  action 
will  take  place  still  more  rapidly.  The  tube  is  removed  from  time 
to  time,  and  a  small  portion  of  the  wax  scraped  off  from  a  part  of 
one  of  the  lines  ;  and  if  the  division  can  be  felt  with  the  finger-nail 
or  the  point  of  a  knife,  the  operation  is  finished  ;  if  not,  the  wax 
must  be  replaced,  and  the  tube  restored  to  the  trough.  When 
sufficiently  etched,  the  tube  is  washed  with  water,  heated  before 
a  fire,  and  the  wax  wiped  off  with  a  warm  cloth. 

The  etching  may  also  be  effected  with  liquid  hydrofluoric  acid, 
by  applying  it  to  the  divisions  on  the  waxed  tube  with  a  brush,  or 
by  placing  the  eudiometer  in  a  gutta-percha  tube  closed  at  one 
end,  and  containing  some  of  the  liquid. 

As  all  glass  tubes  are  liable  to  certain  irregularities  of  diameter, 
it  follows  that  equal  lengths  of  a  graduated  glass  tube  will  not 
contain  exactly  equal  volumes  ;  hence  it  is,  of  course,  impossible 
to  obtain  by  measurement  of  length  the  capacity  of  the  closed  end 
of  the  tube. 

In  order  to  provide  for  this,  the  tube  must  be  carefully  calibrated. 
For  this  purpose  it  is  supported  vertically  (fig.  82),  and  successive 
quantities  of  mercury  poured  in  from  a  measure.  This  measure 
should  contain  about  as  much  mercury  as  ten  or  twenty  divisions 
of  the  eudiometer,  and  is  made  of  a  piece  of  thick  glass  tube,  closed 
at  one  end,  and  with  the  edges  of  the  open  end  ground  perfectly 
flat.  The  tube  is  fixed  into  a  piece  of  wood  in  order  to  avoid 
heating  its  contents  during  the  manipulation.  The  measure  may 
be  filled  with  mercury  from  a  vessel  closed  with  a  stop-cock 
terminating  in  a  narrow  vertical  tube,  which  is  passed  to  the  bottom 
of  the  measure  (fig.  83).  On  carefully  opening  the  stop-cock  the 
mercury  flows  into  the  measure  without  leaving  any  air-bubbles 
adhering  to  the  sides.  A  glass  plate  is  now  pressed  on  the  ground 
edges  of  the  tube,  which  expels  the  excess  of  mercury  and  leaves 
the  measure  entirely  filled.  The  mercury  may  be  introduced  into 
the  measure  in  a  manner  which  is  simpler  and  as  effectual,  though 
perhaps  not  quite  so  convenient,  by  first  closing  it  with  a  glass 
plate,  and  depressing  it  in  the  mercurial  trough,  removing  the 
plate  from  the  tube,  and  again  replacing  it  before  raising  the 
measure  above  the  surface  of  the  mercury.  After  pouring  each 
measured  quantity  of  mercury  into  the  eudiometer,  the  air-bubbles 
are  carefully  detached  from  the  sides  by  means  of  a  thin  wooden 
rod  or  piece  of  whalebone,  and  the  level  of  the  mercury  at  the 
highest  part  of  the  curved  surface  observed. 

In  all  measurements  in  gas  analysis  it  is,  of  course,  essential  that 
the  eye  should  be  exactly  on  a  level  with  the  surface  of  the  mercury, 
for  the  parallax  ensuing  if  this  were  not  the  case  would  produce 
grave  errors  in  the  readings.  The  placing  of  the  eye  in  the  proper 
position  may  be  ensured  in  two  ways.  A  small  piece  of  looking- 
glass  (the  back  of  which  is  painted,  or  covered  with  paper  to  prevent 


CALIBRATION. 


511 


the  accidental  soiling  of  the  mercury  in  the  trough)  is  placed  behind, 
and  in  contact  with  the  eudiometer.  The  head  is  now  placed  in 
such  a  position  that  the  reflection  of  the  pupil  of  the  eye  is  precisely 
on  a  level  with  the  surface  of  the  mercury  in  the  tube  and  the 
measurement  made.  As  this  process  necessitates  the  hand  of  the 
operator  being  placed  near  the  eudiometer,  which  might  cause  the 
warming  of  the  tube,  it  is  preferable  to  read  off  with  a  telescope 
placed  at  a  distance  of  from  two  to  six  feet  from  the  eudiometer. 
The  telescope  is  fixed  on  a  stand  in  a  horizontal  position,  and  the 
support  is  made  to  slide  on  a  vertical  rod.  The  image  of  the  surface 

of  the  mercury  is  brought  to 
the  centre  of  the  field  of  the 
telescope,  indicated  by  the 
cross  wires  in  the  eyepiece, 
and  the  reading  taken. 
The  telescope  has  the 
advantage  of  magnifying 
the  graduations,  and  thus 
facilitating  the  estimation 
by  the  eye  of  tenths  of  the 
divisions.  Fig.  84  repre- 
sents the  appearance  of 
the  tube  and  mercury  as 
seen  by  an  inverting  tele- 
scope. 

By     this      method     the 

—     84  capacity    of    the    tube    at 

different  parts  of  its  length 

is  determined.  If  the  tube  were  of  uniform  bore,  each  measure  of 
mercury  would  occupy  the  same  length  in  the  tube  ;  but  as  this  is 
never  the  case,  the  value  of  the  divisions  at  all  parts  of  the  tube  will 
not  be  found  to  be  the  same. 

From  the  data  obtained  by  measuring  the  space  in  the  tube  which 
is  occupied  by  equal  volumes  of  mercury,  a  table  is  constructed  by 
which  the  comparative  values  of  each  millimetre  of  the  tube  can  be 
found.  The  following  results  were  obtained  in  the  calibration  of 
a  short  absorption  eudiometer  : 

On  the  introduction  of  the  3rd  volume  of  mercury,  the  reading  was  12'8  mm. 
4th  18-4 


5th 
6th 
7th 
8th 


24-0 
29-8 
35-2 
41  0 


Thus  the  standard  volumes  occupied  5'6  mm.  between  12'8  and  18*4 

5-6  „        18-4    „    24-0 

5'8  „        24-0     „     29-8 

5'4  „         29-8     „     35-2 

„  „         5'8  ,,         35'2     ,,     41'0 

If  we  assume  the  measure  of  mercury  to  contain  5 '8  volumes  (the 
greatest  difference  between  two  consecutive  readings  on  the  tube), 
the  volume  at  the  six  points  above  given  will  be  as  follows :— 


512 


GAS    ANALYSTS. 


At  12-8  it  will  be  17-4  or  5-8  x3 
18-4  „       23-2  „   5-8x4 

24-0  „       29-0  „   5-8x5 

29-8  „       34-8  „   5-8x6 

35-2  „       40-6  „   5-8x7 

41-0  „       46-4  „   5-8x8 

Between  the  first  and  second  readings  these  5-8  volumes  are 

contained  in  5-6  divisions,  consequently  each  millimetre  corresponds 

to  — -  =  1  -0357  vol.     This  is  also  the  value  of  the  divisions  between 
5'b 

the  second  and  third  readings.     Between  the  third  and  fourth  1  mm. 
contains   1  vol.  ;  between  the  fourth  and  fifth,   1  mm.   contains 

PC  »Q 

—  =  1-0741  vol.  ;  and  between  the  fifth  and  sixth  mm.  =  l  vol. 
5-4 

From  these  data  the  value  of  each  millimetre  on  the  tube  can 
readily  be  calculated.  Thus  13  will  contain  the  value  of  12-8  + the 
value  of  0-2  of  a  division  at  this  part  of  the  tube,  or  17-4  + (1-0357  x 
0-2)  =  17*60714.  There  is,  however,  no  need  to  go  beyond  the 
second  place  of  decimals,  and,  for  all  practical  purposes,  the  first 
place  is  sufficient.  Thus,  by  adding  or  subtracting  the  necessary 
volumes  from  the  experimental  numbers,  we  find  the  values  of  the 
divisions  nearest  to  the  six  points  at  which  the  readings  were  taken 
to  be — 

or 


13  =  17-61 
18  =  22-79 
24  =  29-00 
30  =  35-00 
35  =  40-38 
41=46-40 


17-6 
22-8 
29-0 
35-0 
40-4 
46-4 


In  a  precisely  similar  manner  the  values  of  the  intermediate 
divisions  are  calculated,  and  we  thus  obtain  the  following  table  : — 


1 

Values. 

i 

Values. 

1 

Values. 

i 

§ 

PH 

w 

10 

14-50 

14-5 

21 

25-89 

25-9 

32 

37-15 

37-1 

11 

15-54 

15-5 

22 

26-93 

26-9 

33 

38-22 

38-2 

12 

16-57 

16-6 

23 

27-96 

28-0 

34 

39-30 

39-3 

13 

17-61 

17-6 

24 

29-00 

29-0 

35  40-38 

40-4 

14 

18-65 

18-6 

25 

30-00 

30-0 

36 

41-40 

41-4 

15 

19-68 

19-7 

26 

31-00 

31-0 

37 

42-40 

42-4 

16 

20-71 

20-7 

27 

32-00 

32-0 

38 

43-40 

43-4 

17 

21-75 

21-8 

28 

33-00 

33-0 

39 

44-40 

44-4 

18 

22-79 

22-8 

29 

34-00 

34-0 

40 

45-40 

45-4 

19 

23-82 

23-8 

30 

35-00 

35-0 

41 

46-40 

46-4 

20 

24-86 

24-9 

31 

36-07 

36-1 

&c. 

&c. 

&c. 

ERROR    OF    MENISCUS. 


513 


If  it  be  desired  to  obtain  the  capacity  of  the  tube  in  cubic  centi- 
metres it  is  only  necessary  to  determine  the  weight  of  the  quantity 
of  mercury  the  measure  delivers,  and  the  temperature  at  which  the 
calibration  was  made,  and  to  calculate  the  contents  by  the  following 
formula  : — 

gx  (1+0-0001815*) 

13-596V 

in  which  g  represents  the  weight  of  the  mercury  contained  in  the 
measure,  t  the  temperature  at  which  the  calibration  is  made, 
0-0001815  being  the  coefficient  of  expansion  of  mercury  for  each 
degree  centigrade,  V  the  volume  read  off  in  the  eudiometer,  and  C 
the  number  of  cubic  centimetres  required.  (13-596  =  sp.  gr.  of 
mercury  at  0°  C.) 

A  correction  has  to  be  made  to  every  number  in  the  table  on 
account  of  the  surface  of  the  mercury  assuming  a  convex  form  in 
the  tube.  During  the  calibration,  the  convexity  of  the  mercury  is 
turned  towards  the  open  end  of  the  tube  (fig.  85),  whilst  in  the 
measurement  of  a  gas  the  convexity  will  be  in  the  opposite  direction 
(fig.  86).  It  is  obvious  that  the  quantity  of  mercury  measured 
during  the  calibration,  while  the  eudiometer  is  inverted,  will  be 
less  than  a  volume  of  gas  contained  in  the  tube  when  the  mercury 
stands  at  the  same  division,  while  the  eudiometer  is  erect.  The 
necessary  amount  of  correction  is  determined  by  observing  the 
position  of  the  top  of  the  meniscus,  and  then  introducing  a  few 
drops  of  a  solution  of  corrosive  sublimate,  which  will  immediately 
cause  the  surface  of  the  mercury  to  become  horizontal  (fig.  87), 
and  again  measuring. 

It  will  be  observed  that  in  fig.  85  the  top  of  the  meniscus  was 
at  the  division  39,  whereas  in  fig.  87,  after  the  addition  of  corrosive 
sublimate,  the  horizontal  surface  of  the  mercury  stands  at  38'7, 
giving  a  depression  of  0-3  mm.  If  the  tube  were  now  placed 
vertical,  and  gas  introduced  so  that  the  top  of  the  meniscus  was  at 


*Fig.  85.  *Fig.  86.  Fig.  87. 

*  In  these  the  mercury  should  just  touch  39. 


2   L 


514 


GAS    ANALYSIS. 


39,  and  if  it  were  now  possible  to  overcome  the  capillarity,  the 
horizontal  surface  would' stand  at  39'3.  The  small  cylinder  of  gas 
between  38'7  and  39'3,  or  0'6  division,  would  thus  escape  measure- 
ment. This  number  O6  is  therefore  called  the  error  of  meniscus, 
and  must  be  added  to  all  readings  of  gas  in  the  eudiometer.  The 
difference,  therefore,  between  the  two  readings  is  multiplied  by 
two,  and  the  volume  represented  by  the  product  obtained — the 
error  of  meniscus — is  added  to  the  measurements  before  finding 
the  corresponding  capacities  by  the  table.  In  the  case  of  the  tube, 
of  which  the  calibration  is  given  above,  the  difference  between  the 
two  readings  was  0'4  mm.,  making  the  error  of  meniscus  0'8. 

All  experiments  in  gas  analysis,  with 
the  apparatus  described,  should  be 
conducted  in  a  room  set  apart  for 
the  purpose,  with  the  window  facing 
the  north,  so  that  the  sun's  rays  cannot 
penetrate  into  it,  and  carefully  pro- 
tected from  flues  or  any  source  of  heat 
which  might  cause  a  change  of  tempera- 
ture of  the  atmosphere.  The  mercury 
employed  should  be  purified,  as  far  as 
possible,  from  lead  and  tin,  which  may 
be  done  by  leaving  it  in  contact  with 
dilute  nitric  acid  in  a  shallow  vessel 
for  some  time,  or  by  keeping  it  when 
out  of  use  under  concentrated  sul- 
phuric acid,  to  which  some  mercurous 
sulphate^has  been  added.  This  mercury 
reservoir  may  conveniently  be  made  of 
a  glass  globe  with  a  neck  at  the  top  and 
a  stop-cock  at  the  bottom  (fig.  88), 
which  is  not  filled  more  than  one-half, 
so  as  to  maintain  as  large  a  surface  as 
possible  in  contact  with  the  sulphuric 
acid.  Any  foreign  metals  (with  the 
exception  of  silver,  gold,  and  platinum) 
which  may  be  present  are  removed  by 
the  mercurous  sulphate,  an  equivalent 
quantity  of  mercury  being  precipitated. 
This  process,  which  was  originated  by 
M.  Deville,  has  been  in  use  for  many 
years  with  very  satisfactory  results,  the 
mercury  being  always  clean  and  dry 
when  drawn  from  the  stop-cock  at  the 
bottom  of  the  globe.  The  mouth  of 
the  globe  should  be  kept  closed  to 
prevent  the  absorption  of  water  by  the 
sulphuric  acid. 
In  all  cases  where  practicable,  gases  should  be  measured  when 


Fig.  88. 


INTRODUCTION    OF   THE    GAS. 


515 


completely  saturated  with  aqueous  vapour  :  to  ensure  this,  the 
top  of  the  eudiometer  and  absorption  tubes  should  be  moistened 
before  the  introduction  of  the  mercury.  This  may  be  done  by 
dipping  the  end  of  a  piece  of  iron  wire  into  water,  and  touching 
the  interior  of  the  clo>sed  extremity  of  the  tube  with  the  point  of 
the  wire. 

In  filling  the  eudiometer,  the  greatest  care  must  of  course  be 
taken  to  exclude  all  air-bubbles  from  the  tubes.  This  may  be 
effected  in  several  ways  :  the  eudiometer  may  be  held  in  an  inverted 
or  inclined  position,  and  the  mercury  introduced  through  a  narrow 
glass  tube  which  passes  to  the  end  of  the  eudiometer  and 
communicates,  with  the  intervention  of  a  stop-cock,  with  a  reservoir 
of  mercury  (fig.  89).  On  carefully  opening  the  stop-cock,  the 
mercury  slowly  flows  into  the  eudiometer,  entirely  displacing  the 
air.  The  same  result  may  be  obtained  by  placing  the  eudiometer 
nearly  in  a  horizontal  position,  and  carefully  introducing  the 
mercury  from  a  test  tube  without  a  rim  (fig.  90).  Any  minute 
bubbles  adhering  to  the  side  may  generally  be  removed  by  closing 
the  mouth  of  the  tube  with  the  thumb,  and  allowing  a  small  air 
bubble  to  rise  in  the  tube,  and  thus  to  wash  it  out.  After  filling  the 
eudiometer  entirely  with  mercury,  and  inverting  it  over  a  trough, 
it  will  generally  be  found  that  the  air-bubbles  have  been  removed. 

For  the  introduction  of  the  gases,  the  eudiometer  should  be  placed 
in  a  slightly  inclined  position,  being  held  by  a  support  attached  to 


Fig.  89. 

the  mercurial  trough  (fig.  91),  and  the  gas  transferred  from  the  tube 
in  which  it  has  been  collected.  The  eudiometer  is  now  put  in  an 
absolutely  vertical  position,  determined  by  a  plumb-line  placed 
near  it,  and  a  thermometer  suspended  in  close  proximity.  It  must 
then  be  left  for  at  least  half  an  hour,  no  one  being  allowed  to  enter 
the  room  in  the  meantime.  After  the  expiration  of  this  period,  the 
operator  enters  the  room,  and,  by  means  of  the  telescope  placed 
several  feet  from  the  mercury  table,  carefully  observes  the  height  of 
the  mercury  in  the  tube,  estimating  the  tenths  of  a  division  with 
the  eye,  which  can  readily  be  done  after  a  little  practice.  He  next 
reads  the  thermometer  with  the  telescope,  and  finally  the  height 
of  the  mercury  in  the  trough  is  read  off  on  the  tube,  for  which  purpose 

2  L  2 


516  GAS   ANALYSIS. 

the  trough  must  have  glass  sides.  The  difference  between  these  two 
numbers  is  the  length  of  the  column  of  mercury  in  the  eudiometer, 
and  has  to  be  subtracted  from  the  reading  of  the  barometer.  It 
only  •  remains  to  take  the  height  of  the  barometer.  The  most 
convenient  form  of  instrument  for  gas  analysis  is  the  siphon 
barometer,  with  the  divisions  etched  on  the  tube.  This  is  placed 
on  the  mercury  table,  so  that  it  may  be  read  by  the  telescope 
immediately  after  the  measurements  in  the  eudiometer.  There  are 
two  methods  of  numbering  the  divisions  on  the  barometer  :  in  one 
the  zero  point  is  at  or  near  the  bend  of  the  tube,  in  which  case  the 
height  of  the  lower  column  must  be  subtracted  from  that  of  the 
higher  ;  in  the  other  the  zero  is  placed  near  the  middle  of  the  tube, 
so  that  the  numbers  have  to  be  added  to  obtain  the  actual  height. 
In  cases^of^extreme  accuracy,  a  correction  must  be  made^for  the 


Fig.  90. 

temperature  of  the  barometer,  which  is  determined  by  a  thermometer 
suspended  in  the  open  limb  of  the  instrument,  and  passing  through 
a  plug  of  cotton  wool.  Just  before  observing  the  height  of  the 
barometer,  the  bulb  of  the  thermometer  is  depressed  for  a  moment 
into  the  mercury  in  the  open  limb,  thus  causing  a  movement  of  the 
mercurial  column,  which  overcomes  any  tendency  that  it  may 
have  to  adhere  to  the  glass. 

In  every  case  the  volume  observed  must  be  reduced  to  the  normal 
temperature  and  pressure,  in  order  to  render  the  results  comparable. 
If  the  absolute  volume  is  required,  the  normal  pressure  of  760  mm. 
must  be  employed  ;  but  when  comparative  volumes  only  are  desired, 
the  pressure  of  1000  mm.  is  generally  adopted,  as  it  somewhat 
simplifies  the  calculation.  In  the  following  formula  for  correction 
of  the  volume  of  gases — 

Vt=the  corrected  volume. 

V=the  volume  found  in  the  table,  and  corresponding  to  the 
observed  height  of  the  mercury  in  the  eudiometer,  the  error  of 
meniscus  being  of  course  included. 

B=the  height  of  the  barometer  (corrected  for  temperature,  if 
necessary)  at  the  time  of  measurement. 

6= the  difference  between  the  height  of  the  mercury  in  the  trough 
and  in  the  eudiometer. 

£=the  temperature  in  centigrade  degrees. 

'T=the  tension  of  aqueous  vapour  in  millimetres  of  mercury  at 
t°.  This  number  is,  of  course,  only  employed  when  the  gas  is 
saturated  with  moisture  at  the  time  of  measurement. 


CORRECTION    OF   THE    VOLUME. 


517 


Then 


Vx(B-6-T) 


760  x  (1+0-003665*) 
when  the  pressure  of  760  mm.  is  taken  as  the  normal  one ;  or, 

Vx(B-6-T) 
1     1000  x  (1+0-003665*) 
when  the  normal  pressure  of  1  metre  is  adopted. 

In  cases  where  the  temperature  at  measurement  is  below  0° 
(which  rarely  happens),  the  factor  1  —0*003665*  must  be  used. 

The  following  table  may  be  of  value  in  gas  analysis  : — 
Density  and  volume  of  Mercury  and  of  Water. 


t°c. 

Mercury. 

Water. 

Weight  of  1  c.c. 

Volume  of  1  gm. 

Weight  of  1  c.c. 

Volume  of  1  gm. 

0 

13-596 

•073551 

•999884 

1-000116 

4 

13-586 

•073605 

1-000013 

•999987 

5 

13-584 

•073617 

1-000003 

•999997 

10 

13-572 

•073681             -999760 

1-000240 

15 

13-559 

•073752             -999173 

1-000828 

20 

13-547 

•073817 

•998272 

1-001731 

25 

13-535 

•073885 

•997133 

1-002875 

30 

13-523 

•073953 

•995778 

1-004240 

Tables  have  been  constructed  containing  the  values  of  T  ;   of 
1000  x  (1+0-003665*),    and    of    760  x  (1+0-003665*),    which    very 


Fig.  91. 


518  GAS   ANALYSIS, 

much  facilitate  the  numerous  calculations  required  in  this  branch  of 
analysis.*     These  will  be  found  at  the  end  of  the  book. 

We  shall  now  be  in  a  position  to  examine  the  methods  employed 
in  gas  analysis.  Some  gases  may  be  determined  directly  ;  that  is, 
they  may  be  absorbed  by  certain  reagents,  the  diminution  of  the 
volume  indicating  the  quantity  of  the  gas  present.  Some  are 
determined  indirectly  ;  that  is,  by  exploding  them  with  other  gases, 
and  measuring  the  quantities  of  the  products.  Some  gases  may  be 
determined  either  directly  or  indirectly,  according  to  the  circum- 
stances under  wrhich  they  are  found. 

1.     GASES    DETERMINED    DIRECTLY. 

A.  Gases  Absorbed  by  Crystallized  Sodium  Phosphate 

and  by  Potassium  Hydrate  :— 

Hydrochloric  acid,  HC1, 
Hydrobromic  acid,  HBr, 
Hydriodic  acid,  HI. 

B.  Gases  Absorbed  by  Potassium  Hydrate,  but  not  by 

Crystallized  Sodium  Phosphate  : — 

Carbonic  anhydride,  CO2, 
Sulphurous  anhydride,  SO2, 
Hydrosulphuric  acid,  H2S. 

C.     Gases  Absorbed  by  neither  Crystallized  Sodium 
Phosphate  nor  Potassium  Hydrate  : — 

Oxygen,  O2, 
Nitric  oxide,  NO, 
Carbonic  oxide,  CO, 

Hydrocarbons  of  the  composition  CnH2ll, 
Hydrocarbons  of  the  formula  (CnH2,1+1)2, 
Hydrocarbons  of  the  formula  CnH2n+2, 
except  Marsh  gas. 

2.     GASES    DETERMINED    INDIRECTLY. 

Hydrogen,  H2, 
Carbonic  oxide,  CO, 
Marsh  gas,  or  Methane,  CH4, 
Ethane,  C2H6, 
Ethyl  hydride,  C2H6, 
Butane,  C4H10, 
Propyl  hydride,  C3H8, 
Butyl  hydride,  C4H10, 
Nitrogen,  N2. 

*  Mr.  Sutton  will  forward  a  copy  of  these  Tables,  printed  separately  for  laboratory 
use,  to  any  one  desiring  them,  on  receipt  of  the  necessary  address. 


GASES    DETERMINED    DIRECTLY. 


519 


DIRECT    DETERMINATIONS. 
Group  A,  consisting  of  Hydrochloric,  Hydrobromic,  and 

Hydriodic  Acids. 

In  Bunsen's  method  the  reagents  for  absorption  are  generally 
used  in  the  solid  form,  in  the  shape  of  bullets.  To  make  the  bullets 
of  sodium  phosphate,  the  end  of  a  piece  of  platinum  wire,  of  about 
one  foot  in  length,  is  coiled  up  and  fixed  in  the  centre  of  a  pistol- 
bullet  mould.  It  is  well  to  bend  the  handles  of  the  mould,  so  that 
when  it  is  closed  the  handles  are  in  contact,  and  may  be  fastened 
together  by  a  piece  of  copper  wire  (fig.  92).  The  usual  practice 


Fig.  92.  Fig.  93. 

is  to  place  the  platinum  wire  in  the  hole  through  which  the  mould 
is  filled ;  but  it  is  more  convenient  to  file  a  small  notch  in  one  of 
the  faces  of  the  open  mould,  and  place  the  wire  in  the  notch  before 
the  mould  is  closed.  In  this  manner  the  wire  is  not  in  the  way 
during  the  casting,  and  it  is  subsequently  more  easy  to  trim  the 
bullet.  Some  ordinary  crystallized  sodium  phosphate  is  fused  in 
a  platinum  crucible  (or  better,  in  a  small  piece  of  wide  glass  tube, 
closed  at  one  end  and  with  a  spout  at  the  other,  and  held  by  a 
copper- wire  handle),  and  poured  into  the  bullet  mould  (fig.  93). 
When  quite  cold,  the  mould  is  first  gently  warmed  in  a  gas-flame, 
opened,  and  the  bullet  removed.  If  the  warming  of  the  mould  is 
omitted,  the  bullet  is  frequently  broken  in  consequence  of  its 
adhering  to  the  metal.  Some  chemists  recommend  the  use  of 
sodium  sulphate  instead  of  phosphate,  which  may  be  made  into 
balls  by  dipping  the  coiled  end  of  a  piece  of  platinum  wire  into  the 
salt  fused  in  its  water  of  crystallization.  On  removing  the  wire, 
a  small  quantity  of  the  salt  will  remain  attached  to  the  wire.  When 
this  has  solidified,  it  is  again  introduced  for  a  moment,  and  a  larger 
quantity  will  collect ;  and  this  is  repeated  until  the  ball  is 
sufficiently  large.  The  balls  must  be  quite  smooth,  in  order  to 
prevent  the  introduction  of  any  air  into  the  eudiometer.  When 
the  bullets  are  made  in  a  mould,  it  is  necessary  to  remove  the  short 
cylinder  which  is  produced  by  the  orifice  through  which  the  fused 
salt  has  been  poured. 

In  the  determination  of  these  gases,  it  is  necessary  that  they  should 
be  perfectly  dry.     This  may  be  attained  by  introducing  a  bullet  of 


520 


GAS    ANALYSIS. 


fused  calcium  chloride.  After  the  lapse  of  about  an  hour,  the  bullet 
may  be  removed,  the  absorption  tube  placed  in  a  vertical  position, 
with  thermometer,  etc.,  arranged  for  the  reading,  and  left  for  half 
an  hour  to  acquire  the  temperature  of  the  air.  When  the  reading 
has  been  taken,  one  of  the  bullets  of  sodium  phosphate  or  sodium 
sulphate  is  depressed  in  the  trough,  wiped  with  the  fingers  while 

under  the  mercury  in  order  to 
remove  any  air  that  it  might  have 
carried  down  with  it,  and  introduced 
into  the  absorption  tube,  which  for 
this  purpose  is  inclined  and  held  in 
one  hand,  while  the  bullet  is  passed 
into  the  tube  with  the  other.  Care 
must  be  taken  that  the  whole  of  the 
platinum  wire  is  covered  with 
mercury  while  the  bullet  remains  in 
the  gas,  otherwise  there  is  a  risk  of 
air  entering  the  tube  between  the 
mercury  and  the  wire  (fig.  94). 

After  standing  for  an  hour,  the 
bullet  is  withdrawn  from  the 
absorption  tube.  This  must  be 
done  with  some  precaution,  so  as  to 
prevent  any  gas  being  removed  from 
Fig.  94.  the  tube.  It  is  best  done  by  drawing 

down  the  bullet  by  a  brisk  move- 
ment of  the  wire,  the  gas  being  detached  from  the  bullet  during  the 
rapid  descent  of  the  latter  into  the  mercury.  The  bullet  may  then 
be  more  slowly  removed  from  the  tube.  As  sodium  phosphate  and 
sodium  sulphate  contain  water  of  crystallization,  and  a  correspond- 
ing proportion  of  this  is  liberated  for  every  equivalent  of  sodium 
chloride  formed,  care  must  be  taken  that  the  bullets  are  not  too 
small,  else  the  water  set  free  will  soil  the  sides  of  the  eudiometer, 
especially  if  there  is  a  large  volume  of  gas  to  be  absorbed.  As 
a  further  precaution,  drive  off  some  of  the  water  of  crystallization 
before  casting  the  bullet.  When  the  bullet  has  been  removed,  the 
gas  must  be  dried  as  before  with  calcium  chloride  and  again 
measured.  If  two  or  more  of  the  gases  are  present  in  the  mixture 
to  be  analyzed,  the  sodium  phosphate  ball  must  be  dissolved  in 
water,  and  the  chlorine,  bromine,  and  iodine  determined  by  the 
ordinary  analytical  methods.  If  this  has  to  be  done,  care  must  be 
taken  that  the  sodium  phosphate  employed  is  free  from  chlorine. 

Group  B.    Gases  absorbed  by  Potassium  Hydrate,  but  not  by 
Sodium  Phosphate. 

Carbonic  anhydride,   sulphuretted  hydrogen,  and 
sulphurous   anhydride. 

IF  the  gases  occur  singly,  they  are  determined  by  means  of  a  bullet 


EXAMPLE    OF   AN   ANALYSIS.  521 

of  caustic  potash  made  in  the  same  manner  as  the  sodium  phosphate 
balls.  The  caustic  potash  employed  should  contain  sufficient  water 
to  render  the  bullets  so  soft  that  they  may  be  marked  with  the 
nail  when  cold.  Before  use  the  balls  must  be  slightly  moistened 
with  water  ;  and  if  large  quantities  of  gas  have  to  be  absorbed,  the 
bullet  must  be  removed  after  some  hours,  washed  with  water,  and 
returned  to  the  absorption  tube.  The  absorption  may  extend  over 
twelve  or  eighteen  hours.  In  order  to  ascertain  if  it  is  completed, 
the  potash  ball  is  removed,  washed,  again  introduced,  and  allowed 
to  remain  in  contact  with  the  gas  for  about  an  hour.  If  no 
diminution  of  volume  is  observed  the  operation  is  finished. 

The  following  analysis  of  a  mixture  of  air  and  carbonic  anhydride 
will  serve  to  show  the  mode  of  recording  the  observations  and  the 
methods  of  calculation  required. 


Analysis  of  a  Mixture  of  Air  and  Carbonic  Anhydride. 

I.     Gas  Saturated  with  Moisture. 

Height  of  mercury  in  trough           .  171 '8  mm. 
Height  of    mercury  in   absorption    eudi- 
ometer     .....  89-0  mm. 
Column  of  mercury  in  tube,  to  be  sub- 


tracted from  the  height  of  barometer  =  6=      82-8  mm. 

Height  of  mercury  in  eudiometer  .  89 -0  mm. 

Correction  for  error  of  meniscus  =  0'8  mm. 


89-8  mm. 

Volume  in  table  corresponding  to  89-8 

mm =V  =  96-4 

Temperature  at  which  the  reading  was 

made  '".'•',.-*.  .  .  .  .  =£=12-2° 

Height  of  barometer  at  time  of  observa- 
tion   =  B  =  765-25  mm. 

Tension  of  aqueous  vapour  at  1£*2°        =T=     10'6  mm. 

Vx(B-ft-T) 
1     1000  x  (1+0-0036650" 

96-4  x  (765-25 -82-8 -10-6)  _ 
1000  x  [1  +  (0-003665  x  12-2)] 
96-4x671-85 

1000x1-044713" 

log.   96-4  =  1-98408 

log.  671-85  =  2-82727 

4-81135 

log.  (1000  x  1-044713)  =3-01900 

1-79235= log.  61-994=V1 

Corrected  volume  of  air  and  C02=Vj  =  61-994. 


522  GAS   ANALYSIS. 

After    absorption    of    carbonic    anhydride    by   bullet    of 
potassium   hydrate. 

Gas  Dry. 

Height  of  mercury  in  trough  .  172*0  mm. 

Height     of     mercury     in     absorption 

eudiometer        ....  62*5  mm. 

Column  of  mercury  in  eudiometer  =  b  =  109-5  mm. 

Height  of  mercury  in  eudiometer  . 
Correction  for  error  of  meniscus  = 


63-3  mm. 
Volume   in   table   corresponding   to   63*3 

mm.          .....          =V  =  69*35 

Temperature         .          .          .          .          =  t=   10*8° 

Barometer  .....          =B  =  766*0  mm. 

_          Vx(B-6) 

1"~1000  x  (1  +0-003665*) = 
69-35  x  (766 -0-109-5) 


1000  x  [1  +  (0-003665  x  10'8)] 
69-35x656-5 
1000x1-039582 

log.    69-35  =  1-84105 

log.  656-5   =2-81723 

4-65828 

log.  (1000  x  1-039582)  =  3-01686 

l-64142=log. 

Corrected  volume  of  air  =  43 -795 
Air  +  C02  =  61-994 
Air  =43-795 

CO2  =  18-199 

61-994  :  18-199  :  :  100  :  x  =percentage  of  C02 

18-199  x  100     o 
x=  -6F995-  =2 


Percentage  of  C02  in  mixture  of  air  and  gas  =  29-355. 
Gas  Moist. 

Height  of  mercury  in  trough  .  174'0  mm. 

Height  of  mercury  in  eudiometer  .  98*0  mm. 

Column  of  mercury  in  tube  .          .          =6  =     76*0  mm. 

Height  of  mercury  in  eudiometer  .  98*0  mm. 

Correction  for  error  of  meniscus     .    .      =  0*8  mm. 

98*8  mm. 


SO2   AND    H2S.  523 

Volume  in  table,  corresponding  to  98 -8 

mm =  V  =  105-6 

Temperature         .          .          .          .  =t  =   12-5° 

Barometer  ....  =  B  =  738-0  mm. 

Tension  of  aqueous  vapour  at  12*5°  =T=   10-8   mm. 

Corrected  volume  of  air  and  carbonic 

anhydride  ....  65'754 

After  absorption  of  CO2. 
II.     Gas  Dry. 

Height  of  mercury  in  trough          .  173*0  mm. 

Height     of      mercury     in     absorption 

eudiometer        ....  70*3  mm. 

Column  of  mercury  in  tube  .          .          =  b  =  102 '7  mm. 
Height  of  mercury  in  eudiometer  .  70*3  mm. 

Correction  for  error  of  meniscus     .  0*8  mm. 

71*1  mm. 

Volume   in   table   corresponding   to   71*1 

mm =V=  77-4 

Temperature         .          .          .          .          =t  =   14-1° 
Barometer  ....  =B  =  733*5  mm. 

Corrected  volume  of  air  =  46-425 
Air+CO2=65-754 
Air  =46-425 

C0,  =  19-329 


65-754  :  19-329  :  :  100  :  29-396. 

i,  ii. 

Percentage  of  C02  in  mixture  of  air  and  gas  29-355     29-396 

If  either  sulphurous  anhydride  or  sulphuretted  hydrogen  occurs 
together  with  carbonic  anhydride,  one  or  two  modes  of  operation 
may  be  followed.  Sulphuretted  hydrogen  and  sulphurous  anhydride 
are  absorbed  by  manganic  peroxide  and  by  ferric  oxide,  which  may 
be  formed  into  bullets  in  the  following  manner.  The  oxides  are 
made  into  a  paste  with  water,  and  introduced  into  a  bullet  mould, 
the  interior  of  which  has  been  oiled,  and  containing  the  coiled  end 
of  a  piece  of  platinum  wire  ;  the  mould  is  then  placed  on  a  sand 
bath  till  the  ball  is  dry.  The  oxides  will  now  be  left  in  a  porous 
condition,  which  would  be  inadmissible  for  the  purpose  to  which 
they  are  to  be  applied  ;  the  balls  are  therefore  moistened  several 
times  with  a  syrupy  solution  of  phosphoric  acid,  care  being  taken 
that  they  do  not  become  too  soft,  so  as  to  render  it  difficult  to 
introduce  them  into  the  eudiometer.  After  the  sulphuretted 
hydrogen  or  sulphurous  anhydride  has  been  removed,  the  gas 
should  be  dried  by  means  of  calcium  chloride.  The  carbonic 
anhydride  can  now  be  determined  by  means  of  the  bullet  of 
potassium  hydrate. 

The  second  method  is  to  absorb  the  two  gases  by  means  of  a  ball 


524  GAS   ANALYSIS. 

of  potassium  hydrate  containing  water,  but  not  moistened  on  the 
exterior,  then  to  dissolve  the  bullet  in  dilute  acetic  acid  which  has 
been  previously  boiled  and  allowed  to  cool  without  access  of  air, 
and  to  determine  the  amount  of  sulphuretted  hydrogen  or  sulphurous 
anhydride  by  means  of  a  standard  solution  of  iodine.  This  process 
is  especially  applicable  when  rather  small  quantities  of  sulphuretted 
hydrogen  have  to  be  determined. 

Group  C.  This  group  contains  the  gases  not  absorbed  by  Potassium 
Hydrate  or  Sodium  Phosphate,  and  consists  of  Oxygen,  Nitric 
Oxide,  Carbonic  Oxide,  Hydrocarbons  of  the  formulae 
CnH2n,  (CnH2n+1)2,  and  CnH2a+2,  except  Marsh  gas. 

OXYGEN  was  formerly  determined  by  means  of  a  ball  of 
phosphorus,  but  it  is  difficult  subsequently  to  free  the  gas  from  the 
phosphorous  acid  produced,  which  exerts  some  tension  and  so 
vitiates  the  results  ;  besides  which,  the  presence  of  some  gases 
interferes  with  the  absorption  of  oxygen  by  phosphorus  ;  and  if 
any  potassium  hydrate  remains  on  the  side  of  the  tube,  from  the 
previous  absorption  of  carbonic  anhydride,  there  is  a  possibility  of 
the  formation  of  phosphoretted  hydrogen,  which  would,  of  course, 
vitiate  the  analysis.  A  more  convenient  reagent  is  a  freshly  pre- 
pared alkaline  solution  of  potassium  pyrogallate  introduced  into 
the  gas  in  a  bullet  of  papier-mache.  The  balls  of  papier  mache  are 
made  by  macerating  filter-paper  in  water,  and  forcing  as  much  of 
it  as  possible  into  a  bullet  mould  into  which  the  end  of  a  piece  of 
platinum  wire  has  been  introduced.  In  order  to  keep  the  mould 
from  opening  while  it  is  being  filled,  it  is  well  to  tie  the  handles 
together  with  a  piece  of  string  or  wire,  and  when  charged  it  is 
placed  on  a  sand  bath.  After  the  mass  is  dry  the  mould  may  be 
opened,  when  a  large  absorbent  bullet  will  have  been  produced. 
The  absorption  of  oxygen  by  the  alkaline  pyrogallate  is  not  very 
rapid,  and  it  may  be  necessary  to  remove  the  ball  once  or  twice 
during  the  operation,  and  to  charge  it  freshly. 

Nitric  oxide  cannot  be  readily  absorbed  in  an  ordinary 
absorption  tube ;  it  may,  however,  be  converted  into  nitrous 
anhydride  and  nitric  peroxide  by  addition  of  excess  of  oxygen, 
absorbing  the  oxygen  compounds  with  potassium  hydrate,  and  the 
excess  of  oxygen  by  potassium  pyrogallate.  The  diminution  of 
the  volume  will  give  the  quantity  of  nitric  oxide.  This  process  is 
quite  successful  when  the  nitric  oxide  is  mixed  with  olefiant  gas 
and  ethylic  hydride,  but  it  is  possible  that  other  hydrocarbons 
might  be  acted  on  by  the  nitrous  compounds. 

Carbonic  oxide  may  be  absorbed  by  two  reagents.  If  carbonic 
anhydride  and  oxygen  be  present  they  must  be  absorbed  in  the 
usual  manner,  and  afterwards  a  papier-mache  ball  saturated  with 
a  concentrated  solution  of  cuprous  chloride  in  dilute  hydrochloric 
acid  introduced.  A  ball  of  caustic  potash  is  subsequently  employed 
to  remove  the  hydrochloric  acid  given  off  by  the  previous 


O2,    NO,    CO,    AND    HYDROCARBONS.  525 

reagent,  and  to  dry  the  gas.  Carbonic  oxide  may  also  be  absorbed 
by  Introducing  a  ball  of  potassium  hydrate,  placing  the  absorption 
tube  in  a  beaker  of  mercury  and  heating  the  whole  in  a  water-bath 
to  100°  for  60  hours.  The  carbonic  oxide  is  thus  converted  into 
potassium  formate  and  entirely  absorbed. 

Olefiant  Gas  (Ethylene)  and  other  Hydrocarbons  of  the 
formula  CnH2n  are  absorbed  by  Nordhausen  sulphuric  acid, 
to  which  an  additional  quantity  of  sulphuric  anhydride  has  been 
added.  Such  an  acid  may  be  obtained  by  heating  some 
Nordhausen  acid  in  a  retort  connected  with  a  receiver  containing 
a  small  quantity  of  the  same  acid.  This  liquid  is  introduced  into 
the  gas  by  means  of  a  dry  coke  bullet.  These  bullets  are  made  by 
filling  the  mould,  into  which  the  usual  platinum  wire  has  been 
placed,  with  a  mixture  of  equal  weights  of  finely  powdered  coke 
and  bituminous  coal.  The  mould  is  then  heated  as  rapidly  as 
possible  to  a  bright  red  heat,  and  opened  after  cooling  ;  a  hard 
porous  ball  will  have  been  produced,  which  may  be  employed  for 
many  different  reagents.  It  is  sometimes  difficult  to  obtain  the 
proper  mixture  of  coal  and  coke,  but  when  once  prepared,  the 
bullets  may  be  made  with  the  greatest  ease  and  rapidity.  The 
olefiant  gas  will  be  absorbed  by  the  sulphuric  acid  in  about  an 
hour,  though  they  may  be  left  in  contact  for  about  two  hours  with 
advantage.  If,  on  removing  the  bullet,  it  still  fumes  strongly  in 
the  air,  it  may  be  assumed  that  the  absorption  is  complete.  The 
gas  now  contains  sulphurous,  sulphuric,  and  perhaps  carbonic 
anhydrides  ;  these  may  be  removed  by  a  manganic  peroxide  ball, 
followed  by  one  of  potassium  hydrate,  or  the  former  may  be  omitted, 
the  caustic  potash  alone  being  used.  The  various  members  of  the 
CnH2ll  group  cannot  be  separated  directly,  but  by  the  indirect 
method  of  analysis  their  relative  quantities  in  a  mixture  may  be 
determined. 

The  hydrocarbons  (CnH2n+1)2  and  CnH2n+2  may  be  absorbed 
by  absolute  alcohol,  some  of  which  is  introduced  into  the  absorption 
tube,  and  ignited  for  a  short  time  with  the  gas.  Correction  has 
then  to  be  made  for  the  weight  of  the  column  of  alcohol  on  the 
surface  of  the  mercury,  and  for  the  tension  of  the  alcohol  vapour. 
This  method  only  gives  approximate  results,  and  can  only  be 
employed  in  the  presence  of  gases  very  slightly  soluble  in  alcohol. 

The  time  required  in  the  different  processes  of  absorption  just 
described  is  considerable ;  perhaps  it  might  be  shortened  by 
surrounding  the  absorption  eudiometer  wrth  a  wider  tube,  similar 
to  the  external  tube  of  a  Liebig's  condenser,  through  which 
a  current  of  water  is  maintained.  By  means  of  a  thermometer  in 
the  space  between  the  tubes  the  temperature  of  the  gas  would  be 
known,  and  the  readings  might  be  taken  two  or  three  minutes 
after  the  withdrawal  of  the  reagents.  Besides  this  advantage,  the 
great  precaution  necessary  for  maintaining  a  constant  temperature 
in  the  room  might  be  dispensed  with.  A  few  experiments  made 
some  years  ago  in  this  direction  gave  satisfactory  results. 


526 


GAS    ANALYSIS. 


INDIRECT    DETERMINATIONS. 

GASES  which  are  not  absorbed  by  any  reagents  that  are  applicable 
in  eudiometers  over  mercury,  must  be  determined  in  an  indirect 
manner,  by  exploding  them  with  other  gases,  and  noting  either  the 
change  of  volume  or  the  quantity  of  their  products  of  decomposition  ; 
or  lastly,  as  is  most  frequently  the  case,  by  a  combination  of  these 
two  methods.  Thus,  for  example,  oxygen  may  be  determined  by 
exploding  with  excess  of  hydrogen,  and  observing  the  contraction  ; 
hydrogen  may  be  determined  by  exploding  with  excess  of  oxygen, 
and  measuring  the  contraction  ;  and  marsh  gas  by  exploding  with 
oxygen,  measuring  the  contraction,  and  also  the  quantity  of  carbonic 
anhydride  generated. 

The  operation  is  conducted  in  the  following  manner  : — The  long 
eudiometer  furnished  with  exploding  wires  is  filled  with  mercury, 
(after  a  drop  of  water  has  been  placed  at  the  top  of  the  tube  by 
means  of  an  iron  wire,  as  before  described),  and  some  of  the  gas  to 
be  analyzed  is  introduced  from  the  absorption  eudiometer.  This 
gas  is  then  measured  with  the  usual  precautions,  and  an  excess  of 
oxygen  or  hydrogen  (as  the  case  may  be)  introduced.  These  gases 
may  be  passed  into  the  eudiometer  directly  from  the  apparatus  in 
which  they  are  prepared  ;  or  they  may  be  previously  collected  in 
lipped  tubes  of  the  form  of  absorption  tubes,  so  as  to  be  always 
ready  for  use. 

For  the  preparation  of  the 
oxygen  a  bulb  is  used,  which  is 
blown  at  the  closed  end  of  a 
piece  of  combustion  tube.  The 
bulb  is  about  half  filled  with  dry 
powdered  potassium  chlorate, 
the  neck  drawn  out,  and  bent  to 
form  a  delivery  tube.  The 
chlorate  is  fused,  and  the  gas 
allowed  to  escape  for  some  time 
to  ensure  the  expulsion  of  the 
atmospheric  air  ;  the  end  of  the 
delivery  tube  is  then  brought 
under  the  orifice  of  the  eudio- 
meter, and  the  necessary  quantity 
of  gas  admitted.  When  it  is 
desired  to  prepare  the  oxygen 
beforehand,  it  may  be  collected 
directly  from  the  bulb  ;  or, 
another  method  to  obtain  the 
gas  free  from  air  may  be  adopted 
by  those  who  are  provided  with 
the  necessary  appliances.  This 
is,  to  connect  a  bulb  containing 
potassium  chlorate  with  a  Sprengel's  mercurial  air-pump,  and, 


Fig.  95. 


PREPARATION    OF    OXYGEN    AND    HYDROGEN. 


527 


after  heating  the  chlorate  to  fusion,  to  produce  a  vacuum  in 
the  apparatus.  The  chlorate  may  be  again  heated  until  oxygen 
begins  to  pass  through  the  mercury  at  the  end  of  the  Sprengel, 
the  heat  then  withdrawn,  and  a  vacuum  again  obtained.  The 
chlorate  is  once  more  heated,  and  the  oxygen  collected  at  the 
bottom  of  the  Sprengel.  Of  course  the  usual  precautions  for 
obtaining  an  air-tight  joint  between  the  bulk  and  the  Sprengel 
must  be  taken,  such  as  surrounding  the  caoutchouc  connector 
with  a  tube  rilled  with  mercury. 

The  hydrogen  for  these  experiments  must  be  prepared  by 
electrolysis,  since  that  from  other  sources  is  liable  to  contamin- 
ation with  impurities  which  would  vitiate  the  analysis.  The 
apparatus  employed  by  Buns  en  for  this  purpose  (fig.  95)  consists 
of  a  glass  tube,  closed  at  "the  lower  end,  and  with  a  funnel  at 

the  other,  into  which  a  de- 
livery tube  is  ground,  the 
funnel  acting  as  a  water-joint. 
A  platinum  wire  is  sealed  into 
the  lower  part  of  the  tube  ; 
and  near  the  upper  end  another 
wire,  with  a  platinum  plate 
attached,  is  fused  into  the 
glass.  Some  amalgam  of  zinc 
is  placed  in  the  tube  so  as 
to  cover  the  lower  platinum 
wire,  and  the  apparatus  filled 
nearly  to  the  neck  with  water 
acidulated  with  sulphuric  acid. 
On  connecting  the  platinum 
wires  with  a  battery  of  two  or 
three  cells,  the  upper  wire  being 
made  the  negative  electrode, 
pure  hydrogen  is  evolved  from 
the  platinum  plate,  and,  after 
the  expulsion  of  the  air,  may  be 
at  once  passed  into  the  eudio- 
meter,  or,  if  preferred,  collected 
in  tubes  for  future  use.  Un- 

fortunately, in  this  form  of  apparatus  the  zinc  amalgam  soon 
becomes  covered  with  a  saturated  solution  of  zinc  sulphate,  which 
puts  a  stop  to  the  electrolysis.  In  order  to  remove  this  layer, 
Bunsen  has  a  tube  fused  into  the  apparatus  at  the  surface  of  the 
amalgam  ;  this  is  bent  upwards  parallel  to  the  larger  tube,  and 
curved  downwards  just  below  the  level  of  the  funnel.  The  end 
of  the  tube  is  closed  with  a  caoutchouc  stopper.  On  removing 
the  stopper,  and  pouring  fresh  acid  into  the  funnel,  the  saturated 
liquid  is  expelled. 

Another  form  of  apparatus  for  preparing  electrolytic  hydrogen 
may  readily  be  constructed.  A  six-ounce  wide-mouth  bottle  is 


96- 


528  GAS    ANALYSIS. 

* 

fitted  with  a  good  cork,  or  better,  with  a  caoutchouc  stopper.  In 
the  stopper  four  tubes  are  fitted  (fig.  96).  The  first  is  a  delivery 
tube,  provided  with  a  U-tube,  containing  broken  glass  and  sulphuric 
acid,  to  conduct  the  hydrogen  to  the  mercurial  trough.  The  second 
tube  about  5  centimetres  long,  and  filled  with  mercury,  has  fused 
into  its  lower  end  a  piece  of  platinum  wire  carrying  a  strip  of 
foil,  or  the  wire  may  be  simply  flattened.  The  third  tube  passes 
nearly  to  the  bottom  of  the  bottle,  the  portion  above  the  cork  is 
bent  twice  at  right-angles,  and  cut  off,  so  that  the  open  end  is 
a  little  above  the  level  of  the  shoulder  of  the  bottle  ;  a  piece  of 
caoutchouc  tube,  closed  by  a  compression  cock,  is  fitted  to  the  end 
of  the  tube.  The  fourth  tube  is  a  piece  of  combustion  tube  about 
30  centimetres  in  length,  which  may  with  advantage  be  formed 
into  a  funnel  at  the  top.  This  tube  reaches  about  one-third 
down  the  bottle,  and  inside  it  is  placed  a  narrower  glass  tube, 
attached  at  its  lower  end  by  a  piece  of  caoutchouc  connector  to 
a  rod  of  amalgamated  zinc.  The  tube  is  filled  with  mercury  to 
enable  the  operator  readily  to  connect  the  zinc  with  the  battery ; 
some  zinc  amalgam  is  placed  at  the  bottom  of  the  bottle ; 
and  dilute  sulphuric  acid  is  poured  in  through  the  wide  tube 
until  the  bottle  is  nearly  filled  with  liquid.  To  use  the  apparatus, 
the  delivery  tube  is  dipped  into  mercury,  the  wire  from  the 
positive  pole  of  the  battery  put  into  the  mercury  in  the  tube 
to  which  the  zinc  is  attached,  and  the  negative  pole  connected  by 
means  of  mercury  with  the  platinum  plate.  The  current,  instead 
of  passing  between  the  amalgam  at  the  bottom  of  the  vessel  and 
the  platinum  plate,  as  in  Bunsen's  apparatus,  travels  from  the 
rod  of  amalgamated  zinc  to  the  platinum,  consequently  the  current 
continues  to  flow  until  nearly  the  whole  of  the  liquid  in  the  bottle 
has  become  saturated  with  zinc  sulphate. .  As  soon  as  the  hydrogen 
is  evolved,  of  course  a  column  of  acid  is  raised  in  the  funnel  until 
the  pressure  is  sufficient  to  force  the  gas  through  the  mercury  in 
which  the  delivery  tube  is  placed.  Care  must  be  taken  that  the 
quantity  of  acid  in  the  bottle  is  sufficient  to  prevent  escape  of  gas 
through  the  funnel  tube,  and  also  that  the  delivery  tube  does  not 
pass  too  deeply  into  the  mercury  so  as  to  cause  the  overflow  of  the 
acid.  When  the  acid  is  exhausted,  the  compression  cock  on  the 
bent  tube  is  opened  and  fresh  acid  poured  into  the  funnel ;  the 
dense  zinc  sulphate  solution  is  thus  replaced  by  the  lighter  liquid, 
and  the  apparatus  is  again  ready  for  use. 

A  very  convenient  apparatus  for  •  transferring  oxygen  and 
hydrogen  into  eudiometers  is  a  gas  pipette,  figured  and  described 
(fig.  68,  p.  459). 

It  is  necessary  in  all  cases  to  add  an  excess  of  the  oxygen  or 
hydrogen  before  exploding,  and  it  is  well  to  be  able  to  measure 
approximately  the  amount  added  without  going  through  the  whole 
of  the  calculations.  This  may  be  conveniently  done  by  making 
a  rough  calibration  of  the  eudiometer  in  the  following  manner  : — 
The  tube  is  filled  with  mercury,  a  volume  of  air  introduced  into  it 


PROPORTIONS   OF   OXYGEN  AND   COMBUSTIBLE   GAS. 


529 


from  a  small  tube,  and  the  amount  of  the  depression  of  the  mercury 
noted  ;  a  second  volume  is  now  passed  up,  a  further  depression  will 
be  produced,  but  less  in  extent  than  the  previous  one,  in  consequence 
of  the  shorter  column  of  mercury  in  the  tube.  This  is  repeated 
until  the  eudiometer  is  filled,  and  by  means  of  a  table  constructed 
from  these  observations,  but  without  taking  any  notice  of  the 
variations  of  thermometer  or  barometer,  the  operator  can  introduce 
the  requisite  quantity  of  gas.  It  may  be  convenient  to  make  this 
calibration  when  the  eudiometer  is  inclined  in  the  support,  and  also 
when  placed  perpendicularly,  so  that  the  gas  may  be  introduced 
when  the  tube  is  in  either  position.  A  table  like  the  following  is 
thus  obtained  : — 


Measures. 
1 

2 
3 
4 
5 
6 
7 
&c. 


DIVISIONS. 

Tube 
Inclined. 

27 

45 

61 

75 

88 
100 
109 
&c. 


Tube 
Perpendicular. 

45 

69 

87 
102 
116 
128 
138 


In  explosions  of  hydrocarbons  with  oxygen,  it  is  necessary  to 
have  a  considerable  excess  of  the  latter  gas  in  order  to  moderate 
the  violence  of  the  explosion.  The  same  object  may  be  attained  by 
diluting  the  gas  with  atmospheric  air,  but  it  is  found  that  sufficient 
oxygen  serves  equally  well.  If  the  gas  contains  nitrogen,  it  is 
necessary  subsequently  to  explode  the  residual  gas  by  hydrogen  ; 
and  if  oxygen  only  has  been  used  for  diluting  the  gas,  a  very  large 
quantity  of  hydrogen  must  be  added,  which  may  augment  the 
volume  in  the  eudiometer  to  an  inconvenient  extent.  When 
atmospheric  air  has  been  employed,  this  inconvenience  is  avoided. 
After  the  introduction  of  the  oxygen,  the  eudiometer  is  restored  to 
its  vertical  position,  allowed  to  stand  for  an  hour,  and  the  volume 
read  off. 

The  determination  of  the  quantity  of  oxygen  which  must  be  added 
to  combustible  gases  so  as  to  prevent  the  explosion  from  being  too 
violent,  and  at  the  same  time  to  ensure  complete  combustion,  has 
been  made  the  subject  of  experiment.  When  the  gases  before 
explosion  are  under  a  pressure  equal  to  about  half  that  of  the 
atmosphere,  the  following  proportions  of  the  gases  must  be 
employed  : — 

Volume  of  Volume  of 

Combustible  Gas.  Oxygen. 

Hydrogen      .          ^  1  1-5 

Carbonic  oxide       *  1  1-5 

Marsh  gas      .         «_         »         .         1  5 

2  M 


530 


GAS    ANALYSIS. 


Volume  of 
Combustible  Gas. 

Gases  containing  two  atoms  of 
carbon  in  the  molecule,  as 


Ethane  C2H, 


1 


Volume  of 
Oxygen. 


10 


Gases  containing  three  atoms  of 
carbon  in  the  molecule,  as 

Propyl  hydride,  C3H8        .1  .18 

Gases  containing  four  atoms  of 
carbon  in  the  molecule,  as 

Butane,  C4H10  1  25 

In  cases  of  mixtures  of  two  or  more  combustible  gases 
proportionate  quantities  of  oxygen  must  be  introduced. 

At  the  time  of  the  explosion,  it  is  necessary  that  the 
eudiometer  should  be  carefully  closed  to  prevent  the  loss  of     Fig.  97. 
gas  by  the  sudden  expansion.     For  this  purpose  a  thick 
plate  of  caoutchouc,  three  or  four  centimetres  wide,  is  cemented  on 
a  piece  of  cork  by  means  of  marine  glue,  or  some  similar  substance, 

and  the  lower  surface  of  the  cork  cut  so 
as    to    lie   firmly  at   the   bottom   of   the 
mercurial  trough  (fig.  97).     It  is,  however, 
preferable  to  have  the  caoutchouc  firmly 
fixed  in  the  trough.     As  the  mercury  does 
not  adhere  to   the  caoutchouc,  there  is 
some  risk  of  air  entering  the  eudiometer 
after  the  explosion;   this  is  obviated  by 
rubbing  the  plate  with  some  solution  of 
corrosive  sublimate  before  introducing  it 
into  the  mercury,  which  causes  the  metal 
to  wet  the  caoutchouc  and  removes  all 
air  from  its  surface.      When  the  caout- 
chouc   is    not    fixed   in  the   trough,   the 
treatment  with   the   corrosive   sublimate 
has  to  be  repeated  before  every  experi- 
ment, and  this  soils  the  surface  of  the 
mercury  to  an  inconvenient  extent.     Th 
cushion  is  next  depressed  to  the  bottor 
of  the  trough,  and  the  eudiometer  placei 
on  it  and  firmly  held  down  (fig.  98).     1 
this  is  done  with  the  hands,  the  tube  mus 
be  held   by  that  portion  containing  th 
mercury,  for  it  is  found    that  when  eu 
diometers    burst    (which,    however,    onl; 
happens  when  some  precaution  has  beei 
neglected)  they  invariably  give  way  jus 
at  the  level  of  the  mercury   within  th 
Fig.  98.  tube,  and  serious  accidents  might  occur  i 

the  hands  were  at  this  point.     The  caus 

of   the  fracture  at  this  point  is  the  following  : — Though  the  ga 
is  at  a  pressure  below  that  of  the  atmosphere  before  the  explosion 


THE    EXPLOSION. 


531 


yet  at  the  instant  of  the  passage  of  the  spark,  the  expansion  of  the 
gas  at  the  top  of  the  tube  condenses  the  layer  just  below  it ;  this  on 
exploding  increases  the  density  of  the  gas  further  down  the  tube, 
and,  by  the  time  the  ignition  is  communicated  to  the  lowest 
quantity  of  gas,  it  may  be  at  a  pressure  far  above  that  of  the 
atmosphere.  It  may  be  thought  that  the  explosion  is  so  instant- 
aneous that  this  explanation  is  merely  theoretical ;  but  on  exploding 
a  long  column  of  gas,  the  time  required  for  the  complete  ignition 
is  quite  perceptible,  and  sometimes  the  flash  may  be  observed  to 
be  more  brilliant  at  the  surface  of  the  mercury.  Some  experimenters 
prefer  to  fix  the  eudiometer  by  means  of  an  arm  from  a  vertical 
stand,  the  arm  being  hollowed  out  on  the  under  side,  and  the  cavity 
lined  with  cork. 

If   a  large  quantity  of    incombustible   gas   is   present,  the  in- 
flammability of  the  mixture  may  be  so  much  reduced  that  either 

the  explosion  does  not  take 
place  at  all,  or,  what  may 
be  worse,  only  a  partial 
combustion  ensues.  To  ob- 
viate this,  some  explosive 
mixture  of  oxygen  and 
hydrogen,  obtained  by  the 
electrolysis  of  water,  must 
be  introduced.  The  appa- 
ratus used  by  Bun  sen  for 
this  purpose  is  shown  in  fig. 
99.  The  tube  in  which  the 
electrolysis  takes  place  is 
surrounded  by  a  cylinder 
containing  alcohol,  in  order 
to  prevent  the  heating  of 
the  liquid.  A  convenient 
apparatus  for  the  prepara- 
tion of  this  gas  is  made  by 
blowing  a  bulb  of  about 
four  centimetres  in  diameter 
on  the  end  of  a  piece  of 
narrow  glass  tube,  sealing 
two  pieces  of  flattened  plati- 
num wire  into  opposite  sides 
of  the  globe,  and  bending 
the  tube  so  as  to  form  a 
delivery  tube.  Dilute  sul- 
phuric acid,  containing  about  one  volume  of  acid  to  twenty  of 
water,  is  introduced  into  the  globe,  either  before  bending  the  tube, 
by  means  of  a  funnel  with  a  fine  long  stem,  or,  after  the  bending, 
by  warming  the  apparatus,  and  plunging  the  tube  into  the  acid. 
Care  must  be  taken  that  the  acid  is  dilute,  and  that  the  battery  is 
not  too  strong,  in  order  to  avoid  the  formation  of  ozone,  which 

2  M  2 


Fig.  99. 


532  GAS   ANALYSIS. 


would  attack  the  mercury,  causing  the  sides  of  the  eudiometer  to 
be  soiled,  at  the  same  time  producing  a  gas  too  rich  in  hydrogen. 

The  spark  necessary  to  effect  the  explosion  may  be  obtained 
from  several  sources.  An  ordinary  electrical  machine  or  electro- 
phorus  may  be  used,  but  these  are  liable  to  get  out  of  order 
by  damp.  Buns  en  uses  a  porcelain  tube,  which  is  rubbed  with 
a  silk  rubber,  coated  with  electrical  amalgam ;  by  means  of  this 
a  small  Ley  den  jar  is  charged.  A  still  more  convenient  apparatus 
is  an  induction  coil  large  enough  to  produce  a  spark  of  half  an 
inch  in  length. 

After  the  explosion,   the  eudiometer  is  slightly  raised  from  the 
caoutchouc  plate  to  allow   the  entrance    of  mercury.      When  no 
more  mercury  rushes  in,  the  tube  is  removed  from  the  caoutchouc 
plate,  placed  in  a  perpendicular  position,  and  allowed  to  remain 
for  at  least  an  hour  before  reading.     After  measuring   the   con- 
traction,  it  is  generally  necessary  to  absorb  the  carbonic  anhy- 
dride  formed   by   the   combustion   by  means    of   a  potash  ball, 
in   the  way  previously  described.      In   some  rare  instances  the 
amount  of  water  produced  in  the  explosion  with  oxygen  must 
be   measured.      If   this    has    to   be    done,    the    eudiometer,    the 
mercury,  the  original  gas,  and  the  oxygen  must  all  be  carefully 
dried.     After  the  explosion,   the  eudiometer  is  transferred  to  a 
circular  glass  vessel  containing  mercury,  and  attached  to  an  iron- 
wire  support,  by  which  the  entire  arrangement  can  be  suspended 
in  a  glass  tube  adapted  to  the  top  of  an  iron  boiler,  from  which 
a  rapid  current  of  steam  may  be  passed  through  the  glass  tube,  so 
as  to  heat  the  eudiometer  and  mercury  to  an  uniform  temperature 
of  100°.     From  the  measurements  obtained  at  this  temperature 
the  amount  of  water  produced  may  be  calculated.     If  three  com- 
bustible gases  are  present,  the  only  data  required  for  calculation 
are,  the  original  volume  of  the  gas,  the  contraction  on  explosion, 
and   the   amount   of   carbonic   anhydride   generated.     When   the 
original  gas  contains  nitrogen,   the  residue  after  explosion  with 
excess  of  oxygen  consists  of  a  mixture  of  oxygen  and  nitrogen.     To 
this  an  excess  of  hydrogen  is  added,  and  the  mixture  exploded  ; 
the  contraction  thus  produced  divided  by  3  gives  the  amount  of 
oxygen  in  the  residual  gas,  and  the  nitrogen  is  found  by  difference. 
It  is  obvious  that,  by  subtracting  the  quantity  of  residual  oxygen, 
thus  determined  by  explosion  with  hydrogen,  from  the  amount 
added,  in  the  first  instance,  to  the  combustible  gas,  the  volume  of 
oxygen  consumed  in  the  explosion  may  be  obtained.     Some  chemists 
prefer  to  employ  this  number  instead  of  the  contraction  as  one  of  the 
data  for  the  calculation. 

We  must  now  glance  at  the  mode  of  calculation  to  be  employed  for 
obtaining  the  percentage  composition  of  a  gas  from  the  numbers 
arrived  at  by  the  experimental  observations. 

The  following  table  shows  the  relations  existing  between  the 
volume  of  the  more  important  combustible  gases  and  the  products 
of  the  explosion  : — 


VOLUME   RELATIONS. 


533 


Name  of  Gas. 

Volume  of 
Combustible 
Gas. 

°fl'2 

K*        O 

Contraction 
after 
Explosion 

"Volume  of 
Carbonic 
Anhydride 
produced. 

Hvdroffen,  H« 

1 

0-5 

1-5 

o 

Carbonic  Oxide,  CO      .      .      . 
Methane,  CH4    

1 
1 

0-5 
2 

0-5 

2 

1 
1 

Acetylene,  C2H2      .... 
Ethylene,  C2H4        .... 
Ethane,  CH3,  CH3  .... 
Ethyl  Hydride,  C2H5H      .      . 
Propylene,  C3H6      .... 
Propyl  Hydride,  C3H7H    .      . 
Butylene  C4H8  

1 
1 
1 
1 
1 
1 
1 

2-5 
3 
3-5 
3-5 
4-5 
5 
6 

1-5 
2 
2-5 
2-5 
2-5 
3 
3 

2 
2 
2 
2 
3 
3 
4 

Butane,  C2H5,  C2H5      .      .      . 
Butyl  Hydride,  C4H9H      .      . 

1 
1 

6-5 
6-5 

3-5 
3-5 

4 
4 

As  an  example,  we  may  take  a  mixture  of  hydrogen,  carbonic 
oxide,  and  methane,  which  gases  may  be  designated  by  x,  y,  and  z 
respectively.  The  original  volume  of  gas  may  be  represented  by  A, 
the  contraction  by  C,  and  the  amount  of  carbonic  anhydride  by  D. 

A  will  of  course  be  made  up  of  the  three  components,  or 

A=x+y+z. 

C  will  be  composed  as  follows  :  —  When  a  mixture  of  2  volumes  of 
hydrogen  and  1  volume  of  oxygen  is  exploded,  the  gas  entirely 
disappears.  One  volume  of  hydrogen  combining  with  half  a  volume 
of  oxygen,the  contraction  will  be  1  J  times  the  quantity  of  hydrogen 
present,  or  1J#.  In  the  case  of  carbonic  oxide,  1  volume  of  this 
gas  uniting  with  half  its  volume  of  oxygen  produces  1  volume  of 
carbonic  anhydride,  so  the  contraction  due  to  the  carbonic  oxide 
will  be  half  its  volume,  or  \y.  Lastly,  1  volume  of  marsh  gas  com- 
bining with  2  volumes  of  oxygen  generates  I  volume  of  carbonic 
anhydride,  so  the  contraction  in  this  case  will  be  twice  its  volume, 
or  2x.  Thus  we  have  — 


Since  carbonic  oxide~on  combustion  forms  its  own  volume  of 
carbonic  anhydride,  the  amount  produced  by  the  quantity  present 
in  the  mixture  will  be  y.  Marsh  gas  also  generates  its  own  volume 
of  carbonic  anhydride,  so  the  quantity  corresponding  to  the  marsh 
gas  in  the  mixture  will  be  z.  Therefore 


It  now  remains  to  calculate  the  values  of  x,  y,  and  z  from  the 
experimental  numbers  A,  C,  and  D,  which  is  done  by  the  help  of 
the  following  equations  :  — 


534  GAS   ANALYSIS. 

To  find  x— 


z=A, 


x 
For  y  we  have  — 


=  4D-2C, 
=  3A-3D, 


3y        =  3A-2C|D,  or 
3A-2C  +  D 


=     3 

The  value  of  z  is  thus  found  — 


z=D-y= 

^    3A-2C+D 

or 


z  = 


3 
2C-3A  +  2D. 


By  replacing  the  letters  A,  C,  and  D  by  the  numbers  obtained  by 
experiment,  the  quantities  of  the  three  constituents  in  the  volume  A 
may  easily  be  calculated  by  the  three  formulae  — 

x  =  A  —  D  =  hydrogen, 

3A-2C+D 
V  =  -  o  -  =  carbonic  oxide, 

2C-3A+2D 

—  ^  —    —  =  methane, 
o 

The  percentage  composition  is,  of  course,  obtained  by  the  simple 
proportions  — 

A  :  x  :  :  100     per-cent.  of  hydrogen, 

A  :  y  :  :  100     per-cent.  of  carbonic  oxide, 

A  :  z  :  :  100     per-cent.  of  methane. 

If  the  gas  had  contained  nitrogen,  it  would  have  been  determined 
by  exploding  the  residual  gas,  after  the  removal  of  the  carbonic 
anhydride,  with  excess  of  hydrogen.  The  contraction  observed, 
divided  by  3,  would  give  the  volume  of  oxygen  in  the  residue,  and 
this  deducted  from  the  residue,  would  give  the  amount  of  nitrogen. 
If  A  again  represents  the  original  gas,  and  n  the  amount  of  nitrogen 
it  contains,  the  expression  A—  n  would  have  to  be  substituted  for 
A  in  the  above  equations. 

It  may  be  as  well  to  develop  the  formulae  for  obtaining  the  same 
results  by  observing  the  volume  of  oxygen  consumed  instead  of  the 
contraction.  If  B  represents  the  quantity  of  oxygen,  we  shall  have 


the  values  of  A  and  D  remaining  as  before,  x=A—  D. 


MODE   OF   CALCULATION.  535 

z  is  thus  found  — 


x+y+  z=A, 


3z  =  2B-A,      or 

2B-A 

z  =•  —  -  - 


For  y — 

D=i/  +  z 

y  =  V-Z  = 

^     2B-A 
D--3~'     °r 
3D-2B+A 

y=        -3- 

Thus  we  have —          '      f 

x=A-V 
3D-2B  +  A 

y=-^r 

=2B-A 


Having  thus  shown  the  mode  of  calculation  of  the  formulae,  it  will 
be  well  to  give  some  examples  of  the  formulae  employed  in  some  of 
the  cases  which  most  frequently  present  themselves  in  gas  analysis. 
In  all  cases  — 

A  =  original  mixture, 

C=  contraction, 

D=  carbonic  anhydride  produced. 


1.     Hydrogen  and  Nitrogen. 


Excess  of  oxygen  is  added,  and  the  contraction  on  explosion 
observed  : — 

x=  — 

3A-2C 
y  = — 5 —  ,  or  A-tf. 


2.     Carbonic  Oxide  and  Nitrogen. 
C0=z;  N=y. 

The  gas  is  exploded  with  excess  of  oxygen,  and  the  amount  of 
carbonic  anhydride  produced  is  determined ; 

x=D, 
«=A-D. 


536  GAS   ANALYSIS. 

3.     Hydrogen,  Carbonic  Oxide,  and  Nitrogen. 
H=z;  CO  =  ?/;  N  =  z. 

In   this   case   the   contraction   and   the    quantity   of   carbonic 
anhydride  are  measured  :  — 

2C-D 
x  =-3-> 

y=*>, 

3A-2C-2D 
-IT    - 

4.     Hydrogen,  Methane,  and  Nitrogen. 
H=x;  CH4=2/;  N=z. 
2C-4D 

x=-^r-> 

2/=D, 

z  _3A-2C+D 

3 

5.     Carbonic  Oxide,  Methane,  and  Nitrogen. 
CO=z;  CH4=i/  ;  N=«. 
_4D-2C 
~3       ' 
_2C-P 

y  q  ' 

z  =A-D. 

6.     Hydrogen,  Ethane  (or  Ethyl  Hydride),  and 

Nitrogen. 

H=z;  C2H6=2/;  N=z. 
4C-5D 


D 
2/  —  -g-  » 

_3A-2C+D 
~^~ 

7.     Carbonic    Oxide,   Ethane  (or   Ethyl    Hydride),    and 

Nitrogen. 
C0=a;;  C2H6=2/;  N=z. 


. 

2C-D 

y- 


_ 


3    , 

3A-4P+2C 


FORMULAE.  537 

8.     Hydrogen,  Carbonic  Oxide,  and  Methane. 
H=z;  C0=2/;  CH4=z. 

z=A-D , 

3A-2C+D 

y=-  -3- 

2C-3A+2D 
-3-    - 

9.     Hydrogen,  Carbonio  Oxide,  and  Ethyl   Hydride 
(or  Ethane). 

H=x;  CO=2/;  C2H6=z. 

3A+2C-4D 

~6 ' 

=3A-2C+D 


z  = 


3 
2C-3A+2D 


10.     Carbonic  Oxide,   Methane,   and   Ethyl  Hydride 
(or  Ethane). 


3A-2C+D 
*=-  -3--' 
3A  +  2C-4D 

y=-  -3-  -, 

z=D-A  . 

11.     Hydrogen,   Methane,   and   Acetylene. 
n=x]  CH4=2/;  C2 
5A-3C-D 


y  =2C-3A  , 
D_2C  +  3A 

-- 


12.     Hydrogen,   Methane,    and  Ethyl  Hydride 
'(or  Ethane). 


This  mixture  cannot  be  analyzed  by  indirect  determination,  since 
a  mixture  of  two  volumes  of  hydrogen  with  two  volumes  of  ethyl 


538  GAS    ANALYSIS. 

hydride  (or  ethane)  has  the  same  composition  as  four  volumes  of 
methane  — 

C2H6  -f-  H2  =  2CH4  ; 

and,    consequently,    would   give   rise   to   the   same   products    on 
combustion  with  oxygen  as  pure  methane  — 


2CH4+4O2=2C02+40H2. 

In  this  case  it  is  necessary  to  determine  directly  the  ethyl  hydride 
(or  ethane)  in  a  separate  portion  of  the  gas  by  absorption  with 
alcohol,  another  quantity  of  the  mixture  being  exploded  with 
oxygen,  and  the  amount  of  carbonic  anhydride  produced  measured. 
If  the  quantity  absorbed  by  alcohol  =  E,  then 


x  =- 
y=D-2E, 

2=E. 

13.     Hydrogen,  Carbonic  Oxide,  Propyl  Hydride. 
H=*;  C0=2/;  C3H8=2. 
3A+4C-5D 


x  = 


9 
3A-2C+D 


2C-3A+2D 
-§- 

14.     Carbonic   Oxide,   Methane,  and  Propyl  Hydride. 

f~^r\ ™  .    f^TT    .    f~i  TT    « 

_3A-2C+D 

=3A+4C-5D 

y  ~ 


6 

D-A 


z  = 


15.     Carbonic    Oxide,   Ethyl  Hydride   (or  Ethane),   and 
Propyl  Hydride. 

C0=*;  C2H6=i/;  C3H8=z. 

_3A-2C+D 

X  =        ~3 ' 

_3A+4G-5P 
3 

4D-3A-2C 

z  = ^ . 


FORMULAE.  539 

16.     Methane,    Ethyl  Hydride    (or  Ethane),    and 
Propyl  Hydride. 

CH4=#;  C2H6=i/;  C3H8=z. 

As  a  mixture  of  two  volumes  of  methane  and  two  of  propyl 
hydride  has  the  same  composition  as  four  of  ethyl  hydride  (or 
ethane) — 

CH4 + C3H8 = 2C2H6, 

the  volume  absorbed  by  alcohol,  which  consists  of  ethyl  hydride 
(or  ethane)  and  propyl  hydride,  must  be  determined,  and  another 
portion  of  the  gas  exploded,  and  the  contraction  measured.  If  E 
represents  the  volume  absorbed — 

x  =A-E, 

y  =4A-2C  +  2E, 

z  =2C+4A-E. 

17.  Hydrogen,   Carbonic   Oxide,   and  Butane  (or  Butyl 

Hydride). 

H~  .    C^Ci      m  •    C*  TT          ^ 
=  X  ,    UU=2/,    L/4±110  =  2. 

_A+2C-2D 
_3A-2C+P 

y=        3       , 

_2C+2D-3A 
~I2~ 

18.  Nitrogen,  Hydrogen,  Carbonic  Oxide,  Ethyl  Hydride 

(or  Ethane),  and  Butyl  Hydride  (or  Butane). 

N=»;  H=w;  C0=z;  C2H6=2/;  C4H10=z. 

In  one  portion  of  the  gas  the  ethyl  hydride  (or  ethane)  and 
the  butyl  hydride  (or  butane)  are  absorbed  by  alcohol ;  the  amount 
absorbed  =  E. 

A  second  portion  of  the  original  gas  is  mixed  with  oxygen  and 
exploded,  the  amount  of  contraction  and  of  carbonic  anhydride 
being  measured. 

The  residue  now  contains  the  nitrogen  and  the  excess  of  oxygen  ; 
to  this  an  excess  of  hydrogen  is  added,  the  mixture  exploded,  and 
the  contraction  measured.  From  this  the  quantity  of  nitrogen  is 
thus  obtained.  Let — 

G= excess  of  oxygen  and  nitrogen, 
v= excess  of  oxygen, 
n= nitrogen, 
Cx= contraction  on  explosion  with  hydrogen. 


540  GAS/  ANALYSIS. 

Then— 

G=v+n, 


Li 


3      ' 

From  these  data  the  composition  of  the  mixture  can  be  determined  — 
2C-D-3E 


w 


_2C-3A+2D-6E+3yfc 
6 


MODIFICATIONS    AND    IMPROVEMENTS    OF 
THE    FOREGOING    PROCESSES. 

IN  the  method  of  gas  analysis  that  we  have  been  considering, 
the  calculations  of  results  are  somewhat  lengthy,  as  will  be  seen 
by  a  reference  to  the  example  given  of  the  analysis  of  a  mixture  of 
air  and  carbonic  anhydride  (page  521).  Besides  this,  the  operations 
must  be  conducted  in  a  room  of  uniform  temperature,  and  con- 
siderable time  allowed  to  elapse  between  the  manipulation  and  the 
readings  in  order  to  allow  the  eudiometers  to  acquire  the  temperature 
of  the  surrounding  air  ;  and,  lastly,  the  absorption  of  gases  by  solid 
reagents  is  slow.  These  disadvantages  are  to  a  great  extent  counter- 
balanced by  the  simplicity  of  the  apparatus  and  of,  the  manipulation. 

From  time  to  time  various  chemists  have  proposed  methods  by 
which  the  operations  are  much  hastened  and  facilitated,  and  the 
calculations  shortened.  It  will  be  necessary  to  mention  a  few  of 
these  processes,  which,  however,  require  special  forms  of  apparatus. 

Williamson  and  Russell*  have  described  an  apparatus,  by 
means  of  which  the  gases  in  the  eudiometers  are  measured  under 
a  constant  pressure,  the  correction  for  temperature  being  eliminated 
by  varying  the  column  of  mercury  in  the  tube  so  as  to  compensate 
for  the  alteration  of  volume  observed  in  a  tube  containing  a  standard 
volume  of  moist  air.  In  this  case  solid  reagents  were  employed  in 
the  eudiometers. 

*  Proceedings  of  the  Royal  Society,  9,  218. 


RUSSELL'S  APPARATUS.  541 

In  1864  they  published*  a  further  development  of  this  method, 
in  which  the  absorptions  were  conducted  in  a  separate  labor- 
atory vessel,  by  which  means  the  reagents  could  be  employed 
in  a  pasty  condition  and  extended  over  a  large  surface.  And  in 
1868  Russellf  improved  the  apparatus,  so  that  liquid  reagents 
could  be  used  in  the  eudiometers,  and  the  analysis  rapidly 
executed. 

The  gutta-percha  mercury  trough  employed  is  provided  with 
a  deep  well,  into  which  the  eudiometer  can  be  depressed  to  any 
required  extent,  and  on  the  surface  of  the  mercury  a  wide  glass 
cylinder,  open  at  both  ends  and  filled  with  water,  is  placed.  The 
eudiometer  containing  the  gas  to  be  examined  is  suspended  within 
the  cylinder  of  water  by  means  of  a  steel  rod  passing  through 
a  socket  attached  to  a  stout  standard  firmly  fixed  to  the  table.  In 
a  similar  manner,  a  tube  containing  moist  air  is  placed  by  the  side 
of  the  eudiometer.  The  clamp  supporting  this  latter  tube  is 
provided  with  two  horizontal  plates  of  steel,  at  which  the  column 
of  the  mercury  is  read  off.  When  a  volume  of  gas  has  to  be 
measured,  the  pressure  tube  containing  the  moist  air  is  raised  or 
lowered,  by  means  of  an  ingeniously  contrived  fine  adjustment, 
until  the  mercury  stands  very  nearly  at  the  level  of  one  of  the 
horizontal  steel  plates.  The  eudiometer  is  next  raised  or  lowered 
until  the  column  of  mercury  within  it  is  at  the  same  level.  The 
final  adjustment  to  bring  the  top  of  the  meniscus  exactly  to  the 
lower  edge  of  the  steel  bar  is  effected  by  sliding  a  closed  wide  glass 
tube  into  the  mercury  trough.  Thus  we  have  two  volumes  of  gas 
under  the  same  pressure  and  temperature,  and  both  saturated  with 
moisture.  If  the  temperature  of  the  water  in  the  cylinder  increased, 
there  would  be  a  depression  of  the  columns  in  both  tubes  ;  but  by 
lowering  the  tubes,  and  thus  increasing  the  pressure  until  the 
volume  of  air  in  the  pressure  tube  was  the  same  as  before,  it  would 
be  found  that  the  gas  in  the  eudiometer  was  restored  to  the  original 
volume.  Again,  if  the  barometric  pressure  increased,  the  volumes 
of  the  gases  would  be  diminished  ;  but,  by  raising  the  tubes  to  the 
necessary  extent,  the  previous  volumes  would  be  obtained.  There- 
fore, in  an  analysis,  it  is  only  necessary  to  measure  the  gas  at  a 
pressure  equal  to  that  which  is  required  to  maintain  the  volume  of 
moist  air  in  the  pressure  tube  constant.  The  reagents  are  intro- 
duced into  the  eudiometer  in  the  liquid  state  by  means  of  a  small 
syringe  made  of  a,  piece  of  glass  tube  about  one-eighth  of  an  inch 
in  diameter.  For  this  purpose  the  eudiometer  is  raised  until  its 
open  end  is  just  below  the  surface  of  the  mercury,  and  the  syringe, 
which  is  curved  upwards  at  the  point,  is  depressed  in  the  trough, 
passed  below  the  edge  of  the  water  cylinder,  and  the  extremity  of 
the  syringe  introduced  into  the  eudiometer.  When  a  sufficient 
quantity  of  the  liquid  has  been  injected,  the  eudiometer  is  lowered 
and  again  raised,  so  as  to  moisten  the  sides  of  the  tube  with  the 
liquid,  and  thus  hasten  the  absorption.  Ten  minutes  was  found  to 

*  J.  C.  S.  17,  238.  t  J.  C.  S.  21,  128. 


542  GAS   ANALYSIS. 

be  a  sufficient  time  for  the  absorption  of  carbonic  anhydride  when 
mixed  with  air. 

To  remove  the  liquid  reagent,  a  ball  of  moistened  cotton  wool  is 
employed.  The  ball  is  made  in  the  following  manner : — A  piece  of 
steel  wire  is  bent  into  a  loop  at  one  end,  and  some  cotton  wool 
tightly  wrapped  around  it.  It  is  then  dipped  in  water  and  squeezed 
with  the  hand  under  the  liquid  until  the  air  is  removed.  The  end 
of  the  steel  wire  is  next  passed  through  a  piece  of  glass  tube,  curved 
near  one  end  and  the  cotton  ball  drawn  against  the  curved  extremity 
of  the  tube.  The  ball,  saturated  with  water,  is  now  depressed  in 
the  mercury  trough,  and,  after  as  much  of  the  water  as  possible  has 
been  squeezed  out  of  it,  it  is  passed  below  the  eudiometer,  and, 
by  pushing  the  wire,  the  ball  is  brought  to  the  surface  of  the  mercury 
in  the  eudiometer  and  rapidly  absorbs  all  the  liquid  reagent,  leaving 
the  meniscus  clean.  The  ball  is  removed  with  a  slight  jerk,  and 
gas  is  thus  prevented  from  adhering  to  it.  It  is  found  that  this 
mode  of  removing  the  liquid  can  be  used  without  fear  of  altering 
the  volume  of  the  gas  in  the  eudiometer. 

Carbonic  anhydride  may  be  absorbed  by  a  solution  of  potassium 
hydrate,  and  oxygen  by  means  of  potassium  hydrate  and  pyrogallic 
acid.  The  determination  of  ethylene  is  best  effected  by  means  of 
fuming  sulphuric  acid  on  a  coke  ball,  water  and  dilute  potassium 
hydrate  being  subsequently  introduced  and  removed  by  the  ball  of 
cotton  wool. 

Doubtless  this  mode  of  using  the  liquid  reagents  might  be  em- 
ployed with  advantage  in  the  ordinary  process  of  analysis  to 
diminish  the  time  necessary  for  the  absorption  of  the  gases.  By 
this  process  of  Russell's  the  calculations  are  much  shortened  and 
facilitated,  the  volumes  read  off  being  comparable  among  themselves ; 
this  will  be  seen  by  an  example,  taken  from  the  original  memoir, 
of  the  determination  of  oxygen  in  air — 

Volume  in  Table 

corresponding 

to  reading. 

Volume  of  air  taken    .          .          .  130-3          132-15 

Volume  after  absorption  of  oxygen  j 

by  potassium  hydrate  and  pyro-  /          103-5          104-46 
gallic  acid        .          .          .          .1 
132-15 
104-46 

27-69  volumes  of  oxygen  in  132-15  of  air 

132-15  :  27-69  :  :  100  :  20-953  percentage  of  oxygen  in  air. 

Russell*  has  also  employed  his  apparatus  for  the  analysis  of 

carbonates.     For  this  purpose  he  adapted  a  graduated  tube,  open  at 

both  ends,  to  a  glass  flask  by  means  of  a  thick  piece  of  rubber  tube. 

Into   the  flask  a  weighed  quantity  of  a  carbonate  was  placed, 

together  with  a  vessel  containing  dilute  acid.     The  position  of  the 

mercury  in  the  graduated  tube  was  first  read  off,  after  which  the 

flask  was  shaken  so  as  to  bring  the  acid  and  carbonate  in  contact, 

*  J.  C.S.  [N.S.J6,  310. 


FRANKLAND    AND    WARD  S    APPARATUS. 


543 


and  the  increase  in  volume  was  due  to  the  carbonic  anhydride 
evolved.     The  results  thus  obtained  are  extremely  concordant. 

In  eight  experiments  with  sodium  carbonate  the  percentage  of 
carbonic  anhydride  found  varied  from  41-48  to  41-61,  theory 
requiring  41-51. 

Thirteen  experiments  with  calc-spar  gave  from  43-52  to  43-86, 
the  theoretical  percentage  being  44-0  ;  and  in  nine  other  analyses 
from  43-58  to  43-90  were  obtained. 

Two  experiments  were  made  with  manganic  peroxide,  oxalic 
acid,  and  sulphuric  acid,  and  gave  58-16  and  58-10  per  cent,  of 
carbonic  anhydride. 

Some  determinations  of  the  purity  of  magnesium  were  also  per- 
formed by  dissolving  the  metal  in  hydrochloric  acid  and  measuring 
the  resulting  hydrogen.  Four  operations  gave  numbers  varying 
between  8-26  and  8-28.  The  metal  should  yield  8-3,3. 

Russell*  has  also  employed  this  process  for  the  determination 
of  the  combining  proportions  of  nickel  and  cobalt. 

Regnault  and  Reisetf 
described  an  apparatus  by 
which  absorptions  could  be 
rapidly  conducted  by  means 
of  liquid  reagents  brought  in 
contact  with  the  gases  in  a 
laboratory  tube.  The  measure- 
ments are  made  in  a  graduated 
tube,  which  can  be  placed  in 
communication  with  the 
laboratory  tube  by  means  of 
fine  capillary  tubes  provided 
with  stop-cocks,  the  lower  end 
of  the  measuring  tube  being 
connected  by  an  iron  socket 
and  stop-cock  with  another 
graduated  tube  in  which  the 
pressure  to  which  the  gas  is 
subjected  is  measured.  The 
measuring  and  pressure  tubes 
are  surrounded  by  a  cylinder 
of  water.  An  apparatus 
similar  in  principle  to  this  has 
recently  been  constructed  by 
Frankland,  and  is  fully  de- 
scribed in  the  section  on  Water 
Analysis  (fig.  64,  p.  454). 

Frankland  and  WardJ 
made  several  important  im- 

They 


Fig.  100. 
provements  in  the  apparatus  of  Regnault   and   Reiset. 


*  J.  C.  S.  [N.S.]  7,  294. 


t  Ann.  Chim.  Phys.  [3]  26,  333. 


I  J.  C.  S.  6,  197. 


544  GAS   ANALYSIS. 

introduced  a  third  tube  (fig.  100),  closed  at  the  top  with  a  stopper, 
which  is  made  to  act  as  a  barometer,  to  indicate  the  tension  of  the 
gas  in  the  measuring  tube,  thus  rendering  the  operation  entirely 
independent  of  variations  of  atmospheric  pressure.  The  correction 
for  aqueous  vapour  is  also  eliminated  by  introducing  a  drop  of 
water  into  the  barometer  as  well  as  into  the  measuring  tube,  the 
pressures  produced  by  the  aqueous  vapour  in  the  two  tubes  thus 
counterbalancing  one  another,  so  that  the  difference  of  level  of  the 
mercury  gives  at  once  the  tension  of  the  dry  gas.  The  measuring 
tube  is  divided  into  ten  equal  divisions  (which,  for  some  purposes, 
require  to  be  calibrated),  and  in  one  analysis  it  is  convenient  to 
make  all  the  measurements  at  the  same  division,  or  to  calculate  the 
tension  which  would  be  exerted  by  the  gas  if  measured  at  the  tenth 
division.  Frankland  and  Ward  also  adapted  an  iron  tube  more 
than  760  mm.  long  at  the  bottom  of  the  apparatus,  which  enables 
the  operator  to  expand  the  gas  to  any  required  extent,  and  thus 
diminish  the  violence  of  the  explosions  which  are  performed  in  the 
measuring  tube.  During  the  operation  a  constant  stream  of  water 
is  kept  flowing  through  the  cylinder,  which  maintains  an  uniform 
temperature. 

By  the  use  of  this  form  of  apparatus  the  calculations  of  analyses 
are  much  simplified.  An  example  of  an  analysis  of  atmospheric 
air  will  indicate  the  method  of  using  the  instrument. 

Volume  of  Air  used.     Determined  at  5th  Division  on 
the  Measuring  Tube. 

Observed  height  of  mercury  in  barometer 
Height  of  5th  Division     ..... 
Tension  of  gas 


Corrected  tension  of  gas  at  10th  division  .          .     145 '00 

Volume  after  Admission  of  Hydrogen.     Determined  at 

6th  Division. 

mm. 

Observed  height  of  mercury  in  barometer  .     772*3 

Height  of  6th  Division 304-Q 

Tension  of  gas      .          .          .     468-3 

0-6 
Corrected  tension  at  10th  division     .          .          .     280-98 

Volume   after   explosion.     Determined  at  5th  Division. 

mm. 

Observed  height  of  mercury  in  barometer  .     763-3 

Height  of  5th  Division 383-Q 

Tension  of  gas      .          .         .' 

Corrected  tension  at  10th  division 


MC  LEOD'S  APPARATUS.  545 

Tension  of  air  with  hydrogen    ....  280*98 

Tension  of  gas  after  explosion  ....  190-15 

Contraction  on  explosion    .          .          .  .       .  90-83 
of  which  one-third  is  oxygen. 

QQ.QO 

— s-  =30-276  =  volumes   of   oxygen   in    145-0   volumes   of   air 

145-0     :     30-276     :  :     100     :     x 
30-276x100     OA_  , 

x~  — IAK  ft —  =20 -88=  percentage  ot  oxygen  in  air. 

If  all  the  measurements  had  been  made  at  the  same  division,  no 
correction  to  the  tenth  division  would  have  been  necessary,  as  the 
numbers  would  have  been  comparable  among  themselves. 

Another  modification  of  Frankland  and  Ward's,  or 
Regnault's  apparatus  has  been  designed  by  Me  Leo  d,*  in  which 
the  original  pressure  tube  of  Regnault's  apparatus,  or  the  filling 
tube  of  Frankland  and  Ward,  is  dispensed  with,  the  mercury 
being  admitted  to  the  apparatus  through  the  stop-cocks  at  the 
bottom. 

The  measuring  tube  A  (fig.  101)  is  900  mm.  in  length,  and  about 
20  mm.  in  internal  diameter.  It  is  marked  with  ten  divisions,  the 
first  at  25  mm.  from  the  top,  the  second  at  50,  the  third  at  100,  and 
the  remaining  ones  at  intervals  of  100  mm.  In  the  upper  part  of  the 
tube,  platinum  wires  are  sealed,  and  it  is  terminated  by  a  capillary 
tube  and  a  fine  glass  stop-cock,  a,  the  capillary  tube  being  bent  at 
right-angles  at  50  mm.  above  the  junction.  At  the  bottom  of  the 
tube,  a  wide  glass  stop-cock  b  is  sealed,  which  communicates,  by 
means  of  a  caoutchouc  joint  surrounded  with  tape  and  well  wired  to 
the  tubes,  with  a  branch  from  the  barometer  tube  B.  This  latter 
tube  is  5  mm.  in  width,  and  about  1200  mm.  long,  and  is  graduated 
in  millimetres  from  bottom  to  top.  At  the  upper  extremity  a  glass 
stop-cock  d  is  joined,  the  lower  end  being  curved  and  connected  by 
caoutchouc  with  a  stop-cock  and  tube  C,  descending  through  the 
table  to  a  distance  of  900  mm.  below  the  joint.  It  is  advisable  to 
place  washers  of  leather  at  the  end  of  the  plugs  of  the  stop-cocks 
c  and  b,  as  the  pressure  of  the  mercury  which  is  to  be  afterwards 
introduced  has  a  tendency  to  force  them  out ;  if  this  should  happen, 
the  washers  prevent  any  great  escape  of  mercury. 

The  two  tubes  are  firmly  held  by  a  clamp  D,  on  which  rests  a  wide 
cylinder  E,  about  55  mm.  in  diameter,  surrounding  the  tubes,  and 
adapted  to  them  by  a  water-tight  caoutchouc  stopper  F.  The 
cylinder  is  maintained  in  an  upright  position  by  a  support  at  its 
upper  end  G,  sliding  on  the  same  rod  as  the  clamp.  Around  the 
upper  part  of  the  barometer  tube  a  siphon  H  is  fixed  by  means 
of  a  perforated  cork,  through  which  the  stop-cock  d  passes.  A 
small  bulb-tube  e,  containing  some  mercury,  is  also  fitted  in  this 
cork,  so  as  to  allow  of  the  air  being  entirely  removed  from  the 

*  J.  CJS.  [N.S.]  7,  313. 

2    N 


546 


GAS   ANALYSIS. 


Fig.   101. 


MC  LEOD'S  APPARATUS.  547 

siphon.  The  siphon  descends  about  100  mm.  within  the  cylinder, 
and  has  a  branch  at  the  top  communicating  by  caoutchouc  with 
a  bent  tube  contained  in  a  wider  one  J  affixed  to  the  support. 
A  constant  current  of  water  is  supplied  to  the  cylinder  through 
a  glass  tube,  which  passes  to  the  bottom,  and  escapes  through 
the  siphon  and  tubes  to  the  drain. 

To  the  end  of  the  narrow  tube  C  is  fastened  a  long  piece  of 
caoutchouc  tube  K,  covered  with  tape,  by  which  a  communication 
is  established  with  the  mercurial  reservoir  L,  suspended  by  a  cord 
so  that  by  means  of  the  winch  M,  it  may  be  raised  above  the  level 
of  the  top  of  the  barometer  tube.  As  the  mercury  frequently  forces 
its  way  through  the  pores  of  the  caoutchouc  tube,  it  is  advisable  to 
surround  the  lower  part  with  a  piece  of  wide  flexible  tube  ;  this 
prevents  the  scattering  of  the  mercury,  which  collects  in  a  tray 
placed  on  the  floor.  Into  the  bottom  of  the  tray  a  screw  must  be 
put,  to  which  the  end  of  the  glass  tube  is  firmly  attached  by  wire. 
The  capillary  stop-cock  a  is  provided  with  a  steel  cap,  by  means  of 
which  it  may  be  adapted  to  a  short  and  wide  laboratory  tube 
capable  of  holding  about  150  c.c.,  and  identical  in  form  with  the  one 
described  in  the  section  on  Water  Analysis  (p.  456).  The  mercurial 
trough  for  the  laboratory  tube  is  provided  with  a  stand  with  rings, 
for  the  purpose  of  holding  two  tubes  containing  gases  that  may  be 
required. 

The  apparatus  is  used  in  the  same  way  as  Frankland  and 
Ward's,  except  that  the  mercury  is  raised  and  lowered  in  the 
tubes  by  the  movement  in  the  reservoir  L,  instead  of  by  pouring  it 
into  the  centre  supply  tube. 

To  arrange  the  apparatus  for  use,  the  reservoir  L  is  lowered  to  the 
ground,  and  mercury  poured  into  it.  The  laboratory  tube  being 
removed,  the  stop-cocks  are  all  opened,  and  the  reservoir  gradually 
raised.  When  the  tube  A  is  filled,  the  stop-cock  a  is  closed,  and  the 
reservoir  elevated  until  mercury  flows  through  the  stop-cock  d  at 
the  top  of  the  barometer.  It  is  convenient  to  have  the  end  of  the 
tube  above  the  stop-cock  so  bent  that  a  vessel  can  be  placed  below 
to  receive  the  mercury.  This  bend  must,  of  course,  be  so  short 
that,  when  the  plug  of  the  stop-cock  is  removed,  the  siphon  will 
pass  readily  over.  When  the  air  is  expelled  from  the  barometer 
tube,  the  stop-cock  is  closed.  A  few  drops  of  water  must  next  be 
introduced  into  the  barometer  ;  this  is  accomplished  by  lowering 
the  reservoir  to  a  short  distance  below  the  top  of  the  barometer, 
and  gently  opening  the  stop-cock  d,  while  a  small  pipette,  from  which 
water  is  dropping,  is  held  against  the  orifice,  the  stop-cock  being 
closed  when  a  sufficient  amount  of  water  has  penetrated  into  the 
tube.  In  the  same  manner,  a  small  quantity  of  water  is  passed 
into  the  measuring  tube.  In  order  to  get  rid  of  any  bubbles  of  air 
which  may  still  linger  in  the  tubes,  the  reservoir  is  lowered  to  the 
ground  so  as  to  produce  a  vacuum  in  the  apparatus  ;  in  this  manner 
the  interior  surfaces  of  the  tubes  become  moistened.  The  reservoir 
is  now  gently  raised,  thus  refilling  the  tubes  with  mercury.  Great 

2  N  2 


548  GAS   ANALYSIS. 

care  must  be  taken  that  the  mercury  does  not  rush  suddenly  against 
the  tops  of  the  measuring  and  barometer  tubes,  which  might  cause 
their  destruction.  This  may  be  avoided  by  regulating  the  flow  of 
mercury  by  means  of  the  stop-cock  c,  which  may  be  conveniently 
turned  by  a  long  key  of  wood,  resting  against  the  upper  table  of 
the  sliding  stand  of  the  mercurial  trough.  When  the  reservoir 
has  again  been  elevated  above  the  top  of  the  barometer,  the  stop- 
cocks of  the  measuring  and  barometer  tubes  are  opened,  and  the 
air  and  water  which  have  collected  allowed  to  escape. 

The  heights  of  the  mercurial  columns  in  the  barometer,  correspond- 
ing to  the  different  divisions  of  the  measuring  tube,  have  now  to  be 
determined.  This  is  done  by  running  out  all  the  mercury  from  the 
tubes,  and  slowly  readmitting  it  until  the  meniscus  of  the  mercury 
just  touches  the  lowest  division  in  the  measuring  tube.  This  may 
be  very  conveniently  managed  by  observing  the  division  through 
a  small  telescope  of  short  focus,  and  sufficiently  close  to  the 
apparatus  to  permit  of  the  key  of  the  stop-cock  c  being  turned, 
while  the  eye  is  still  at  the  telescope.  When  a  reading  is  taken, 
the  black  screen  O  behind  the  apparatus  must  be  moved  by  means 
of  the  winch  P,  until  its  lower  edge  is  about  a  millimetre  above  the 
division.  The  telescope  is  now  directed  to  the  barometer  tube, 
and  the  position  of  the  mercury  carefully  noted.  As  the  tubes 
only  contain  aqueous  vapour,  and  are  both  of  the  same  temperature, 
the  columns  in  the  two  tubes  are  those  which  exactly  counter- 
balance one  another,  and  any  difference  of  level  that  may  be 
noticed  is  due  to  capillarity. 

The  same  operation  is  now  repeated  at  each  division  of  the  tube. 
The  measuring  tube  next  requires  calibration,  an  operation  per- 
formed in  a  manner  perfectly  similar  to  that  described  on  p.  457, 
namely,  by  filling  the  measuring  tube  with  water,  and  weighing  the 
quantities  contained  between  every  two  divisions.  The  eudiometer 
being  filled  with  water,  and  the  stop-cock  b  closed,  the  reservoir  is 
raised  and  the  mercury  allowed  to  rise  to  the  top  of  the  barometer. 
The  capillary  stop-cock  a  having  been  opened,  the  cock  b  is  gently 
turned,  and  the  water  allowed  to  flow  out  until  the  mercury  reaches 
the  lowest  division  of  the  tube.  A  carefully  weighed  flask  is  now 
supported  just  below  the  steel  cap,  the  stop-cock  b  again  opened, 
until  the  next  division  is  reached,  and  the  quantity  of  water  is 
weighed,  the  temperature  of  the  water  in  the  wide  cylinder  being 
observed.  The  same  operation  is  repeated  at  each  division,  and 
by  calculation  the  exact  contents  of  the  tube  in  cubic  centimetres 
may  be  found. 


MC  LEOD'S  APPARATUS.  549 

In  this  manner,  a  table,  such  as  the  following,  is  obtained  :— 


Division 
on 
measuring 
tube. 

Height  of  Mercury  in 
Barometer  tube 
corresponding  to 
division. 

Contents. 

Cubic  Centimetres. 

Log. 

1 

756-9 

8-689 

0-93898 

2 

706-7 

18-162 

1-25917 

3 

606-8 

36-931 

1-56739 

4 

506-5 

55-734 

1-74612 

5 

406-8 

74-430 

1-87175 

6 

306-8 

93-331 

1-97002 

7 

206-9 

112-417 

2-05083 

8 

107-0 

131-634 

2-11937 

9 

7-1 

151-162 

2-17944 

When  a  gas  is  to  be  analyzed,  the  laboratory  tube  is  filled  with 
mercury,  either  by  sucking  the  air  out  through  the  capillary  stop- 
cock while  the  open  end  of  the  tube  stands  in  the  trough,  or,  much 
more  conveniently,  by  exhausting  the  air  through  a  piece  of  flexible 
tube  passed  under  the  mercury  to  the  top  of  the  laboratory  tube, 
the  small  quantity  of  air  remaining  in  the  stop-cock  and  at  the  top 
of  the  wide  tube  being  afterwards  very  readily  withdrawn.  The 
face  of  one  of  the  steel  pieces  is  greased  with  a  small  quantity  of 
resin  cerate,  and,  the  measuring  apparatus  being  full  of  mercury, 
the  clamp  is  adjusted. 

Before  the  introduction  of  the  gas,  it  is  advisable  to  ascertain  if 
the  capillary  tubes  are  clear,  as  a  stoppage  may  arise  from  the 
admission  of  a  small  quantity  of  grease  into  one  of  them.  For  this 
purpose  the  globe  L  is  raised  above  the  level  of  the  top  of  the 
measuring  tube,  and  the  capillary  stop-cocks  opened  ;  if  a  free 
passage  exists,  the  mercury  will  be  seen  to  flow  through  the  tubes. 
The  stop-cock  of  the  laboratory  tube  is  now  closed.  When  all  is 
properly  arranged,  the  gas  is  transferred  into  the  laboratory  tube, 
and  the  stop-cock  opened,  admitting  a  stream  of  mercury.  The 
cock  c  is  gently  turned,  so  as  just  to  arrest  the  flow  of  mercury 
through  the  apparatus,  and  the  reservoir  lowered  to  about  the  level 
of  the  table,  which  is  usually  sufficient.  By  carefully  opening  the 
cock  c,  the  gas  is  drawn  over  into  the  measuring  tube,  and  when 
the  mercury  has  reached  a  point  in  the  capillary  tube  of  the 
laboratory  tube,  about  midway  between  the  bend  and  the  stop- 
cock, the  latter  is  quickly  closed.  It  is  necessary  that  this  stop- 
cock should  be  very  perfect.  This  is  attained  by  grinding  the  plug 
into  the  socket  with  fine  levigated  rouge  and  solution  of  sodium 
or  potassium  hydrate.  By  this  means  the  plug  and  socket  may 
be  polished  so  that  a  very  small  quantity  of  resin  cerate  and  a  drop 
of  oil  renders  it  perfectly  gas-tight.  In  grinding,  care  must  be 
taken  that  the  operation  is  not  carried  on  too  long,  otherwise  the 


550  GAS    ANALYSIS. 

hole  in  the  plug  may  not  coincide  with  the  tubes.  If  this  stop-cock 
is  in  sufficiently  good  order,  it  is  unnecessary  to  close  the  stop-cock 
a  during  an  analysis. 

The  mercury  is  allowed  to  flow  out  of  the  apparatus  until  its 
surface  is  a  short  distance  below  the  division  at  which  the  measure- 
ments are  to  be  taken.  The  selection  of  the  division  depends  on 
the  quantity  of  gas  and  the  kind  of  experiment  to  be  performed 
with  it.  A  saving  of  calculation  is  effected  if  all  the  measurements 
in  one  analysis  are  carried  on  at  the  same  division.  When  the 
mercury  has  descended  below  the  division,  the  cock  c  is  closed,  the 
reservoir  raised,  and  the  black  screen  moved  until  its  lower  edge 
is  about  a  millimetre  above  the  division,  and  the  telescope  so  placed 
that  the  image  of  the  division  coincides  with  the  cross-wires  in 
the  eye-piece.  The  stop-cock  c  is  now  gently  opened  until  the 
meniscus  just  touches  the  division  ;  the  cock  is  closed  and  the 
height  of  the  mercury  in  the  barometer  is  measured  by  means  of 
the  telescope.  The  difference  between  the  reading  of  the  barometer 
and  the  number  in  the  table  corresponding  to  the  division  at  which 
the  measurement  is  taken,  gives  in  millimetres  the  tension  of  the 
gas.  The  volume  of  the  gas  is  found  in  the  same  table,  and  with 
the  temperature  which  is  read  off  at  the  same  time  as  the  pressure, 
all  the  data  required  for  the  calculation  of  the  volume  of  the  gas 
at  0°  and  760  mm.  are  obtained.  No  correction  is  required  for 
tension  of  aqueous  vapour  ;  the  measuring  tube  and  barometer 
tube  being  both  moist,  the  tensions  in  the  tubes  are  counter- 
balanced. Absorptions  are  performed  with  liquid  reagents  by 
introducing  a  few  drops  of  the  liquid  into  the  laboratory  tube, 
transferring  the  gas  into  it,  and  allowing  the  mercury  to  drop  slowly 
through  the  gas  for  about  five  minutes.  The  gas  is  then  passed 
over  into  the  measuring  tube,  and  the  difference  of  tension  observed 
corresponds  to  the  amount  of  gas  absorbed.  It  is  scarcely  necessary 
to  add  that  the  greatest  care  must  be  taken  to  prevent  any  trace 
of  the  reagent  passing  the  stop-cock.  If  such  an  accident  should 
occur,  the  measuring  tube  must  be  washed  out  several  times  with 
distilled  water  at  the  conclusion  of  the  analysis.  If  the  reagent  is 
a  solution  of  potassium  hydrate  it  may  be  got  rid  of  by  introducing 
into  the  tube  some  distilled  water,  to  which  a  drop  of  sulphuric 
acid  has  been  added.  If  this  liquid  is  found  to  be  acid  on  removing 
it  from  the  tube,  it  may  be  presumed  that  all  the  alkali  has  been 
neutralized. 

When  explosions  are  to  be  performed  in  the  apparatus,  the  gas  is 
first  measured  and  then  returned  to  the  laboratory  tube.  A  quantity 
of  oxygen  or  hydrogen,  as  the  case  may  be,  which  is  judged  to  be  the 
proper  volume,  is  transferred  into  the  laboratory  tube,  and  some 
mercury  is  allowed  to  stream  through  the  gases  so  as  to  mix  them 
thoroughly.  The  mixture  is  next  passed  into  the  eudiometer  and 
measured.  If  a  sufficient  quantity  of  the  second  gas  has  not  been 
added,  more  can  readily  be  introduced.  After  the  measurement,  it 
may  be  advisable  to  expand  the  mixture,  in  order  to  diminish  the 


EXAMPLE    OF   AN   ANALYSIS.  551 

force  of  the  explosion.  This  is  done  by  allowing  mercury  to  flow 
out  from  the  tube  into  the  reservoir.  When  the  proper  amount  of 
expansion  has  been  reached,  the  stop-cocks  a  and  b  are  closed.  To 
enable  the  electric  spark  to  pass  between  the  wires,  it  is  necessary  to 
lower  the  level  of  the  water  in  the  cylinder.  For  this  purpose,  the 
bent  glass  tube  at  the  extremity  of  the  siphon  is  made  to  slide 
easily  through  the  cork  which  closes  the  top  of  the  wide  tube  J. 
By  depressing  the  bent  tube,  the  water  flows  out  more  rapidly  than 
before,  and  the  level  consequently  falls.  When  the  surface  is 
below  the  eudiometer  wires,  a  spark  from  an  induction-coil  is 
passed,  exploding  the  gas.  The  siphon  tube  is  immediately  raised, 
and,  when  the  water  in  the  cylinder  has  reached  its  original  level, 
the  gas  is  cool  enough  for  measurement.  900  c.c.  of  mercury  are 
amply  sufficient  for  the  whole  apparatus  ;  and  as  there  is  no  cement 
used  to  fasten  the  wide  tubes  into  iron  sockets,  a  great  difficulty  in 
the  original  apparatus  is  avoided. 

The  following  details  of  an  analysis,  in  which  absorptions  only 
were  performed,  will  show  the  method  employed.  The  gas  was 
a  mixture  of  nitrogen,  oxygen,  and  carbonic  anhydride,  and  the 
measurements  were  all  made  at  division  No.  1  on  the  eudiometer, 
which  has  been  found  to  contain  8*6892  c.c. 

Original  Gas. 

The  absorbing  liquids  required  are  :— 

For  carbon  dioxide  :  a  solution  of  caustic  potash  of  sp.  gr.  1*20. 
For  oxygen  :  the  same  solution,  to  which  some  saturated  solution 

of  pyrogallol  is  added. 
Temperature  of  water  in  cylinder,  15 '4°.  mm- 

Height  of  mercury  in  barometer  tube 980*5 

,,  ,,  corresponding  to  Division  No.  1  (see 

Table)     .        .        .        .    -   .  .?      ....      756-9 

Pressure  of  the  gas     -.        .        .        .        .        .        .        .        .      223-6 

After  absorption  of  the  carbonic  anhydride  by  solution  of 

potassium  hydrate — 
Height  of  mercury  in  barometer  tube         ....      941-7 

,,  ,,  corresponding  to  Division  No.  1      .      756-9 

Pressure  of  the  gas  after  removal  of  carbonic  anhydride    . 

Pressure  of  original  gas      .        .        .        .        .  =      .        , 

,,  gas  after  removal  of  carbonic  anhydride  . 

Tension  of  carbonic  anhydride          ,        .        .        .        .       V  38-8 

After  absorption  of  the  oxygen  by  potassium  pyrogallate — 

Height  of  mercury  in  barometer  tube         .        .                .  885-4 

,,             ,,                 corresponding  to  Division  No.  1  .  756-9 

Pressure  of  nitrogen    .        .        •       ,?        •        •           •-•••        •  128*5 

Pressure  of  oxygen  and  nitrogen      .        .        .        .....  184*8 

,,             nitrogen ,  .     .  .        .        •  128-5 

,,             oxygen     .       v     '-.  ,    " .        .v      .        ..      .,     *  56-3 


552 


GAS   ANALYSIS. 


These  measurements,  therefore,  give  us  the  following  numbers  : 


mm. 

128-5 
56-3 

^38-8 
223-6 


Pressure  of  nitrogen    . 
„  oxygen     . 

,,  carbonic  anhydride 

,,  original  gas 

If  the  percentage  composition  of  the  gas  is  required,  it  is  readily 
obtained  by  a  simple  proportion,  the  temperature  having  remained 
constant  during  the  experiment  :— 

223-6 

223-6 


223-6 


128-5 
56-3 

38-8 


100 
100 
100 


57-47  per  cent.  N 
25-18  per  cent.  O 
17-35  per  cent.  CO2 
lOCHK) 


8-6892x56-3 


760  x  LI  +  (0-003665  xl5.4)] 

8-6892x38-8 
760  x[l  +  (0-003665  xl5-4) 

8-6892x223-6 


If,  however,  it  is  necessary  to  calculate  the  number  of  cubic 
centimetres  of  the  gases  at  0°  and  760  mm.,  it  is  done  by  the 
following  formulae  :— 

8-6892x128-5 

1'39  C'C'  °f 


=0'61  C'C'  °f 


°'42  C'C'  °f  Carbonic 


'c'  of  the  origmal  gas' 


If  many  of  the  calculations  are  to  be  done,  they  may  be  very 
much  simplified  by  constructing  a  table  containing  the  logarithms 
of  the  quotients  obtained  by  dividing  the  contents  of  each  division 
of  the  tube  by  760  x  (1+OO03665Z).  The  following  is  a  very  short 
extract  from  such  a  table  :  — 


T°. 

Division  No.  1. 
8-6892 

Division  No.  2. 
18-1621 

Lug-  760  x  (1  +  80- 

Log'760x(l  +  80. 

15-0 

2-03492 

2-35511 

•1 

2-03477 

2-35496 

•2 

2-03462 

2-35481 

•3 

2-03447 

2-35466 

•4 

2-03432 

2-34451 

By  adding  the  logarithms  of  the  tensions  of  the  gases  to  those  in 
the  above  table,  the  logarithms  of  the  quantities  of  gases  are 
obtained  ;  thus  : — 


EXAMPLE    OF   AN    ANALYSIS. 


553 


Log.  corresponding  to  Division  No.  1, 

and  15-4° 

Log.  128 '5=  pressure  of  nitrogen 
Log.  of  quantity  of  nitrogen 

Volume  of  nitrogen  at  0°  and  760 
mm.     •  \ :, 

Log.  56*3=pressure  of  oxygen 

Log.  of  quantity  of  oxygen 

Volume  of  oxygen  at  0°  and  760 
mm. 


Log.  38-8  =  pressure  of  carbonic  anhy- 
dride   

Log.  of  quantity  of  carbonic  anhy- 
dride   

Volume  of  carbonic  anhydride  at 
0°  and  760  mm.  .        .     .  .        . 

Log.  223* 6=  pressure  of  original  gas 
Log.  of  quantity  of  original  gas 
Volumo  of  original  gas  at  0°  and 
780  mm.        .        .        . 

Nitrogen    .        .        . 
Oxygen      ...      .        ... 

Carbonic  anhydride      .."--.        . 
Total  . 


2-03432 
2-10890 
0-14322=log.  1'39 

1-39       c.c. 
2-03432   : 
1-75051 
1-78483  =log.  0-61 

0-61       c.c. 
2-03432 

1-58883 

1-62315 =log.  0-42 

0-42       c.c. 

2-03432 

2-34947 

0-38379 -log.  2-42 

2-42  c.c. 
1-39  c.c. 
0-61  c.c. 
0-42  c.c. 
2T42  c.c. 


The  following  example  of  an  analysis  of  coal  gas  will  show  the 
mode  of  working  with  this  apparatus,  and  the  various  operations  to 
be  performed  in  order  to  determine  the  carbonic  anhydride,  oxygen, 
hydrocarbons  absorbed  by  Nordhausen  sulphuric  acid,  hydrogen, 
methane,  carbonic  oxide,  and  nitrogen. 

The  measuring  tube  and  laboratory  tube  were  first  filled  with 
mercury,  some  of  the  gas  introduced  into  the  laboratory  tube,  and 
passed  into  the  apparatus. 

The  gas  was  measured  at  the  second  division. 

Height  of  mercury  in  the  barometer  tube      .        .  989-0 

„   •          ,,  „  measuring  tube      .        .  706-8 

Pressure  of  the  gas  at  16-6°  282-2 

Two  or  three  drops  of  a  solution  of  potassium  hydrate  were  now 
placed  in  the  laboratory  tube,  and  the  gas  passed  from  the 
measuring  tube,  the  mercury  being  allowed  to  drop  through  the  gas 
for  ten  minutes.  On  measuring  again — 

Height  of  mercury  in  barometer      .  •',  ...  ..1  ^      .      984-0 


554  GAS   ANALYSIS. 

Some  saturated  solution  of  pyrogallic  acid  was  introduced  into 
the  laboratory  tube,  and  the  gas  left  in  contact  with  the  liquid 
for  ten  minutes.  On  measuring  — 

Height  of  mercury  in  barometer      ....  983*6 

Height  of  mercury  when  measuring  original  gas  .  989-0 

„             ,,               after  absorption  of  C02  -.        .  984-0 

Pressure  of  CO2  5-Q 

,,             „               after  absorption  of  C02  -.        .  884-0 

,,             „               after  absorption  of  O        .        .  983-6 

Pressure  of  0  0-4 

The  volumes  of  the  gases  being  proportional  to  their  pressures, 
it  is  a  simple  matter  to  obtain  the  percentages  of  carbonic 
anhydride  and  oxygen  in  the  original  gas. 

Original  gas.     COa 

282-2     :     5-0     :  :     100     :     1-77  per  cent.  C02 

Original  gas.      O 

282-2     :     0-4     :  :     100     :     Q-14  per  cent.  0 


By  subtracting  1-91  from  100,  we  obtain  the  remainder,  98-09, 
consisting  of  the  hydrocarbons  absorbed  byNordhausen  sulphuric 
acid,  hydrogen,  carbonic  oxide,  marsh  gas,  and  nitrogen  ;  thus  :— 
Original  gas.        .        .      •  .v      .        .      V      .        .    100-00 
O  and  CO2    .  '      ........        1-91 

C^H,,,.  H2.  CO.  CH4.  N2.     .        .        .        .       .       .    "9fr09 

While  the  gas  remains  in  the  measuring  tube,  the  laboratory  tube 
is  removed,  washed,  dried,  filled  with  mercury,  and  again  attached 
to  the  apparatus.  Much  time  is  saved  by  replacing  the  laboratory 
tube  by  a  second,  which  was  previously  ready.  As  a  minute 
quantity  of  gas  is  lost  in  this  operation,  in  consequence  of  the 
amount  between  the  stop-cocks  being  replaced  by  mercury,  it  is 
advisable  to  pass  the  gas  into  the  laboratory  tube,  then  transfer  it 
to  the  eudiometer,  and  measure  again. 

On  remeasuring,  the  mercury  in  the  barometer 

stood  at     .        .        .        .        .        ,      .'.       .        .      983-3 
The  mercury  in  the  measuring  tube        .  %     .        .      706-8 


Pressure  of  CnH2n.  H2.  CO.  CH4.  N2.       276-5 

The  gas  is  again  passed  into  the  laboratory  tube,  and  a  coke  ball, 
soaked  in  fuming  sulphuric  acid,  left  in  contact  with  the  gas  for  an 
hour  ;  the  bullet  is  then  withdrawn,  and  some  potassium  hydrate 
introduced  and  left  in  the  tube  for  ten  minutes,  in  order  to  remove 
the  vapours  of  sulphuric  anhydride,  and  the  sulphurous  and  carbonic 
anhydrides  formed  during  the  action  of  the  Nordhausen  acid  on 
the  gas.  The  gas  is  now  measured  again. 


EXAMPLE    OF   AN   ANALYSIS.  555 

Height  of  mercury  in  barometer  tube     .        .        .  969-3 
,,             ,,             „             ,,           before  absorbing 

CnH2n 983-3 

„     after  .                        .  969-3 

Pressure  of  CnH2n  14-Q 

The  percentage  of  these  hydrocarbons  is  thus  found  : — 
Gas  containing  CnH2tl.  H2.  CO.  CH4.  N2. 

276-5     :     14-0     :  :     98-09     :     4-97  per  cent.  CnH2n. 

It  now  remains   to   determine   the   hydrogen,   carbonic   oxide, 
methane,   and  nitrogen  in   a  portion   of   the  residual  gas.     The 
laboratory  tube  is  therefore  removed,  some  of  the  gas  allowed  to 
escape,   and  another  laboratory  tube  adapted  to  the  apparatus. 
The  portion  of  gas  remaining  is  expanded  to  a  lower  ring  (in  this 
special  case  to  the  third  division),  and  the  tension  measured  : — 
Height  of  mercury  in  the  barometer  tube      .        .      642-2 
,,  ,,  ,,  measuring  tube      .        .      606-7 

Pressure  of  residue        35-5 

An  excess  of  oxygen  has  now  to  be  added.  For  this  purpose  the 
gas  is  passed  into  the  laboratory  tube,  and  about  five  times  its 
volume  of  oxygen  introduced  from  a  test  tube  or  gas  pipette.  The 
necessary  quantity  of  oxygen  is  conveniently  determined  by  the  aid 
of  rough  graduations  on  the  laboratory  tube,  which  are  made  by 
introducing  successive  quantities  of  air  from  a  small  tube  in  the 
manner  previously  described  for  the  calibration  of  the  eudiometers. 

After  the  introduction  of  the  oxygen,  the  mixed  gases  are  passed 
into  the  eudiometer  and  measured. 

Height  of  mercury  in  the  eudiometer  after  addition 

of  O    .        .       *.        .        .    ;  ^       .       V       •        •      789-5 

The  mixture  has  now  to  be  exploded,  and  when  the  pressure  is 
considerable,  it  is  advisable  to  expand  the  gas  so  as  to  moderate  the 
violence  of  the  explosion.  When  sufficiently  dilated,  the  stop-cock 
at  the  bottom  of  the  eudiometer  is  closed,  the  level  of  the  water 
lowered  beneath  the  platinum  wires  by  depressing  the  siphon,  and 
the  spark  passed.  The  explosion  should  be  so  powerful  that  it 
should  be  audible,  and  the  flash  visible  in  not  too  bright  daylight. 

The  stop-cock  at  the  bottom  of  the  eudiometer  is  now  opened,  and 
the  gas  measured. 

Height  of  mercury  in  barometer  after  explosion  .      732-5 

The  difference  between  this  reading  and  the  previous  one  gives 
the  contraction  produced  by  the  explosion  : 

Height  of  mercury  in  barometer  before  explosion       789-5 

after         „  732-5 

Contraction  =  C       57-0 


556  GAS   ANALYSIS. 

It  is  now  necessary  to  determine  the  amount  of  carbonic  anhydride 
formed.  This  is  done  by  absorbing  with  potassium  hydrate  as 
before  described. 

Height  of  mercury  in  barometer  tube  after  absorb- 
ing CO2 715-8 

This  number  deducted  from  the  last  reading  gives  the  carbonic 
anhydride. 

Height  of  mercury  in  barometer  after  exploding  .      732-5 

„  „  „  „      after  absorbing  CO 2      715-8 

Carbonic  anhydride  =D        16*7 

It  now  remains  to  determine  the  quantity  of  oxygen  which  was 
not  consumed  in  the  explosion,  and  which  excess  now  exists  mingled 
with  the  nitrogen.  For  this  purpose,  a  volume  of  hydrogen  about 
three  times  as  great  as  that  of  the  residual  gas  is  added,  in  the  same 
way  as  the  oxygen  was  previously  introduced,  and  the  pressure  of 
the  mixture  determined. 

Height  of  mercury  in  barometer  after  adding  H  1031 '3 
This  mixture  is  exploded  and  another  reading  taken. 
Height  of  mercury  in  barometer  after  exploding 

with  H 706-7 

This  number  subtracted  from  the  former,  and  the  difference 
divided  by  3,  gives  the  excess  of  oxygen. 

Height  of  mercury  in  barometer  before  exploding 

with  H 1031-3 

Height  of  mercury  in  barometer  after  exploding 

with  H 706-7 

3)  ^2^6 
Excess  of  oxygen      108-2 

In  order  to  obtain  the  quantity  of  nitrogen  in  the  gas  analyzed, 
this  number  has  to  be  deducted  from  the  volume  of  gas  remaining 
after  the  explosion  with  oxygen  and  the  removal  of  the  carbonic 
anhydride. 

Height  of  mercury  in  barometer  after  absorbing 

CO2 .  715-8 

,,             ,,            in  eudiometer  at  division  No.  3  606-7 

Nitrogen  and  excess  of  oxygen         ....  109-1 

Excess  of  oxygen         . 108-2 

Nitrogen  Q-9 

We  have  now  all  the  data  necessary  for  the  calculation  of  the 
composition  of  the  coal  gas.  It  is  first  requisite  to  calculate  the 
proportion  of  the  combustible  gas  present  in  the  coal  gas,  which  is 
done  by  deducting  the  sum  of  the  percentages  of  gas  determined  by 
absorption  from  100. 


EXAMPLE    OF   AN    ANALYSIS.  557 

Percentage  of  carbonic  anhydride    .        .        .  1-77 

„                 oxygen         .        .        ...        .  0-14 

CnH2n           .        ...        .    -.-.  4j97_ 

C02.  O2.  C  H2u  6-88 

Original  gas  .        .        .        ....        .        .  100-00 

C02.  Oa.  C   H2a.         ...     -  .       ||     .        .  6-88 

H2.  CO.  CH4.  N2  93-12 

The  formulae  for  the  calculation  of  the  analysis  of  a  mixture  of 
hydrogen,  carbonic  oxide,  and  methane,  are  (see  page  534) — 

Hydrogen          =#=*A  —  D 

3A-2C+D 

Carbonic  oxide —y  =  — 


Methane  =z  = 


3 
2C-3A+2D 


3 

A  =  35-5-0-9=34-6 
C  =  57-0 
D  =  16-7 
A=  34-6 
P  =  16-7 

17-9=a;=  Hydrogen  in  35-5  of  the  gas  exploded 

with  oxygen. 

A=  34-6  C  =  57-0 

_  3  _2 

A  =  103:8  2C  =  114-0 

D  =   16-7 


3A+D=l20r5 

2C  =  114-0 

3)     6-5 


_ 
=  2-17  =y  =  Carbonic  oxide  in  35-5  of  the  gas. 


D=   16-7 
_2 

2D  =  ^33T4 
2C  =  114-0 
=  147-4 
3A  =  103-8 


3)  43-6=2D  +  2C-3A 

OT)    I    Op Q  A     ~ 

— ^—     —"as   14-53  =z=Methane  in  35-5  of  the  gas. 

These  numbers  are  readily  transformed  into  percentages,  thus  : — 
35-5  :  17-9     :  :  93-12  :  46-95  per  cent,  of  Hydrogen. 
35-5  :     2-17  :  :  93-12  :     5-68  per  cent,  of  Carbonic  oxide. 
35-5  :  14-53  :  :  93-12  :  38-12  per  cent,  of  Methane. 
35-5  :     0-9     :  :  93-12  :     2-36  per  cent,  of  Nitrogen. 


558  GAS   ANALYSIS. 

This  completes  the  calculations,   the  results  of  which  are  as 
follows  : — 

Hydrogen      .        .       ..        .        .46-95 

Methane        .        .        .        .        .      38-12 

Cn  H2n  .          .        .        .        .        .        4-97 

Carbonic  oxide     .        .        .        .        5-68 

Carbonic  anhydride     .        .        .        1-77 

Oxygen 0-14 

Nitrogen        .        .        .        .        .        2-36 

99799 

It  is  obvious  that  this  analysis  is  not  quite  complete,  since  it  does 
not  give  any  notion  of  the  composition  of  the  hydrocarbons  ab- 
sorbed by  the  Nordhausen  acid.  To  determine  this,  some  of  the 
original  gas,  after  the  removal  of  the  carbonic  anhydride  and 
oxygen,  is  exploded  with  oxygen,  and  the  contraction  and  carbonic 
anhydride  produced  are  measured.  The  foregoing  experiments 
have  shown  the  effect  due  to  the  hydrogen,  carbonic  oxide,  and 
methane,  the  excess  obtained  in  the  last  explosion  being  obviously 
caused  by  the  hydrocarbons  dissolved  by  the  sulphuric  acid,  and 
from  these  data  the  composition  of  the  gas  may  be  calculated. 

It  may  be  remarked  that  analyses  of  this  kind  were  performed 
with  the  apparatus  at  the  rate  of  two  a  day  when  working  for 
seven  hours. 

It  may  be  useful  to  show  how  this  analysis  appears  in  the 
laboratory  note-book  : 

Analysis  of  Coal  Gas. 
989-0  V.0v  989-0  984-0 

93 


282 -2 J    g  5-0  =  C02       0-4  =  0 

984-0     Aft.  absorb.  C02  ~~282-2  :  5-0  :  :  100  :   1-77  C0a 

282-2  :  0-4  :  :   100  :  0-14  0 

983-6    Aft.  absorb.  O  1-91 

100-00 
983-3     Remeasured 

98-09  Cn  H2n.  H.  CO.  CH4. 

969-3  Aft.  absorb.  Cn  H2n  "    983  "3          983-3 
706-8          969-3 


276-5  14-0  Cn  H2n 

642-2  *) 
606-7  I  Portion  of 

C     Residue  276-5  :  14'0  :  :  98'09  :  4'97  Cn  H2n 

35-5; 

C02=l-77 

789-5     with  O  35-5  =H.  CO.  CH4.  N.  O=0'14 

0-9  =N  CnH2n=4-97 

732-5     Aft.  expl.  3£6  =H.  CO.  CH4  =A  6-88 


715-8  Aft.  absorb.  C02 

1031-3     with  H 
706-7     Aft.  expl. 


EXAMPLE    OF   AN   ANALYSIS. 

789-5 
732-5 

57 -Q  =  con  traction 
103T3  715-8 

706-7  606-7 


559 


732-5 
715-8 

16-7=CO,=:D 


3)324-6 
108-2=0 

H=a:=A-D       =   17'9 
=  2-17 


109-1  =N+O 
108-2=0 


O'l)  =N 


34-60 


34-6  =A 
16-7  =D 

17-9  =z=H 


57-0=C 

2 


114-0=20 


34-6     =A 


103-8     =3A 
16-7      =D 


120-5 
114-0 


=3A+D 
=20 


3)     6-5     =3A+D-2C 
2-17=y=CO 


100-00 

6-88  CO.  0.  CUH21 


93-12  H.  CO.  CH4.  N. 
H 

CH4 
CnH2 
CO 
C02 

o 

N 


35-5 
35-5 
35-5 
35-5 


16-7 
2 


33-4 
114-0 


D 


=2D 
=20 


147-4     =2C+2D 
103-8     =3A 

3)  43-6     =2D+2C-3A 


14-53 

=2=CH4 

17-9     :  : 

93.12  : 

"  46-95  H 

2-17  :  : 

93.12  : 

5-68  CO 

14-53  :  : 

93-12  : 

36-12  CH4 

0-9     :  : 

93-12  : 

2-36  N 

46-95 

38-12 

4-97 

5-68 

1-77 

0-14 

2-36 

99-99 


It  is  assumed,  in  the  above  example,  that  the  temperature  of  the 
water  in  the  cylinder  remained  constant  throughout  the  period 
occupied  in  performing  the  analysis.  As  this  very  rarely  happens, 
the  temperature  should  be  carefully  read  off  after  every  measurement 
of  the  gas  and  noted,  in  order  that  due  correction  be  made  for  any 
increase  or  decrease  of  volume  which  may  result  in  consequence. 


THOMAS'S    MODIFIED    GAS    APPARATUS. 

In  the  Chemical  Society's  Journal  for  May,  1879,  Thomas 
described  an  apparatus  for  gas  analysis  (fig.  102)  which  has  the 
closed  pressure  tube  of  Frankland  and  Ward,  and  is  supplied 
with  mercury  by  means  of  the  flexible  rubber  tube  arrangement  of 


560 


GAS    ANALYSIS. 


Me  Leod.  The  manner  in  which  this  apparatus  is  filled  with 
mercury  and  got  into  order  for  working  is  so  similar  to  that  already 
described  that  no  further  reference  need  be  made  thereto. 


Fig.  102. 

The  eudiometer  is  only  450  mm.  long  from  shoulder  to  shoulder, 
and  the  laboratory  tube  and  mercury  trough  are  under  the  command 
of  the  operator  from  the  floor  level.  The  eudiometer  has  divisions 
20  mm.  apart,  excepting  the  uppermost,  which  is  placed  as  close 
beneath  the  platinum  wires  as  is  convenient  to  obtain  a  reading. 


THOMAS'S  APPARATUS.  561 

The  method  explained  in  sequel  of  exploding  combustible  gases 
under  reduced  pressure,  without  adding  excess  of  gas  to  modify  the 
force  of  the  explosion,  permits  the  shortening  of  the  eudiometer  as 
above,  and  enables  the  apparatus  to  be  so  erected  that  a  long 
column  of  the  barometer  tube  shall  stand  above  the  summit  of  the 
eudiometer.  By  means  of  such  an  arrangement  a  volume  of  gas 
may  be  measured  under  nearly  atmospheric  pressure,  and  as  this 
pressure  is  equal  to  more  than  700  mm.,  plus  tension  of  aqueous 
vapour,  the  sensitiveness  of  the  apparatus  is  considerably  aug- 
mented. The  barometer  tube  is  1000  mm.  in  length,  having  about 
700  mm.  lines  above  Division  2  on  the  eudiometer.  The  steel 
clamp  and  facets  forming  the  connections  between  the  eudiometer 
and  detachable  laboratory  tube  of  the  apparatus  previously  described 
are  dispensed  with,  as  in  this  form  the  eudiometer  and  laboratory 
vessels  are  united  by  a  continuous  capillary  tube,  12  mm.  (outside) 
diameter,  and  one  three-way  glass  tap  is  employed  in  lieu  of  the 
two  stop-cocks.  The  arrangement  is  simple.  The  glass  tap  is 
hollow  in  the  centre,  and  through  this  hollow  a  communication  is 
made  with  the  capillary,  by  means  of  which  either  the  laboratory 
tube  or  the  eudiometer  can  be  washed  out.  As  the  laboratory 
vessel  is  not  disconnected  for  the  removal  of  the  reagent  used  in  an 
absorption,  it  is  supported  by  a  clamp,  as  shown  in  the  drawing  ; 
and  when  it  requires  washing  out  the  mercury  trough  is  turned  aside 
in  order  that  an  enema  syringe  may  be  used  for  injecting  a  stream 
of  water.  A  few  drops  of  water  are  let  fall  into  the  hollow  of  the 
tap,  and  blown  through  the  capillary  tube  three  times  in  succession, 
so  as  to  get  rid  of  the  absorbent  remaining  in  the  capillary,  then  the 
syringe  is  brought  into  play  once  more,  the  excess  of  water  removed 
by  wiping,  and  the  trough  turned  back  into  position.  The  laboratory 
tube  may  be  refilled  with  mercury,  as  described  on  page  549,  but 
it  will  be  found  much  more  serviceable  if  a  double-acting  syringe, 
connected  to  a  bulb  apparatus  (to  catch  any  mercury  that  may 
come  over),  and  then  to  the  orifice  of  the  hollow  in  the  tap  by 
a  ground  perforated  stopper,  be  used,  as  this  will  obviate  the 
destructive  effect  of  heavy  suction  upon  the  gums  and  teeth.  The 
mercury  trough  is  supported  upon  a  guide  which  travels  over  the 
upright  U,  and  is  turned  aside  for  the  purpose  of  washing  out  the 
laboratory  vessel  in  the  following  manner  : — The  spiral  spring  is 
depressed  by  means  of  the  tension  rods  until  the  slot  is  brought 
below  the  stud  fixed  in  the  upright  U  ;  and  the  top  ferrule  holding 
the  guide  rods  being  movable,  the  trough  can  b'e  turned  round  out 
of  the  way,  but  is  prevented  from  coming  in  contact  with  the 
glass  water-cylinder  by  an  arrangement  in  the  top  of  the  guide, 
which  comes  against  the  stud  in  the  upright.  The  height  of  the 
trough  can  be  accurately  adjusted  by  the  screw  in  the  top  of  the 
lever  guide.  When  the  trough  is  in  position,  the  clamp  holding  the 
laboratory  vessel  may  be  loosed  when  necessary. 

The  eudiometer  and  barometer  tubes  pass  through  an  india- 
rubber  stopper,  as  in  Me  Leod's  apparatus,  but  are  not  supported 


562  GAS   ANALYSIS. 

by  the  clamp  C,  which  here  simply  bears  the  water-cylinder.  No 
glass  stop-cocks  are  used,  or  glass  work  of  any  kind  employed  in  the 
construction  of  the  lower  portion  of  the  apparatus.  The  lower  end 
of  the  eudiometer  has  a  neck  of  the  same  outside  diameter  as  the 
barometer  tube  (9*5  mm.),  and  both  tubes  are  fixed  into  the  steel 
block  X,  without  rigidity,  by  the  usual  steam  cylinder  gland 
arrangement,  small  india-rubber  rings  being  used  to  form  the  pack- 
ing. The  steel  block  is  fixed  to  the  table  by  a  nut  screwed  upon  the 
f-inch  hydraulic  iron  tube,  which  runs  to  the  bottom  of  the  table. 
The  tap  in  the  steel  block  is  so  devised  that  it  first  cuts  off  connection 
with  the  barometer  tube,  in  order  that  the  gas  may  be  drawn  over 
from  the  laboratory  vessel  into  the  eudiometer  without  risking  the 
fracture  of  the  upper  end  of  the  barometer  tube  by  any  sudden 
action  of  the  mercury.  This  precaution  is  necessary,  as  during  the 
transferring  of  the  gas  the  mercury  in  the  barometer  tube  is  on  the 
point  of  lowering,  to  leave  a  vacuous  space  in  the  summit  of  the 
tube.  By  moving  the  handle  a  little  further  on  the  quadrant 
a  communication  is  made  with  both  tubes  and  the  reservoir  for  the 
purpose  of  bringing  the  gas  into  position,  so  as  to  take  a  reading  ; 
then  the  handle  is  drawn  a  little  further  to  cut  off  the  reservoir 
supply,  whilst  there  is  a  way  still  left  between  the  eudiometer  and 
barometer  tubes,  and  if  the  handle  be  drawn  forward  a  little  more, 
all  communication  is  cut  off  for  the  purpose  of  exploding. 

The  windlass  P,  for  raising  and  lowering  the  mercury  reservoir  L, 
is  placed  beneath  the  table,  in  order  that  it  may  be  under  command 
from  a  position  opposite  the  laboratory  vessel,  and  it  is  furnished 
with  a  spring  ratchet  motion,  so  as  to  be  worked  by  one  hand.  The 
water-cylinder  should  be  four  inches  in  diameter,  and  the  casing 
tube  of  the  barometer  as  wide  as  practicable,  so  that  the  temperature 
of  the  apparatus  may  be  maintained  as  constant  as  possible.  To 
attain  an  accurate  result  it  is  as  essential  to  keep  the  barometer 
tube  of  uniform  temperature  as  the  eudiometer,  since  the  tension  of 
aqueous  vapour  varies  proportionately.  The  stream  of  water  from 
the  service  main  is  run  into  the  casing  tube  at  the  upper  end  of  the 
barometer,  and,  whilst  the  water-cylinder  is  filling,  the  tap  at  the 
bottom  is  opened  slightly,  so  that  water  may  run  out  very  slowly. 
When  the  water-cylinder  is  full,  the  upright  tube  G  acts  as  a  siphon, 
and  sucks  out  the  excess  of  water  from  the  top  of  the  cylinder,  thus 
keeping  up  the  circulation  at  the  point  where  it  is  most  required. 
For  a  further  detailed  description  of  the  apparatus  see  J.  C.  S., 
May,  1879. 

There  are  only  two  working  taps  upon  this  apparatus — the  three- 
way  glass  tap  between  the  eudiometer  and  laboratory  tube,  and  the 
steel  cap  at  the  lower  ends  of  the  barometer  and  eudiometer.  The 
steel  cap  is  greased  with  a  little  beef -tallow  (made  from  clean  beef- 
suet),  or  with  real  Russian  tallow  ;  it  will  last  for  twelve  months 
without  further  attention.  A  moderately  thick  washer  of  india- 
rubber,  placed  between  the  steel  washer  and  the  nut  at  the  end  of 
the  steel  tap,  adds  greatly  to  the  steady  working  of  the  needle  on 


THOMAS'S  APPARATUS. 


563 


the  quadrant.     Moderately  soft  resin  cerate  is  best  for  the  glass 
tap. 

When  filling  the  laboratory  vessel  with  mercury,  suction  is 
maintained  until  the  mercury  has  reached  some  height  in  the  hollow 
of  the  three-way  tap.  The  remainder  of  the  hollow  space  is  re- 
plenished by  pouring  the  mercury  from  a  small  crucible  ;  any  water 
that  may  be  present  is  then  removed,  and  the  small  stopper  inserted. 
When  the  laboratory  vessel  has  to  be  washed  out  after  an  absorption, 
the  gas  is  transferred  to  the  eudiometer  until  the  absorbent  gets 
within  a  quarter  of  an  inch  of  the  stop-cock.  The  mechanical 
arrangement  should  be  so  manageable  that  this  nicety  of  adjustment 
can  be  accomplished  with  ease.  Much  depends,  of  course,  upon  the 
care  bestowed  in  cerating  the  tap,  so  that  the  capillary  is  not 
carelessly  blocked  up.  As  soon  as  the  gas  has  passed  over  to  the 
extent  required,  turn  the  three-way  tap  until  the  through-way  is 
at  right-angles  to  the  capillary,  and  the  way  to  the  hollow  of 
the  tap  is  in  communication  with  the  laboratory  vessel,  then 
take  out  the  little  stopper  from  the  hollow,  so  that  the  mercury 
shall  flow  out,  and  allow  the  laboratory  vessel  to  become  emptied 
whilst  the  reading  of  the  volume  of  the  gas  is  being  taken.  The 
best  arrangement  for  washing  out  the  laboratory  tube  is  a  "  siphon 
enema,"  fig.  103  (Dr.  Higginson's  principle,  which  may  be 
obtained. of  any  druggist),  adapting  in  the  place  of  the  usual  nozzle 
a  bent  glass  tube.  This  syringe  is  constant  in  its  action,  as  it  fills 
itself  when  the  pressure  is  released,  if  the  tube  at  the  lower  end  is 
placed  in  a  vessel  of  water.  The  laboratory  vessel  can  be  washed 
out  and  refilled  in  a  very  little  time,  as  it  is  already  connected,  and 
for  all  ordinary  absorptions  it  is  sufficient  to 
wipe  the  vessel  out  once  by  passing  up  a  fine 
towel  twisted  on  a  round  stick.  When 
Cn  H2tl  gases  are  to  be  absorbed  by  fuming 
sulphuric  acid,  the  water  should  be  carefully 
blown  out  of  the  capillary  tube  into  the 
laboratory  vessel,  which  must  be  repeatedly 
dried.  A  few  drops  of  strong  sulphuric  acid 
were  at  first  run  into  the  hollow  of  the  tap 
and  then  through  the  capillary  whilst  the 
laboratory  vessel  was  full  of  mercury,  in 
order  to  remove  any  moisture  remaining,  but 
it  has  since  been  found  unnecessary,  as  the 
drying  can  be  performed  thoroughly  without. 
To  calibrate  the  eudiometer  with  water, 
introduce  the  quantity  required  through  the 
hollow  in  the  stopper,  then  remove  the 
latter,  and  collect  the  water  in  a  light  flask 
from  the  bottom  of  the  tap-socket. 

In  the  same  paper  (J.  C.  S.,  May,  1879), 
Thomas     pointed     out    that    it    was    not 


Fig.  103. 


essential  to  add  excess  of  either  oxygen  or  hydrogen  for  the  purpose 

2  o  2 


564 


GAS    ANALYSIS. 


of  modifying  the  force  of  the  explosion  when  combustible  gases 
were  under  analysis,  and  it  is  necessary  to  take  advantage  of  this 
when  working  with  so  short  an  eudiometer.  The  method  is,  how- 
ever, applicable  to  all  gas  apparatus  having  a  reasonable  length  of 
barometer  column  above  the  eudiometer  ;  in  fact,  the  exploding 
pressures  were  first  worked  out  and  employed  in  an  apparatus  on 
Me  Leod's  model.  When  the  percentage  of  oxygen  in  a  sample 
of  air  has  to  be  determined  by  explosion,  only  one-half  its  volume 
of  hydrogen  is  required,  and  the  pressure  need  not  be  reduced 
below  400  mm.  If  much  more  than  one-half  volume  of  hydrogen 
has  been  added  by  accident,  explode  under  atmospheric  pressure. 
When  the  excess  of  oxygen  used  in  an  analysis  has  to  be  determined, 
add  2\5  times  its  volume  of  hydrogen,  and  reduce  the  pressure  to 
180  mm.  of  mercury  before  exploding.  After  adding  the  hydrogen 
and  taking  the  reading,  the  gas  is  expanded  by  lowering 
the  mercurial  reservoir  until  a  column  of  mercury,  measuring  the 
number  of  mm.  in  length  just  referred  to  and  in  the  following 
table,  stands  above  the  meniscus  of  the  mercury  in  the  eudiometer. 
This  column  can  be  read  off  quite  near  enough  by  the  eye,  as  there 
is  no  risk  of  breaking  the  apparatus  by  the  force  of  the  explosion  if 
the  pressure  is  20  mm.  greater  than  that  given  ;  but  if  the  gas  under 
analysis  is  all  combustible,  it  is  better  to  explode  at  a  slightly  less 
pressure  than  to  exceed  that  recommended.  It  follows,  naturally, 


Name  of  Gas. 

Volume  of 
Combustible 
Gas. 

Volume  of 
Oxygen  to  be 
added. 

*w 
00       bO 

£££;§ 

P-P  o^ 

IP! 

Ok          o 

Hydrogen    
Carbonic  Oxide      .... 
Methane      
Acetylene    
Ethylene      

1 

1 

1 

2-5 
3 
3-5 

200  mm. 
200  mm. 
170  mm. 
150  mm. 
145  mm. 

Ethane  and  Hydride  of  Ethyl 
Propvl   . 

4 
5 

140  mm. 
135  mm. 

Hydride  of  Propyl 
Butyl     
Butane  and  Hydride  of  Butyl 

1 

5-5 

6 

7 

130  mm. 
125  mm. 
120  mm. 

that  the  exploding  pressure  will  depend  upon  the  proportion  of 
combustible  gas  introduced  ;  and  experience  alone  can  enable  one 
to  determine  with  any  degree  of  exactness  what  that  pressure  must 
be,  as  no  general  law  can  be  laid  down.  For  instance,  if  more  than 
three  volumes  of  hydrogen  were  added  to  one  of  oxygen,  the 
exploding,  pressure  should  exceed  200  mm.  ;  and  if  much  nitrogen 
or  other  gas  were  present  that  did  not  take  a  part  in  the  reaction, 
the  pressure  should  be  still  more  increased.  As  a  consequence,  the 


EXPLODING   PRESSURES. 

same  experience  is  necessary  when  dealing  with  explosive  gases  by 
the  other  method,  because  the  addition  of  too  much  inert  gas,  with 
a  view  to  modify  the  force  of  the  explosion,  may  lead  to  imperfect 
combustion,  inasmuch  as  the  cooling  effect  of  the  tube  and  gas  can 
reduce  the  temperature  below  that  required.  In  all  instances,  when 
the  approximate  composition  of  the  gas  is  known,  it  is  not  difficult 
to  determine  the  quantity  of  oxygen  or  hydrogen,  as  the  case  may 
be,  which  is  required  for  explosion,  or  the  pressure  under  which  the 
gas  should  be  exploded.  In  order  to  do  this  systematically,  it  is 
always  well  to  remember  certain  points  observed  during  the  stages 
of  the  analysis.  The  gas  in  the  laboratory  vessel,  before  being 
transferred  to  the  eudiometer,  occupies  a  certain  volume  in  a  position 
between  (or  otherwise)  the  calibration  divisions.  After  transferring 
and  reading  off,  bear  in  mind  the  number  of  millimetres  which  the 
volume  represents  ;  and  calculate,  as  the  gas  is  being  re-transferred 
to  the  laboratory  vessel  to  be  mixed  with  that  employed  in  the 
explosion,  the  height  at  which  the  mercury  should  stand  in  the 
barometer  tube  when  measuring  the  mixed  gases,  and  how  much 
of  the  laboratory  vessel  was  occupied  on  a  previous  occasion  when 
a  similar  reading  was  obtained.  If  this  is  done,  one  can  realize  at 
once,  after  reading  off  the  volume  of  the  mixed  gases,  the  proportion 
of  combustible  gas  added,  and  the  pressure  under  which  the  gas 
has  been  measured.  Another  glance  at  the  volume  which  the  gas 
occupies  in  the  eudiometer,  with  a  comparison  of  the  pressure 
recorded  upon  the  barometer  tube,  enables  one,  after  a  little  practice, 
at  once  to  expand  the  mixture  to  the  point  at  which  it  will  explode 
with  satisfactory  results.  It  is  not  expedient  to  place  too  much 
reliance  upon  the  marks  showing  equal  volumes  upon  the  laboratory 
vessel,  especially  when  dealing  with  small  quantities  of  gas  ;  and 
a  comparison  of  the  volumes  obtained  in  reading  before  and  after 
the  addition  of  oxygen  or  hydrogen  is  always  prudent,  in  order  to 
see  that  sufficient  gas  has  been  added,  as  well  as  to  enable  one  to 
judge  the  pressure  under  which  the  gas  should  be  exploded. 

NOTE. — Meyer  and  Seubert  (Z.  a.  C.  24,  414)  have  designed  a  gas  apparatus 
similar  in  many  respects  to  that  of  M  c  L  e  o  d  and  Thomas,  but  of  simpler  con- 
struction, and  especially  adapted  for  explosions  under  diminished  pressure. 


SO  BEAU'S    GAS    APPARATUS. 

This  form  of  instrument  is  shown  in  fig.  104,  and  is  described  in 
a  paper  read  by  W.  H.  Sodeau  before  the  Newcastle  Section  of  the 
Society  of  Chemical  Industry,  and  is  printed  in  full  in  the  journal  of 
that  Society  (xxii.  187).  It  is  an  improved  form  of  an  instrument 
previously  devised  by  Macfarlane  and  Caldwell,  and  in  its 
present  state  is  adapted  for  gas  analysis  of  the  highest  accuracy.* 
In  addition  to  this,  its  cost  is  much  less  than  most  of  those  which 
have  been  previously  described. 

*  Brady  and  Martin  of  Newcastle -on-Tyne  are  the  original  makers. 


566 


GAS   ANALYSIS. 


DESCRIPTION  OF  THE  APPARATUS. — The  measuring  parts  are 
fitted  into  a  glass  water  jacket,  which  is  held  in  position  by  means 
of  a  cork  at  A  and  a  band-clip  at  B.  The  measuring  tube  M  is  of 
50  c.c.  capacity  graduated  in  -^  c.c.  Its  upper  end  terminates  in 
a  capillary  bearing  a  three-way  stop-cock.  When  examining 
samples  which  leave  a  large  residue  after  absorption,  etc.,  it  is 
convenient  to  replace  the  usual  tube  of  uniform  diameter  by  one 
having  a  bulb  at  its  upper  end.  The  zero-point  is  at  the  outer  side 


104. 


of  the  three-way  stop-cock  N,  which  is  placed  horizontally.  The 
bent  tube  U,  partly  filled  with  water,  can  be  connected  with  the 
capillary  K  through  the  stop-cock.  The  level  tube  L  is  straight, 
with  stop-cock  at  top,  and  communicates  with  the  measuring  tube 
by  a  branch  so  bent  as  to  prevent  any  bubbles  rising  from  below 
from  passing  into  the  measuring  tube.  The  lower  end  is  connected 
to  a  T  piece,  one  end  of  which  has  a  stop-cock  and  leads  to  the 
mercury  reservoir,  and  the  other  is  prolonged  across  the  table  to 
a  point  near  the  reading  telescope,  where  it  terminates  in  a  thick 
piece  of  rubber  tubing  compressed  by  a  screw  clip  having  a  broad 


567 

bearing  surface.  This  is  used  as  a  fine  adjustment  when  levelling 
the  mercury.  The  graduations  are  on  the  side  next  the  telescope, 
and  the  stop-cocks  are  worked  from  the  opposite  side.  This 
arrangement  renders  it  possible  to  have  the  reading  telescope  on 
the  gas  analysis  table  instead  of  on  a  separate  support,  and  so  adds 
to  the  compactness  and  convenience  of  the  apparatus.  The 
graduations  can  be  illuminated  by  an  electric  lamp  behind  a  ground 
glass  screen,  having  its  upper  portions  rendered  opaque  in  order  to 
prevent  troublesome  reflection  of  light  from  the  surface  of  the 
mercury. 

Correction  for  variation  of  temperature  and  pressure  is  simplified 
by  the  use  of  the  "  Kew  principle  "  correction  tube  C.  This  consists 
of  a  cylindrical  bulb  provided  with  a  stop-cock  and  attached  to  a  U 
tube  graduated  on  the  narrow  limb  and  partially  filled  with  water. 
The  volume  of  air  contained  in  the  bulb  is  such  that  the  water  is 
displaced  to  the  extent  of  one  small  division  by  a  change  of 
temperature  and  atmospheric  pressure  which  will  cause  a  gas  to 
experience  an  alteration  of  volume  amounting  to  O'l  per  cent.* 
The  scale  is  observed  by  means  of  a  mounted  lens  and  the  small 
divisions  are  further  sub-divided  into  tenths  by  eye  estimation. 
The  water  is  brought  approximately  to  the  zero  mark  at  the  begin- 
ning of  an  analysis  by  momentarily  opening  the  stop-cock,  and  the 
corrections  are  read  directly  in  percentages  as  easily  as  the 
temperatures  would  be  read  by  means  of  a  thermometer. 

METHOD  OF  PROCEDURE  :  Introduce  the  gas  into  the  measuring  tube  M  by 
lowering  the  reservoir.  Roughly  level  the  mercury  and  turn  the  stop-cock  N  so 
as  to  connect  K  with  the  tube  U.  Place  the  absorption  pipette  in  position  and 
connect  it  to  the  measuring  apparatus  by  thick-  walled  rubber  tubing,  the  glass 
ends  being  made  to  meet. 

Suck  a  little  water  from  U  into  F,  and  allow  mercury  to  run  back  and  fill  the 
capillaries. 

Let  the  capacity  of  the  bulb,  together  with  that  of  the  portion  of  the  tube 
which  is  above  the  zero  point  =X  c.c.,  and  assume  the  atmospheric  pressure  to  be 
760  mm.  Then  if  a  change  which  would  lead  to  a  1  per  cent,  increase  of  volume 
is  to  give  0*5  c.c.  displacement  of  water,  and  this  results  in  a  disturbance  of  level 
amounting  to  N  mm.,  it  follows  that 


No  appreciable  error  is  likely  to  be  introduced  by  the  atmospheric  pressure 
markedly  deviating  from  the  value  assumed  in  this  calculation. 

If  the  internal  diameters  of  the  two  limbs  of  the  U  tube  are  dj  mm.  and  d2  mm. 
respectively,  then 

636-6     636-6      ^  v     SOd^  do2  +3'1  (d^+d.,2) 
~~d7~  +  ~d^'  =~d?d^-6-2(d12+d^) 

EXAMPLE.—  If  dj=6-9  mm.  and  d2  =  15'5  mm.,  then  N  =  13-35  +2'65  =16-0 
and  X=59-4  c.c. 

Next  close  the  stop-cock  leading  to  the  large  mercury  reservoir,  make  sure 

*  The  Absorption  Pipette  differs  from  that  ofMacfarlane  and  C  a  1  d  w  e  1  1  in  two 
important  points.  First,  the  lower  bulb  E,  of  about  80  c.c.  capacity,  is  inclined  so 
that  the  unabsorbed  gas  may  readily  be  returned  to  the  measuring  vessel  without 
tilting  the  whole  apparatus.  Second,  the  connection  is  made  by  means  of  a  three- 
way  stop  -cock  so  that  the  capillary  G  may  be  connected  either  with  E  containing  the 
absorbent  over  mercury  or  with  F  containing  clean  mercury.  The  ends  of  the 
capillaries  G  and  K  must  be  free  from  appreciable  unevenness.  It  is  important  that 
the  bore  of  these  tubes  should  neither  exceed  1'5  mm.  nor  be  less  than  1  mm.,  and 
their  external  diameter  should  be  about  6  mm. 


568  GAS   ANALYSIS. 

that  the  level  tube  is  in  free  communication  with  the  atmosphere,  and  level 
accurately  by  means  of  the  screw  clip,  which  acts  as  a  fine  adjustment  and  which 
should  be  alongside  the  reading  telescope.  Finally,  read  the  correction  tube.  It 
is  convenient  to  enter  the  readings  thus  : — 

Correction.  Reading.  Corrected  Reading. 

+0-04  per  cent.  49'97  c.c.  49*99  c.c. 

The  gas  is  next  sent  over  into  the  absorption  pipette,  followed  by  sufficient 
mercury  to  clear  the  capillaries,  and  the  pipette  shaken  with  the  stop-cock  closed 
until  absorption  is  complete.  Run  over  a  little  more  mercury  in  order  to  clear 
absorbent  from  the  capillary  between  E  and  H.  Send  the  soiled  mercury  in  the 
capillaries  into  U.  When  the  gas  reaches  N  turn  the  stop-cock  and  run  it  into 
the  measuring  tube,  controlling  the  rate  by  H.  The  absorbent  is  readily  stopped 
when  it  reaches  H  and  the  capillary  is  cleared  of  gas  by  means  of  clean  mercury 
from  F,  this  being  stopped  as  soon  as  it  reaches  N.  It  is  advisable  to  turn  the 
stop-cock  N  so  as  to  place  K  in  communication  with  U  whilst  removing  a  pipette. 
By  means  of  a  retort  stand  a  small  evaporating  basin  may  be  supported  about 
2  inches  below  G  in  order  to  catch  drops  of  mercury. 

If,  as  in  the  determination  of  carbon  monoxide,  it  is  necessary  to  subject  the  gas 
to  more  than  one  treatment  with  an  absorbent,  the  first  pipette  is  brought  into 
direct  connection  with  the  second  and  the  gas  transferred.  With  fuming 
sulphuric  acid  as  an  absorbent,  no  mercury  is  used  in  E,  a  U  tube  with  pumice 
and  strong  sulphuric  acid  is  attached  to  D  to  prevent  access  of  moisture,  and  the 
mercury  from  the  capillaries  is  driven  into  F.  The  explosion  pipette  is  similar 
to  Dittmar's,  but  with  the  special  stop -cock  and  mercury  bulb  as  in  the 
absorption  pipettes. 

The  phosphorus  pipette  consists  of  an  ordinary  Orsat's  phosphorus  pipette 
fitted  with  a  horizontal  stop-cock  and  fixed  in  a  tin  water  vessel  with  aperture  for 
thermometer. 

The  Advantages  of  the  apparatus  as  compared  with  that  of 
Macfarlane  and  CaldwelPs  are — 

An  accurate  correction  tube,  which  really  saves  time  and  trouble. 

A  means  of  accurately  adjusting  the  level  of  the  mercury  without  taking 

one's  eye  from  the  reading  telescope. 
Greater  cleanliness  of  the  mercury  in  the  measuring  tube  towards  the  end 

of  an  analysis. 
The  possibility  of  washing  out  the  measuring  tube,  in  case  of  accident,  at 

any  stage  whilst  the  gas  is  in  one  of  the  pipettes. 
Direct  transference  from  pipette  to  pipette  when  desired. 
The  measuring  tube  constant  in  position. 

A  good  illumination  for  reading  always  obtainable  without  trouble. 
Explosion  in  a  separate  pipette. 

Advantages,  as  compared  with  the  Dittmar  or  similar  apparatus — 

Transference  direct  from  measuring  tube  to  pipette,  instead  of  to  and  from 

an  intermediate  tube. 
Easier  manipulation  and  greater  cleanliness,  especially  as  regards  the  fatal 

introduction  of  absorbent  into  the  measuring  tube. 
Pipettes  giving  more  surface  and  better  agitation. 
A  considerable  reduction  in  the  amount  of  mercury  required. 

SIMPLER    METHODS    OF    GAS    ANALYSIS. 

ALL  the  sets  of  apparatus  previously  described  are  adapted  to 
secure  the  greatest  amount  of  accuracy,  regardless  of  speed  or  of  the 
time  occupied  in  carrying  out  the  various  intricate  processes 
involved. 

For  industrial  and  technical  purposes  the  demand  for  something 


ORSAT-LUNGE    APPARATUS. 


569 


requiring  less  time  and  care,  even  at  the  sacrifice  of  some  accuracy, 
has  been  met  by  a  large  number  of  designs  for  apparatus  of  a  simpler 
class,  among  which  may  be  mentioned  those  of  Orsat,  Bunte, 
Winkel,  Hempel,  Stead,  Lunge,  etc.  Many  of  these  are 
arranged  to  suit  the  convenience  of  special  industries,  and  will 
not  be  described  here. 

The  most  useful  apparatus  for 
general  purposes  is  either  that  of 
Hempel  or  Lunge,  both  of 
which  will  be  shortly  described. 
Fuller  details  as  to  these  and 
other  special  kinds  of  apparatus 
are  contained  in  Winkler's 
Handbook  of  Technical  Gas 
Analysis,  translated  by  Lunge.* 
The  general  principles  upon 
which  these  various  sets  of 
apparatus  are  based,  and  the 
calculation  of  results,  are  the 
same  as  have  been  described  in 
preceding  pages  ;  and  of  course 
due  regard  must  be  had  to 
tolerable  equality  of  temperature 
and  pressure,  and  the  effects  of 
cold  or  warm  draughts  of  air  upon 
the  apparatus  whilst  the  manipu- 
lations are  carried  on.  If  the 


Fig.  106. 


operator  is  not  already  familiar  with  methods  of  gas  analysis,  a  study 
of  the  foregoing  sections  will  be  of  great  assistance  in  manipulating 
the  apparatus  now  to  be  described. 

Orsat-Lunge  Gas  Apparatus. — Fig  106.  shows  the  outline  of 
this  instrument.  The  modification  of  the  original  Orsat  instrument, 
by  Lunge,  is  a  contrivance  for  burning  hydrogen  and  other  gases 
by  heated  palladium  asbestos.  It  is  so  well  known  and  so  constantly 
in  use  that  no  detailed  description  is  needed  here,  but  another  form 
of  the  apparatus,  which  is  intended  for  the  determination  of  unburnt 
products  in  chimney  gases,  has  been  devised  byW.  H.Sodeau.f 
It  is  shown  in  fig.  107,  and  the  description  is  as  follows  :— 

In  experimental  work  on  the  economic  application  of  fuel,  it  is  very  desirable 
to  know,  whilst  a  trial  is  actually  proceeding,  not  only  what  excess  of  air  is  being 
employed,  but  also  the  amount  of  unburnt  gases.  J  Combining  these  data  with 
the  indications  of  a  thermo junction  placed  at  the  base  of  the  funnel,  one  can 
follow  throughout  the  trial  the  total  amount  of  heat  (potential  as  well  as  actual) 
which  is  passing  up  the  chimney.  With  a  given  boiler,  for  example,  if  one  takes 
-re  of  the  exit  gases,  the  evaporation  will  practically  take  care  of  itself.  The 
el  employed  represents  a  certain  total  amount  of  heat,  and  the  loss  by  radiation 


*Gurney  and  Jackson,  2nd  edition,  1902.  t  C.  N.  89,  61. 

J  When  working  with  very  limited  furnace  space,  as  in  naval  water-tube  boilers  of 
the  "  Express  "  type,  a  reduction  of  the  "  excess  "  of  air  may  cause  large  amounts  of 
combustible  gases  to  escape  unburnt,  and  so  lead  to  decreased,  instead  of  increased, 
evaporative  efficiency. 


570  GAS   ANALYSIS. 

from  the  boiler  will  be  practically  uniform  ;  hence  the  evaporation  per  pound  of 
fuel  may  be  ganged  by  what  remains  after  deducting  the  various  losses  from  the 
calorific  value  of  the  fuel,  or,  in  other  words,  by  working  up  the  chimney  gas 
results  as  in  a  "  heat  balance  sheet." 

This  method  of  checking  the  evaporative  efficiency  is  especially  advantageous 
in  short  experimental  runs,  as  feed  water  is  not  always  read  with  much  accuracy, 
and  all  that  passes  through  the  main  stop  valve  may  not  actually  be  steam. 

The  actual  analytical  problem  may  be  reduced  to  the  rapid  determination  of 
carbon  dioxide,  carbon  monoxide,  and  hydrogen  (including  hydrocarbons  if 
present).  It  is,  however,  customary  to  determine  the  oxygen  in  chimney  gases, 
whilst  the  determination  of  hydrogen  is  usually  omitted.  The  desirability  of 
determining  the  amount  of  hydrogen  is  illustrated  by  the  table  below,  in  which  are 
given  a  few  analyses  of  the  products  of  incomplete  combustion  obtained  in  some 
experiments  with  a  1000  horse-power  water-tube  boiler  of  the  "Express"  type. 
With  Welsh  coal  the  loss  as  unburnt  hydrogen  usually  amounted  to  nearly  a  third 
of  the  loss  as  carbon  monoxide,  whilst  with  petroleum  the  proportion  averaged 
about  two-thirds.  This  difference  was  no  doubt  mainly  due  to  the  larger  proportion 
of  total  hydrogen  present  when  oil-fuel  was  employed. 

Examples  of  Incomplete  Combustion. 

Crude  Texas  oil  (steam 
Welsh  coal  sprayed). 

Carbon  dioxide,  per  cent.  ..     9-0~~lT-0       9^9       9  "2          6'l"~~  9  "5       9-0       7  -9 
Carbon  monoxide,  per  cent.      2'15     2-8       1'65     1-3          2'1       1-5       I'O      0'6 
Hydrogen,  per  cent.  ..     0'65    0'55     0'47     0'4          1-2       I'O      0*6      0'4 

Pounds  air  per  pound  fuel      17'2     147     17'75  19'5        25'7     19-4521-3525-0 
Excess  air  per  cent,  above 

theoretical  ..          .  .  55  "0     32  '3     60  '0     75  "7        84        39        53        79 

Evaporation       units*       per 
pound    of    fuel    lost    as 

combustible  gases  ..     2  -34    2  -26     1-71     1'51        3  '65     2  '05     1'45     1*06 

Air  being  practically  constant  in  composition  it  is  unnecessary  to  determine  the 
oxygen  in  chimney  gases  if  the  approximate  composition  of  the  fuel  is  known, 
for  it  is  then  easy  to  work  out  simple  formulae  by  means  of  which  those  data 
which  are  of  practical  importance  may  be  calculated  with  fair  accuracy  from  the 
percentages  of  carbon  dioxide,  carbon  monoxide,  and  hydrogen  alone. 
Take,  for  example,  two  analyses  of  fuels  :  — 

Welsh  coal.         Texas  petroleum. 
Carbon         ........     88'2  85'0 

Sulphur       ........       0-77  1'34 

Hydrogen    ........       2*6  12-0 

Ash,  oxygen,  &c  .......       8  -43  1'66 

100-0  100-0 

Theoretical  amount  of  air  for  1  pound  fuel         11*1  Ib.  13  '95  Ib. 

Using  C02,  CO,  and  H2  respectively  to  denote  the  percentages  of  carbon  dioxide, 
carbon  monoxide,  and  hydrogen  in  the  chimney  gases,  the  following  formulae 
may  be  obtained  by  calculation  from  the  equations  for  combustion  :  — 

Welsh  coal.  Texas  petroleum. 

1.     Pounds  air  per  pound  of  fuel  — 

204  -CO  206  -CO    A0 

+0'84 


co2co 

2.  Excess  air  per  cent,  above  theoretical  — 

1837  -9  CO  1476  -7-4  CO 

C02+C0    ~y7°  C02+C0 

3.  Evaporation  units  lost  as  unburnt  gases  — 

Welsh  coal,  Texas  petroleum. 


9-33 


C02+C0  C02+CO 

*  An  "evaporation  unit"  is  the  amount  of  heat  required  to  convert  one  pound  of 
water  at  100°  C.  into  steam  at  the  same  temperature, 


ORSAT-LTJNGE-SODEAU   APPARATUS. 


571 


These  formulae  will,  of  course,  apply  only  to  fuels  having  the  composition  given 
above.  For  ordinary  purposes  the  terms  including  CO  may  be  omitted  from  the 
numerators  of  formulae  (1)  and  (2).  Curves  can  then  be  plotted  having  C02  +CO 
on  one  axis,  and  either  "  pounds  air  per  Ib.  of  fuel "  or  "  excess  air  per  cent, 
above  theoretical "  on  the  other,  so  that  the  meaning  of  an  analysis  can  be  seen 
at  a  glance. 


Fig.  107. 

An  ordinary  0  r  s  a  t  apparatus  affords  a  ready  means  of  determining  the  carbon 
dioxide,  but  the  absorption  of  carbon  monoxide  by  means  of  that  somewhat 
objectionable  re-agent,  cuprous  chloride,  involves  the  previous  removal  of  oxygen 
by  means  of  phosphorus  or  alkaline  pyrogallol,  absorbents  which  act  exceedingly 
slowly  when  too  cold.  This  method  takes  no  account  of  free  hydrogen  (or 
saturated  hydrocarbons).  The  author  decided  to  discard  absorption  by  cuprous 
chloride  in  favour  of  a  combustion  method,  and  not  finding  the  capillary  com- 
bustion tube  of  palladini/ed  asbestos  (as  fitted  in  the  Orsat -Lunge  apparatus) 
quite  suitable  for  use  in  the  stokehold,  finally  adopted  an  Orsat  apparatus 
modified  as  shown  in  the  accompanying  figure,  in  which  the  main  feature  is  an 
adaptation  of  the  Winkler  combustion  pipette.* 

A  large  glass  stop-cock,  s,  of  4  to  5  mm.  clear  bore,  is  attached  to  the  base  of 
the  apparatus,  one  end  being  connected  to  the  measuring  tube  and  the  other  to 
the  reservoir  M  R.  The  pipette,  K,  is  of  the  ordinary  form,  and  contains  caustic 
potash  solution  (one  part  potash  to  two  of  water).  A  similar  pipette,  p,  con- 
taining phosphorus  or  alkaline  pyrogallol,  may  be  used  if  it  is  desired  to  determine 

*  The  Winkler  pipette,  a  development  of  Coquillon's  "  Grisoumeter,"  is 
described  and  figured  in  Winkler  and  Lunge's  "Technical  Gas  Analysis," 
pp. 151-155. 


572  GAS   ANALYSIS. 

the  oxygen.  The  combustion  pipette,  c,  may  be  made'from  the  commercial  form 
of  W inkier  pipette  by  simply  cutting  off  the  U-tube  and  sealing  on  a  straight 
piece  of  capillary  tube  of  suitable  length.  An  ordinary  Hemp  el  pipette  for 
solid  absorbents  may  be  altered  in  a  similar  manner,  the  neck  at  the  bottom  being 
closed  by  a  two-hole  rubber  stopper  through  which  passes  a  pair  of  unlacquered 
brass  electrodes  bridged  across  by  a  platinum  spiral  made  by  coiling  about  4  cm. 
of  platinum  wire  of  about  0'3  mm.  diameter  around  a  needle  1'3  mm.  thick. 

When  the  combustion  pipette  is  employed  with  mixtures  rich  in  combustible 
gases  the  coil  must  be  near  the  top  of  the  bulb  in  order  that  serious  explosions 
may  be  avoided,  but  for  the  analysis  of  any  ordinary  chimney  gases  the  coil  may 
be  placed  much  lower  in  order  to  reduce  the  heating  of  the  glass.  The  gas  may 
then  be  returned  as  soon  as  the  current  is  cut  off,  without  waiting  for  the  glass 
to  cool. 

A  fixed  bulb*  as  in  the  Hempel  (orOrsat)  pipette,  may  be  employed  to 
receive  the  displaced  water  unless  it  is  desired  to  carry  out  the  rapd  determination 
of  carbon  monoxide  and  hydrogen  together,  as  described  below,  when  the  bulb,  c, 
should  be  connected  by  means  of  india-rubber  tubing  to  the  reservoir,  c  E,  con- 
sisting of  a  small  aspirator  bottle,  similar  to  that  connected  to  the  measuring 
tube.  When  in  use  the  spiral  is  raised  to  a  white  heat  by  means  of  a  two-cell 
accumulator,  the  current  being  conveniently  adjusted  to  the  right  strength  by 
passing  it  through  a  few  feet  of  the  tinned  iron  wire,  about  -fa"  diameter,  which 
is  commonly  sold  in  penny  skeins. 

In  order  to  eliminate  parallax  the  measuring  tube  is  read  by  means  of  a  lens 
mounted  in  conjunction  with  an  eye-cap,  and  sliding  on  a  brass  rod,  as  employed 
in  connection  with  the  "  Kew  principle  "  correction  tube.f  Before  each  reading 
is  taken,  the  water  in  the  measuring  tube  is  allowed  to  drain  down  for  one  minute, 
as  indicated  by  the  sand-glass  used  for  timing  the  absorptions  and  combustions. 

The  U-tube  filled  with  glass  wool  ordinarily  supplied  with  the  Or  sat 
apparatus  being  somewhat  apt  to  clog  if  dense  black  smoke  is  produced,  it  is 
conveniently  replaced  by  a  simple  T  piece  having  a  small  plug  of  glass  wool  in 
the  limb  through  which  the  sample  enters  the  apparatus.  In  this  way  only  the 
actual  samples  are  filtered,  and  the  solid  particles  in  the  main  stream  pass  direct 
to  the  aspirator. 

METHOD  OF  PROCEDURE  :  The  measuring  off  of  the  sample  is  effected  in  the 
usual  manner,  except  that  it  is  convenient  to  close  S  instead  of  pinching  the 
india-rubber  tube  when  adjusting  the  water  to  the  zero  mark  prior  to  bringing 
the  gas  to  atmospheric  pressure. 

Carbon  dioxide  is  determined  by  sending  the  greater  part  of  the  gas  over  into  K 
and  back  again,  then  leaving  it  for  one  minute  to  complete  the  absorption,  and 
measuring  again  as  usual ;  the  decrease  gives  the  percentage  of  carbon  dioxide. 
The  current  is  next  switched  on,  and  the  gas  passed  over  into  the  combustion 
pipette,  c,  where  it  remains  for  one  minute,  being  then  returned  to  the  measuring 
tube  (after  switching  off  the  current),  and  the  contraction  noted. 

The  carbon  dioxide  produced  is  then  determined  by  a  one-minute  absorption 
in  the  potash  pipette,  K ;  the  decrease  gives  the  percentage  of  carbon  monoxide. 

As  carbon  monoxide  unites  with  half  its  volume  of  oxygen  to  form  its  own 
volume  of  carbon  dioxide,  it  follows  that  half  the  amount  of  the  carbon  monoxide 
found  must  be  deducted  from  the  contraction  during  combustion.  Two -thirds 
of  the  corrected  contraction  equals  the  percentage  of  hydrogen. 

For  example,  if  the  contraction  on  combustion  amounted  to  2 '3  c.c.  and  the 
resulting  carbon  dioxide  to  1  *6  c.c.,  then  the  gas  contained  1  '6  per  cent,  of  carbon 

monoxide  and  f  ^2'3  -  -^  \  =|  x  1'5  =  1*0  per  cent,  of  hydrogen.     It  should  be 

noted  that  the  only  absorbent  employed  is  one  which  is  readily  obtained  and 
seldom  needs  renewing.  The  determination  of  carbon  dioxide,  carbon  monoxide, 
and  hydrogen  by  the  above  method  can  be  completed  in  fifteen  minutes.  Any 
traces  of  hydrocarbons,  if  present,  will  of  course  appear  partly  as  carbon  monoxide 

*  The  ordinary  Hempel  pipette  is  sometimes  supplied  with  a  bulb  too  small  to 
contain  the  water  displaced  by  the  heated  gas. 

t  J.  S.  C.  I.,  1903,  22,  188,  189. 


SIMPLE    TITBATION.  573 

and  partly  as  hydrogen  in  the  above  method  of  analysis.  Their  heat  value  will 
not  be  fully  represented,  but  this  is  an  unimportant  defect,  and  it  exists  more 
markedly  in  the  old  cuprous  chloride  method. 

It  may  occasionally  be  desirable  to  know  the  percentage  of  oxygen  originally 
present  in  the  gas.  This  may  be  found  by  finally  absorbing  with  phosphorus  or 
pyro  in  the  pipette  p,  and  adding  to  the  result  the  amount  of  oxygen  required  to 
burn  the  combustible  gases. 

RAPID  JOINT  DETERMINATION  OF  CARBON  MONOXIDE  AND  HYDROGEN. — This 
may  be  carried  out  by  omitting  the  measurement  immediately  after  combustion, 
and  transferring  the  gas  direct  from  c  to  K  in  the  following  manner : — After  the 
carbon  dioxide  has  been  determined,  open  the  stop-cock  attached  to  c,  and  switch 
on  the  current,  raising  the  reservoir,  M  R,  so  as  to  drive  the  gas  into  c.  Close  s 
when  the  water  has  risen  to  the  top  of  the  measuring  tube,  and  about  one  minute 
later  switch  off  the  current  and  open  the  stop-cock  of  K,  raising  the  reservoir, 
c  R,  so  as  to  drive  the  gas  over  into  the  potash,  and  finally  closing  the  stop-cock 
of  c.  After  allowing  one  minute  for  absorption,  the  stop-cock,  s,  is  opened,  the 
residue  drawn  back  into  the  measuring  tube,  and  the  stop-cock  of  K  then  closed. 
Two-thirds  of  the  contraction  so  produced  equals  the  percentage  of  "  combustible 
gases,"  i.e.,  carbon  monoxide  and  hydrogen,  in  the  sample. 

The  result  so  obtained  is  translated  into  "  evaporation  units  "  by  means  of 
a  formula  similar  to  3,  the  denominator  being  replaced  by  C02  +  (f  x  combustible 
gases)  in  the  case  of  Welsh  coal,  and  by  C02  +  (f  x  combustible  gases)  when  Texas 
oil  is  employed.  The  amount  of  air  is  similarly  found  by  means  of  formulae  or 
curves. 

When  collecting  samples  of  chimney  gas  for  subsequent  analysis  the  sometimes 
troublesome  process  of  saturating  water  during  the  trial  may  be  avoided  by 
appropriately  diluting  a  saturated  solution  of  carbon  dioxide.  Thus,  if  the  gas  is 
expected  to  contain  about  12  per  cent,  of  carbon  dioxide,  the  sample  bottles 
should  be  filled  with  a  mixture  of  seven  parts  tap- water  and  one. part  of  water 
saturated  with  carbon  dioxide.  The  flow  of  water  from  the  sample  bottle  may  be 
conveniently  regulated  by  attaching,  say,  a  yard  of  tubing  (in  order  to  give  a 
fairly  uniform  head),  to  the  end  of  which  is  connected  a  "  wash-bottle  jet  " 
which  has  previously  been  found  to  permit  the  emptying  to  take  place  in  the 
required  time.  The  jet  is  less  likely  to  clog  if  used  in  the  reversed  position. 

Simple  Titration  of  Gases. — Many  instances  occur  in  which  the 
amount  of  a  given  constituent  in  a  gaseous  mixture  can  be 
determined  by  aspirating  the  sample  through  a  solution  which 
effects  selective  absorption  of  the  constituent  to  be  determined. 
Analysis  of  the  solution  enables  the  weight  and  volume  of  the 
absorbed  gas  to  be  calculated  ;  the  volume  of  the  residual  gases 
with  which  it  was  associated  is  obtained  by  measuring  the  volume 
of  water  discharged  by  the  aspirator  employed  or  by  passing  the 
gases  through  a  gas  meter.  Two  methods  of  testing  are  in  general 
use  : — 

1.  The  Continuous  Method,  whereby  a  portion  of  the  gas  is 
aspirated  slowly  and  continuously    over  a   long  period   through 
absorbing  vessels  by  means  of  a  water  jet  pump  or  water  aspirator 
of  large  size. 

2.  The  Intermittent  Method,  whereby  separate  tests  are 'taken 
at  intervals  throughout  the  day  or  from  day  to  day. 

The  former  method  is  in  use  in  the  larger  chemical  works  where 
an  exact  measure  of  loss  is  desired  by  day  and  night ;  the  latter  is 
in  general  use  in  all  works  where  testing  is  done,  and  is  employed 
by  the  Inspectors  appointed  under  The  Alkali,  etc.  Works 
Regulation  Act,  1906,  for  their  routine  testing  to  control  the  escape 


574  GAS   ANALYSIS. 

of  acid  and  other  gases  scheduled  as  "  noxious  and  offensive  gases  " 
by  the  Act. 

In  the  Intermittent  Method  the  standard  solutions  employed  are 
generally  so  arranged  as  to  minimise  calculations,  the  volume  of 
standard  solution  used  for  titration  giving  directly  the  weight  of 
constituent  in  grains  per  cubic  foot  of  escaping  gases.  The  absorbing 
vessels  used  in  the  works  are  generally  of  glass  with  an  aspirator 
attached  of  unknown  capacity.  The  Alkali  Inspectors  prefer 
a  collapsible  rubber  aspirator,  such  as  the  Fletcher's  bellows 
aspirator,  as  being  more  portable  and  less  liable  to  breakage.  The 
absorbing  liquid  in  this  case  is  placed  inside  the  aspirator  ;  gas  and 
liquid  are  brought  into  contact  by  vigorous  shaking,  and  the 
latter  subsequently  expelled  for  titration.  The  capacity  of  the 
bellows  on  expansion  being  known,  the  weight  and  volume  of  the 
condensed  products  can  be  readily  calculated. 

In  testing  chlorine-exits  a  rubber  enema-pump  is  often  employed 
to  draw  the  gases  through  the  absorbing  solution,  which  is  of  such 
strength  that  a  certain  number  of  deliveries  of  the  bulb  indicate 
a  definite  quantity  of  chlorine  when  the  colour  of  the  liquid 
changes. 

Detailed  descriptions  of  the  various  methods  of  determining  the 
amount  of  acid  and  other  noxious  constituents  of  gaseous  mixtures 
by  selective  absorption  will  be  found  in  Lunge's  "Sulphuric 
Acid  and  Alkali,"  and  in  Lunge's  "Technical  Chemists' Handbook." 
The  reader  should  also  consult  the  Alkali  Reports,  especially  that 
for  1902. 

Normal  Solutions  for  Gas  Analysis. — In  the  titration  of  gases 
by  these  methods,  particularly  on  the  Continent,  the  custom  is  to 
use  special  normal  solutions,  1  c.c.  of  which  represents  1  c.c.  of  the 
absorbable  gas  in  a  dry  condition  and  at  760  mm.  pressure  and  0°C 
temperature.  These  solutions  must  not  be  confounded  with  the 
usual  normal  solutions  used  in  volumetric  analysis  of  liquids  or 
solids.  For  instance,  a  normal  gas  solution  for  chlorine  would  be 
made  by  dissolving  4-4917  gm.  of  As203,  with  a  few  grams  of 
sodium  carbonate  to  the  Ijtre,  and  a  corresponding  solution  of 
iodine  containing  11-522  gm.  per  litre,  in  order  that  1  c.c.  of  either 
should  correspond  to  1  c.c.  of  chlorine  gas.  1  c.c.  of  the  same 
iodine  solution  would  also  represent  1  c.c.  of  dry  S02,  and  so  on. 

A  very  convenient  bottle  for  the  titration  of  certain  gases  is 
adopted  by  Hesse.  It  is  made  in  a  conical  form,  like  an 
Erlenmeyer's  flask,  and  has  a  mark  in  the  short  neck,  down  to 
which  is  exactly  fitted  a  caoutchouc  stopper  having  two  holes, 
which  will  either  admit  the  jet  of  a  burette  or  pipette,  or  may  be 
securely  closed  by  solid  glass  rods.  The  exact  content  of  the 
vessel  up  to  the  stopper  is  ascertained,  a  convenient  size  being 
about  500  or  600  c.c.  The  exact  volume  is  marked  upon  the 
vessel. 

In  the  case  of  gases  not  affected  by  water,  the  bottle  is  filled  with 
that  liquid  and  a  portion  displaced  by  the  gas,  and  the  stopper  with 


HEMPEL  S    BURETTE. 


575 


its  closed  holes  inserted.  If  water  cannot  be  used,  the  gas  is  drawn 
into  the  empty  bottle  by  means  of  tubes  with  an  elastic  pump.  The 
absorbable  constituent  of  the  gas  is  then  determined  with  an  excess 
of  the  standard  solution  run  in  from  a  pipette  or  burette.  During 
this  operation  a  volume  of  the  gas  escapes  equal  to  the  volume  of 
standard  solution  added,  which  must  of  course  be  deducted  from 
the  contents  of  the  absorbing  vessel.  The  gas  and  liquid  are  left 
to  react  with  gentle  shaking  until  complete.  The  excess  of  standard 
solution  is  then  found  residually  by  another  corresponding  standard 
solution  ;  and  in  the  case  of  using  gas  normal  solutions,  the  difference 
found  corresponds  to  the  volume  of  the  absorbed  constituent  of 
the  gas  in  c.c.  ;  and  from  this,  and  from  the  total  volume  of  gas 
employed,  may  be  calculated  the  per- 
centage, allowing  for  the  correction 
mentioned.  This  arrangement  may  be 
used  for  C02  in  air,  using  normal  gas 
barium  hydrate  and  a  corresponding 
normal  gas  oxalic  acid  with  phenolphtha- 
lein.  The  normal  oxalic  acid  should  con- 
tain 5'6314  gm.  per  litre,  in  order  that 
1  c.c.  may  represent  1  c.c.  of  C02.  The 
baryta  solution  must  correspond,  or  its 
relation  thereto  found  by  blank  experi- 
ment at  the  time.  The  arrangement  is 
also  available  for  HC1  in  gases,  using 
a  normal  gas  silver  solution  containing 
4'8488  gm.  Ag  per  litre,  as  absorbent,  with 
a  corresponding  solution  of  thiocyanate 
(p.  145)  and  ferric  indicator  ;  or  the  HC1 
may  be  absorbed  by  potash,  then  acidified 
with  HNO3.  and  the  titration  carried  out 
by  the  same  process  ;  or  again,  an  alkali 
carbonate  may  be  used,  and  the  titration 
made  with  a  normal  gas  silver  solution 
using  the  chromate  indicator  (p.  142). 

H  e  m  p  e  1 '  s  Gas  Burette. — This  consists  of  two  tubes  of  glass 
on  feet>  one  of  which  is  graduated  to  100  c.c.  in  i  c.c.  (the  burette 
proper) ,  and  the  other  plain  (the  pressure  tube).  They  are  connected 
at  the  feet  by  an  elastic  tube,  much  in  the  same  way  as  Lunge's 
nitrometer.  The  arrangement  is  shown  in  fig.  108. 

The  illustration  shows  the  burette  with  three-way  stop-cock  at 
bottom,  which  is  necessary  in  the  case  of  gases  soluble  in  water, 
or  where  any  of  the  constituents  are  affected  thereby.  If  this  is 
not  the  case,  a  burette  without  such  stop-cock  is  substituted 
(fig.  109).  The  elastic  tube  should  not  be  in  one  piece,  but  connected 
in  the  middle  by  a  short  length  of  glass  tube  to  admit  of  ready 
disconnection. 

Fig.  109  illustrates  not  only  the  original  Hemp  el  burette  with 
pressure  tube,  but  also  the  method  of  connection  with  the  gas 


Fig.  108. 


576 


GAS    ANALYSIS. 


pipette,  and   the  way  in  which  the  elastic  tube  is  joined  by  the 
intervening  glass  tube.* 

Hemp  el,  with  great  ingenuity,  has  devised  special  pipettes  to 
be  used  in  connection  with  the  burette,  which  render  the 
instrument  very  serviceable  for  general  gas  analysis.  The  pipette 


B 


109. 


shown  in  fig.  109  is  known  as  the  simple  absorption  pipette,  and 
serves  for  submitting  the  gas  originally  in  the  burette  to  the  action 
of  some  special  absorbent.  With  a  series  of  these  pipettes  the  gas 
is  submitted  to  the  action  of  special  absorbents,  one  after  another, 

*  The  same  chemist  has  since  designed  a  gas  burette  which  has  the  advantage  of 
being  unaffected  by  the  fluctuating  temperature  and  pressure  of  the  atmosphere. 
This  is  effected  by  connecting  the  measuring  apparatus  with  a  space  free  from  air,  but 
saturated  with  aqueous  vapour.  A  figure  showing  the  arrangement  is  given  in  C.  N. 
56,  254.  These  simpler  forms  of  gas  apparatus  in  great  variety,  including  various 
forms  of  the  nitrometer,  are  kept  in  stock  by  most  of  the  dealers  in  apparatus  in  the 
kingdom. 


HEMPEL'S  BURETTE. 


577 


until  the  entire  composition  is  ascertained.     The  connections  must 
in  all  cases  be  made  of  best  stout  rubber,  and  bound  with  wire. 

Collection  and  measurement  of  the  Gas  over  Water.  —  Both  tubes 
are  filled  completely  with  water  (preferably  already  saturated 
mechanically  with  the  gas),  care  being  taken  that  all  air  is  driven  out 
of  the  elastic  tube.  The  clip  is  then  closed  at  the  top  of  the  burette, 
and  the  bulk  of  the  water  poured  out  of  the  pressure  tube,  the  elastic 
tube  being  pinched  meanwhile  with  the  finger  and  thumb  to  prevent 
air  entering  the  burette.  The  latter  is  then  connected  by  a  small 
glass  tube  with  the  source  of  the  gas  to  be  examined,  when,  by  lower- 
ing the  pressure  tube,  the  gas  flows  in  and  displaces  the  water  from 
the  burette  into  the  pressure  tube.  The  pressure  is  then  regulated 
by  raising  or  lowering  either  of  the  tubes  until  the  water  is  at  the 
same  level  in  both,  when  the  volume  of  gas  is  read  off.  It  is  con- 
venient of  course  to  take  exactly  100  c.c.  of  gas  to  save  calculation. 
Collection  and  measurement  of  the  Gas  without  Water.  —  In  this 
case  the  three-way  tap  burette  (fig.  108)  is  dried  thoroughly  by  first 
washing  with  alcohol,  then  ether,  and  drawing  air  through  it. 
The  three-way  tap  is  then  closed,  the  upper  tube  connected  with  the 
gas  supply,  and  the  burette  filled  either  by  the  pressure  of  the  gas, 
or  by  using  a  small  pump  attached  to  the  three-way  cock  to  draw 
out  the  air  and  fill  the  burette  with  the  gas.  When  full  the  taps 
are  turned  off,  and  connection  made  with  the  pressure  tube,  which 
is  then  filled  with  water,  the  tap  opened  so  that  the  water  may  flow 
into  the  burette  and  absorb  the  soluble  gases  present.  As  the  burette 
holds  exactly  100  c.c.  between  the  three-way  tap  and  the  upper  clip, 
the  percentage  of  soluble  gas  is  shown  directly  on  the  graduation. 

The  method  of  Absorption.  —  In 
the  case  of  the  simple  pipette  fig. 
109,  a  is  filled  with  the  absorbing 
liquid,  which  reaches  into  the 
siphon  bend  of  the  capillary  tube  ; 
the  bulb  b  remains  nearly  empty. 
In  order  to  fill  the  instrument,  the 
liquid  is  poured  into  6,  and  the  air 
sucked  out  of  a  by  the  capillary 
tube.  It  is  convenient  to  keep  a 
number  of  these  pipettes  filled 
with  various  absorbents,  well 
corked,  and  labelled. 

Another  pipette  of  similar  char- 
acter is  shown  in  fig.  110,  and  is 
adapted  for  solid  reagents,  such  as 
stick  phosphorus  in  water.  The 
instrument  has  an  opening  at  the  bottom,  which  can  be  closed 
with  a  caoutchouc  stopper.  This  pipette  is  also  used  for  absorb- 
ing CO  2  by  filling  it  with  plugs  of  wire  gauze  and  caustic  potash 
solution,  so  as  to  expose  a  large  active  surface  when  the  liquid 
is  displaced  by  the  gas. 

2  P 


HO. 


578  GAS   ANALYSIS. 

To  make  an  absorption,  the  capillary  U-tube  is  connected  with 
the  burette  containing  the  measured  gas  by  a  small  capillary  tube 
(fig.  110),  the  pinch-cock  of  course  being  open,  then  by  raising  the 
pressure  tube,  the  gas  is  driven  over  into  the  cylindrical  bulb,  where 
it  displaces  a  portion  of  the  liquid  into  the  globular  bulb.  When  the 
whole  of  the  gas  is  transferred,  the  pinch-cock  is  closed,  and  the 
absorption  promoted  by  shaking  the  gas  with  the  reagent.  When 
the  action  is  ended,  communication  with  the  burette  is  restored, 
and  the  gas  siphoned  back  by  means  of  the  pressure  tube  into  the 
burette  to  be  measured. 

The  double  absorption  Pipette,  shown  in  fig.  Ill,  is  of  great 
utility  in  preserving  absorbents  which  would  be  acted  on  by  the 
air,  such  for  instance  as  alkaline  pyrogallol,  cuprous  chloride,  etc. 
The  bulb  next  the  siphon  tube  is  filled  with  the  absorbent,  the  next 
is  empty,  the  third  contains  water,  and  the  fourth  is  empty.  When 
the  gas  is  passed  in,  the  intermediate  water  passes  on  to  the  last 
bulb  to  make  room  for  the  gas,  thus  shutting  off  all  contact  with 
the  atmosphere,  except  the  small  amount  in  the  second  bulb.  An 
arrangement  is  also  made  for  the  use  of  solid  reagents,  by  substitut- 
ing for  the  globe  next  the  U  capillary  tube  a  cylindrical  bulb  as  in 
fig.  110. 

Hydrogen  Pipette. — The  hydrogen  gas  necessary  for  explosions 
or  combustions  is  produced  from  a  hollow  rod  of  zinc  fixed  over 
a  glass  rod  passed  through  the  rubber  stopper  (fig.  110).  The  bulb 
being  filled  with  dilute  acid,  gas  is  generated,  and  as  it  accumulates 
the  acid  is  driven  into  the  next  bulb  and  the  action  ceases. 

Explosion  Pipette. — Another  arrangement  provides  for  explosions 
by  the  introduction  into  a  thicker  bulb  of  measured  volumes  of  the 
gas,  of  air,  and  of  hydrogen.  The  bulb  being  shut  off  with  a  stop-cock, 
a  spark  is  passed  through  wires  sealed  into  the  upper  portion  of  the 
bulb. 

Pipette  with  Capillary  Combustion  Tube. — This  simple  arrange- 
ment consists  of  a  short  glass  capillary  tube  bent  at  each  end  in 
a  right-angle,  into  which  an  asbestos  fibre  impregnated  with  finely 
divided  palladium  is  placed,  so  as  to  allow  of  the  passage  of  the  gas.* 

*  To  prepare  palladium  asbestos,  dissolve  about  1  gm.  palladium  in  aqua  regia, 
evaporate  to  dryness  on  water-bath  to  expel  all  acid.  Dissolve  in  a  veiy  small 
quantity  of  water,  and  add  5  or  6  c.c.  of  saturated  solution  of  sodium  formate,  then 
sodium  carbonate  until  strongly  alkaline.  Introduce  into  the  liquid  about  1  gm.  soft, 
long-fibred  asbestos,  which  should  absorb  the  whole  liquid.  The  fibre  is  then  dried  at 
a  gentle  heat,  and  finally  in  the  water-bath  till  perfectly  dry ;  it  is  then  soaked  in 
a  little  warm  water,  put  into  a  glass  funnel,  and  all  adhering  salts  washed  out  care- 
fully without  disturbing  the  palladium  deposit.  The  asbestos  so  prepared  contains 
about  50  per  cent.  Pd,  and  in  a  perfectly  dry  state  is  capable  of  causing  the  combin- 
ation of  H  and  O  at  ordinary  temperature,  but  when  used  in  the  capillary  tube  it  is 
preferable  to  use  heat  as  mentioned.  The  capillary  combustion  tubes  are  about  1  mm. 
bore  and  5  mm.  outside  diameter,  with  a  length  of  about  15  cm.  The  fibre  is  placed 
into  them  before  bending  the  angles,  as  follows  : — Lay  a  few  loose  fibres,  about  4  cm. 
long,  side  by  side  on  smooth  filter  paper,  moisten  with  a  drop  or  two  of  water,  then  by 
sliding  the  finger  over  them  twist  into  a  kind  of  thread  about  the  thickness  of 
darning  cotton.  The  thread  is  taken  carefully  up  with  pincers  and  dropped  into  the 
tube  held  vertically,  then  by  aid  of  water  and  gentle  shaking  moved  into  position  in 
the  middle  of  the  tube.  The  tube  is  then  dried  in  a  warm  place,  and  finally  the  ends 
bent  at  right-angle  for  a  length  of  3  J  to  4  cm.  Platinum  asbestos  may  be  prepared  in 
the  same  way,  using,  however,  only  from  half  to  one-fourth  the  quantity  of  metal. 


HEMPEL'S  ABSORPTION  PIPETTES. 


579 


The  gas  being  mixed  with  a  definite  volume  of  air  in  the  burette,  and 
the  measure  ascertained  (not  more  than  25  c.c.  of  gas  and  60  or  70  c.c. 
of  air),  the  asbestos  tube  is  heated  gently  with  a  small  gas  flame  or 
spirit  lamp,  and  the  pinch-cocks  being  opened,  the  mixture  is  slowly 
passed  through  the  asbestos  and  back  again,  the  operation  being 
repeated  so  long  as  any  combustible  gas  remains.  No  explosion 
need  be  feared.  The  residue  of  gas  ultimately  obtained  is  then 
measured,  and  the  contraction  found  ;  from  this  the  volume  of  gas 
burned  is  ascertained  either  directly,  or  by  the  previous  removal 
of  CO2  formed  by  the  combustion  with  the  potash  pipette.  H  is 
very  easily  burned,  CO  less  easily.  Ethylene,  benzene,  and 
acetylene  require  a  greater  heat  and  longer  time.  CH4  is  not 
affected  by  the  method,  even  though  mixed  with  a  large  excess  of 
combustible  gases. 

In  order  to  illustrate  the  working  of  the  whole  set  of  apparatus,  the  analysis 
of  a  mixture  containing  most  or  all  of  the  gases  likely  to  be  met  with  hi  actual 
testing  is  given  from  a  paper  contributed  by  Dr.  W.  Bott.*  The  mixture  of 
gases  consists  of  C02,  0,  CO,  C2H4,  CH4,  H  and  N.  A  sample  of  this  gas — say 
100  c.c. — is  collected  and  measured  in  the  gas  burette.  The  C02  is  next  absorbed 
by  passing  the  gas  into  a  pipette  (fig.  109)  containing  a  solution  of  1  part  of  KHO 
in  2  parts  of  water.  To  ensure  a  more  rapid  absorption,  the  bulb  shown  in  fig.  110 
containing  the  caustic  potash  may  be  partly  filled  with  plugs  of  wire  gauze.  The 
absorption  of  the  C02  is  almost  instantaneous.  It  is  only  necessary  to  pass  the 
gas  into  the  apparatus  and  siphon  it  back  again  to  be  measured.  The  contraction 
produced  gives  directly  the  percentage  of  C02  since  100  c.c.  were  used  at  starting. 


Fig.  111. 

The  remaining  gas  contains  0,  CO,  H,  C2H4,  CH4,  N.  The  oxygen  is  next  absorbed. 
This  may  be  effected  ha  two  ways — by  means  of  moist  phosphorus  or  by  an 
alkaline  solution  of  pyrogallic  acid.  The  former  method  is  by  far  the  more  elegant 
of  the  two,  but  not  universally  applicable.  The  absorption  is  done  in  a  pipette 
(fig.  1 10),  the  corked  bulb  of  which  is  filled  with  thin  sticks  of  yellow  phosphorus 
surrounded  by  water.  The  gas  to  be  tested  is  introduced  in  the  usual  manner, 
and  by  displacing  the  water  comes  into  contact  with  the  moist  surface  of  the 
phosphorus,  which  speedily  absorbs  all  the  oxygen  from  it.  The  absorption 


*  J.  S.  C.  I.  4,  160. 


2  p  2 


580  GAS   ANALYSIS. 

proceeds  best  at  about  15-20°  C.,  and  is  complete  in  ten  minutes.  The  small 
quantity  of  P203  formed  by  the  absorption  dissolves  in  the  water  present,  and 
thus  the  surface  of  the  phosphorus  always  remains  bright  and  active.  This  neat 
and  accurate  method  is  not  however  universally  applicable  ;  the  following  are  the 
conditions  under  which  it  can  be  used  : — The  oxygen  in  the  gas  must  not  be  more 
than  50  per  cent.,  and  the  gas  must  be  free  from  ammonia,  C2H4  and  other  hydro- 
carbons, vapour  of  alcohol,  ether  and  essential  oils.  In  the  instance  chosen,  the 
phosphorus  method  would  hence  not  be  applicable,  as  the  mixture  contains 
C2H4  ;  therefore  pyrogallol  must  be  used.  The  absorption  is  carried  out  in  the 
compound  absorption  pipette  (fig.  Ill),  the  bulb  of  which  is  completely  filled  with 
an  alkaline  solution  of  pyrogallol  made  by  dissolving  1  part  (by  volume)  of  a 
25  per  cent,  pyrogallol  solution  in  6  parts  of  a  60  per  cent,  solution  of  caustic 
potash.  The  absorption  is  complete  in  about  five  minutes,  but  may  be  hastened 
by  shaking.  The  remainder  of  the  gas  now  contains  C2H4,  CO,  CH4,  H,  N,  and 
the  next  step  is  to  absorb  the  C£H4  by  means  of  fuming  S03,  the  CH4  being  sub- 
sequently determined  by  explosion.  In  choosing  the  latter  method  a  portion,  say 
half,  of  the  residual  gas  is  taken  for  the  determination  of  hydrogen.  The  absorption 
of  the  hydrogen  is  based  on  the  fact  that  palladium  black  is  capable  of  completely 
oxidizing  hydrogen  when  mixed  with  excess  of  air,  and  slowly  passed  over  the  metal 
at  the  ordinary  temperature.  About  1£  gm.  of  palladium  black  are  placed  in 
a  small  U-tube  plunged  into  a  small  beaker  of  cold  water,  and  the  gas,  mixed  with 
an  excess  of  air  (which,  of  course,  must  be  accurately  measured),  is  passed  slowly 
through  the  tube  two  or  three  times,*  the  tube  at  the  time  being  connected  with 
an  ordinary  absorption  pipette  filled  with  water  or  else  with  the  KOH  pipette, 
which  in  this  case,  of  course,  simply  serves  as  a  kind  of  receiver.  Finally  the  gas 
is  siphoned  back  into  the  burette  and  measured — two -thirds  of  the  contraction 
correspond  to  the  amount  of  H  originally  present  in  the  mixture  of  gas  and  air. 
The  CH4  is  not  attacked  by  ordinary  30  per  cent.  S0.j  Nordhausen  acid  during  the 
absorption  of  the  C2H4.  The  acid  is  contained  in  an  absorption  pipette  (fig.  1 10), 
the  bulb  of  which  is  filled  with  pieces  of  broken  glass  so  as  to  offer  a  larger  absorbing 
surface  to  the  gas.  The  absorption  is  complete  in  a  few  minutes,  but  the 
remaining  gas  previous  to  measuring  should  be  passed  into  the  KOH  pipette  and 
back  again,  so  as  to  free  it  from  fumes  of  S03.  Residual  gas  :  CO,  CH4,  H,  N. 
The  CO  is  next  absorbed  by  means  of  an  ammoniacal  solution  of  cuprous  chloride 
in  a  compound  absorption  pipette.  The  gas  has  to  be  shaken  with  the  absorbent 
for  about  three  minutes.  It  must  be  borne  in  mind  that  Cu2Cl2  solution  also 
absorbs  oxygen,  and,  according  to  Hem  pel,  considerable  quantities  of  C2H4, 
hence  these  gases  must  be  removed  previously.  Residue  :  CH4,  H,  N.  Both  CH4 
and  H  may  now  be  determined  either  by  exploding  with  an  excess  of  air  in  the 
explosion  pipette  and  measuring  (1)  the  contraction  produced,  and  (2)  the  amount 
of  C02  formed  (by  means  of  the  KOH  pipette)  ;  or,  according  to  Hem  pel ,  absorb 
the  hydrogen  first  of  all  as  described  above — provided  the  U-tube  be  kept  well 
cooled  with  water,  inasmuch  as  that  at  about  200°  C.  a  mixture  of  air  and  CH4 
is  also  acted  upon  by  palladium.  The  presence  of  CO,  vapours  of  alcohol,  benzene 
and  hydrochloric  acid  also  interfere  with  the  absorption  by  palladium. 

The  palladium  may  be  used  for  many  consecutive  experiments,  but  must  be 
kept  as  dry  as  possible.  After  it  has  been  used  for  several  absorptions  it  may  be 
regenerated  by  plunging  the  tube  into  hot  water  and  passing  a  current  of  dry 
air  through  it. 

Having  determined  the  hydrogen,  the  CH4  in  the  remaining  portion  of  the  gas 
has  to  be  determined.  This  contains  CH4,  N  and  H,  the  amount  of  the  latter 
being  known  from  the  previous  experiment.  The  gas  is  mixed  with  the  requisite 
quantity  of  air  and  hydrogen,  introduced  into  the  explosion  pipette  and  fired  by 
means  of  a  spark.  The  water  resulting  from  the  combustion  condenses  in  the 
bulb  of  the  pipette,  whilst  the  C02  formed  is  absorbed  by  the  KOH  solution 
present.  Hence  the  total  contraction  produced  corresponds  to  : 

a.     The  hydrogen  present  in  the  original  gas+£  its  vol.  of  0  (the  quantity 
requisite  for  complete  combustion). 
*6.     The  known  quantity  of  hydrogen  added  +  £  its  vol.  of  "0. 

*  Instead  of  this  the  H  may  be  oxidized  in  the  tube  containing  the  palladium 
asbestos  fibre  previoiisly  described. 


fcJJDSON-HEMPEL  PIPETTES.  S8l 

<*.     The  CH4  present  +  2  vols.  of  0  requisite  for  its  combustion. 
CH4  +  04  =  (C02+2H20) 

2          4      disappears. 

Since  a  and  b  are  known,  or  can  be  readily  calculated  from  the  previous  data,  by 
subtracting  (a  +b)  from  the  total  contraction  it  is  possible  to  obtain  C  —  (a  +6)  —c 
contraction  due  to  CH4  alone,  and  one-third  of  this  is  equal  to  the  volume  of  CH4 
present,  as  will  readily  be  seen  from  the  above  equation. 
The  remaining  nitrogen  is  obtained  by  difference. 

Improved  arrangement  of  H  e  m  p  e  1 '  s  Pipettes  for  storing 
and  using  absorbents. — P.  P.  Bedson  has  designed  an  arrangement 
of  pipettes  which  he  uses  in  connection  with  aDittmar's  measuring 
apparatus,  but  which  may  of  course  be  used  with  other  forms  of 
gas  apparatus,  by  suitable  connections.  The  pipettes  are  shown 
in  fig.  112,  and  their  use  may  be  described  as  follows  : — A  capillary 
tube  with  a  three-way  cock  A  is  fused  to  the  Hemp  el  pipette — 
the  capillary  is  drawn  out  and  bent  so  as  to  pass  into  the  mercury 
trough.  The  tap  A  can  be  placed  in  connection  with  C,  to  which  is 
attached  a  movable  mercury  reservoir  D.  In  working,  e.g.,  trans- 
ferring gas  to  E,  the  absorbent  fills  E  and  the  capillary  of  tap  A. 
By  raising  D  the  vessel  C  and  capillary  B  are  entirely  filled  with 
mercury.  B,  of  course,  is  immersed  in  the  mercury  trough.  Having 
filled  B  with  mercury,  the  test  tube  containing  the  gas  to  be 
examined  is  brought  over  the  end  of  B  and  some  gas  drawn  into  C 
by  depressing  D.  The  tap  is  then  turned  to  put  the  tube  in 
connection  with  E,  and  the  gas  forced  into  E  by  depressing  the  tube 
in  trough.  By  raising  and  lowering  the  tube  the  gas  can  be  brought 
into  intimate  contact  with  the  absorbent  and  absorption  thus 

promoted.  To  bring  all  the  gas 
into  E,  D  is  again  used  and  the 
remainder  of  gas  drawn  into  C  by 
depressing  D ;  then  by  turning 
the  tap  round  the  gas  from  C  can 
be  forced  into  E  ;  the  tap  is  then 
turned  so  as  to  put  the  capillary 
and  E  in  connection,  and  the  gas 
flows  into  E  with  a  small  portion 
in  capillary  B,  retained  by  the 
column  of  mercury  filling  the  bent 
limb. 

The  gas  may  be  left  thus  for 
some  hours  ;  and  to  transfer  it  to 
the  tube,  C  and  E  are  placed  in 
connection  by  suitably  turning  the 
tap  ;  then  by  depressing  D  some 
gas  is  drawn  into  C  and  the  tap 
turned  so  as  to  put  0  and  the 
tube  in  connection. 

By    carefully    raising    D    the 


582 


GAS   ANALYSIS. 


mercury  is  washed  out  of  B  and  some  of  the  gas  passes  into  the  tube. 
With  B  clear  of  mercury  and  filled  with  gas,  the  tube  and  E  are 
placed  in  connection  and  the  gas  flows  out  of  E  into  the  tube. 
When  the  liquid  from  E  has  risen  so  as  to  fill  the  vessel  up  to  the 
tap  (the  capillary  of  the  tap  being  also  filled),  the  tap  is  turned  to 
put  C  and  B  in  connection  ;  then  by  raising  D  all  gas  is  washed  out 
of  C  and  capillary  into  the  tube  used  for  its  collection  and 
transferred  to  the  measuring  tube. 

Beds  on  also  attaches  to  the  measuring  apparatus  a  vessel  con- 
taining a  known  volume  of  air  at  known  temperature  and  pressure, 
as  recommended  by  Lunge,  so  as  to  dispense  with  the  otherwise 
necessary  corrections.     Further  details 
as    to     the     various    uses    to     which 
Hemp  el's    gas    pipettes     and     other 
simple    forms    of   gas   apparatus   may 
be  adapted,  will  be  found  inHempel's 
Gas  Analysis  (Macmillan). 

THE    NITROMETER    AND 
GAS-VOLUMETER. 

(For  list  of  conversion  factors,  see  p.  285). 
THE  nitrometer  has  been  incidentally 
alluded  to  (page  286)  as  being  useful 
for  the  determination  of  nitric  acid  in 
the  form  of  nitric  oxide.  It  was, 
indeed,  for  this  purpose  that  the 
instrument  was  originally  contrived, 
more  especially  for  ascertaining  the 
proportion  of  nitrogen  acids  in  vitriol 
by  Crum's  process. 

The  instrument  has  also  been 
found  extremely  useful  for  general 
technical  gas  analysis,  and  for  the  rapid 
testing  of  such  substances  as  manganese 
peroxide,  hydrogen  peroxide,  bleaching 
powder,  urea,  etc.  The  apparatus  in  its 
simplest  form  is  shown  in  fig.  113,  and 
consists  of  a  graduated  measuring  tube 
fitted  at  the  top  with  a  three-way  stop- 
cock and  a  glass  cup  or  funnel ;  the 
graduation  extends  from  the  tap  down- 
wards to  50  c.c.  usually,  and  is  divided 
into  yL  c.c.  The  plain  tube  known  as 
the  pressure  tube,  is  about  the  same 
size  as  the  burette,  and  is  connected 
with  the  latter  by  means  of  stout  elastic 
tubing  bound  securely  with  wire.  Both 
tubes  are  held  in  clamps  on  a  stand,  and  Fig.  113. 


583 

it  is  advisable  to  fix  the  burette  itself  into  a  strong  spring  clamp, 
so  that  it  may  be  removed  and  replaced  quickly. 

One  great  advantage  over  many  other  kinds  of  technical  gas 
apparatus  which  pertains  to  this  instrument  is  that  it  is  adapted 
for  the  use  of  mercury,  thus  ensuring  more  accurate  measurements, 
and  enabling  gases  soluble  in  water,  etc.,  to  be  examined. 

Another  form  of  the  same  instrument  is  designed  by  Lunge  for 
the  determination  of  the  nitric  acid  in  saltpetre  and  nitrate  of  soda, 
where  a  larger  volume  of  nitric  oxide  is  dealt  with  than  in  many 
other  cases.  In  this  instrument  a  bulb  is  blown  on  the  burette 
just  below  the  tap,  and  the  volume  contents  of  this  bulb  being 
found,  the  graduation  showing  its  contents  begins  on  the  tube  at 
the  point  where  the  bulb  ends,  and  thence  to  the  bottom  ;  the 
pressure  tube  also  has  a  bulb  at  bottom  to  contain  the  mercury 
displaced  from  the  burette.  Illustrations  of  this  form  of  nitrometer 
will  be  found  further  on. 

The  following  description  of  the  manipulation  required  for  the 
determination  of  nitrogen  acids  in  vitriol  applies  to  the  ordinary 
nitrometer,  and  applies  equally  to  the  determination  of  nitrates  in 
water  residues  and  the  like  (see  page  286)  : — 

The  burette  a  is  filled  with  mercury  in  such  quantity  that,  on  raising  6  and 
keeping  the  tap  open  to  the  burette,  the  mercury  stands  quite  in  the  tap-hole 
and  about  two  inches  up  the  tube  b.  The  tap  is  now  closed  completely, 
and  from  0*5  to  5  c.c.  of  the  nitrous  vitriol  (according  to  strength)  poured 
into  the  cup.  b  is  then  lowered  and  the  tap  cautiously  opened  to  the  burette, 
and  shut  quickly  when  all  the  acid  except  a  mere  drop  has  run  in,  carefully  avoiding 
the  passage  of  any  air.  3  c.c..  of  strong  pure  H2S04  are  then  placed  in  the  cup 
and  drawn  in  as  before,  then  a  further  2  or  3  c.c.  of  the  acid  to  rinse  all  traces  of 
the  sample  out  of  the  cup.  a  is  then  taken  out  of  its  clamp,  and  the  evolution  of 
gas  started  by  inclining  it  several  times  almost  to  a  horizontal  position  and 
suddenly  righting  it  again,  so  that  the  mercury  and  acid  are  well  mixed  and  shaken 
for  a  minute  or  two,  until  no  further  gas  is  evolved.  The  tubes  are  so  placed  that 
the  mercury  in  6  is  as  much  higher  than  that  in  a  as  is  required  to  balance  the  acid 
in  a  ;  this  takes  about  one  measure  of  mercury  for  6 '5  measures  of  acid.  When 
the  gas  has  assumed  the  temperature  of  the  room,  and  all  froth  subsided,  the 
volume  is  read  off,  and  also  the  temperature  and  pressure  from  a  thermometer 
and  barometer  near  the  place  of  operation.  The  level  should  be  checked  by  opening 
the  tap,  when  the  mercury  level  ought  not  to  change.  If  it  rises,  too  much  pressure 
has  been  given,  and  the  reading  must  be  increased  a  trifle.  If  it  sinks,  the  reading 
must  be  slightly  diminished.  A  good  plan  is  to  put  a  little  acid  into  the  cup  before 
opening  the  tap ;  this  will  be  drawn  in  if  pressure  is  too  low,  or  blown  up  if  it  is 
too  high.  These  indications  will  serve  as  a  guide  for  a  more  correct  second 
determination . 

To  empty  the  apparatus  ready  for  another  trial,  lower  a  and  open  the  tap, 
then  raise  b  so  as  to  force  both  gas  and  acid  into  the  cup  ;  by  opening  the  tap 
then  outwards,  the  bulk  of  the  acid  can  be  collected  in  a  beaker,  the  last  drops 
being  wiped  out  with  blotting-paper.  It  is  hardly  necessary  to  say  that  the  tap 
must  be  thoroughly  tight,  and  kept  so  by  the  use  of  a  little  vaseline,  taking  care 
that  none  gets  into  the  bore-hole. 

The  factors  for  nitrogen  etc.,  are  given  on  page  285. 

It  is  evident  that  the  nitrometer  can  be  made  to  replace  H  e  m  p  e  1'  s 
burette  if  so  required,  by  attaching  to  the  side  opening  of  the  three- 
way  tap  the  various  pipettes  previously  described,  or  smaller 


584 


GAS  ANALYSIS. 


pipettes  of  the  same  kind  to  be  used  with  mercury,  as  described  by 
Lunge.* 

The  instrument  will  also  be  found  useful  for  collecting, 
measuring,  and  analyzing  the  gases  dissolved  in  water  or  other 
liquids.  An  illustration  of  this  method  is  given  by  Lunge  and 
Schmidt  f  in  the  examination  of  a  sample  of  water  from  the  hot 
spring  at  Leuk  in  Switzerland. 

The  determination  of  the  dissolved  gases  was  made  in  the  nitro- 
meter, arranged  as  shown  in  figs.  114  and  115  :— 


Fig.   114. 


Fig.  115. 


The  flask  A  is  completely  filled  with  the  water  ;  an  india-rubber  plug  with  a 
capillary  tube  (a)  passing  through  it  is  then  inserted  in  the  flask,  and  the  tube  is 
thereby  completely  filled  ]  with  water.  The  whole  is  then  weighed,  and  the 
difference  between  this  and  the  weight  of  the  empty  flask  and  tube  gives  the 
amount  of  water  taken.  The  end  of  the  capillary  tube  is  then  connected  to  the 
side  tube  of  the  nitrometer  by  the  tube  6.  The  nitrometer  is  then  completely 
filled  with  mercury,  and  when  the  tubes  are  quiet,  the  flask  and  measuring  tube 
of  the  nitrometer  are  quickly  placed  in  connection,  without  the  introduction  of 
the  slightest  trace  of  air.  The  water  in  the  flask  is  then  slowly  heated  to  boiling. 
Some  water  as  well  as  the  dissolved  gases  collect  in  the  measuring  tube  of  the 
nitrometer.  The  tube  N  of  the  nitrometer  should  be  lowered  in  order  that  the 
boiling  may  take  place  under  reduced  pressure.  After  boiling  for  five  to  ten 
minutes,  the  stop-cock  is  quickly  turned  through  180°,  so  that  the  flask  is  placed 
in  communication  with  the  cup  B  containing  mercury,  and  the  flame  removed. 

Since  the  mercury  stands  lower  in  N  than  in  M,  it  is  not  possible  for  any  loss 
of  gas  to  take  place  at  the  moment  of  turning  the  tap.  It  is  also  impossible  for 
any  gas  or  steam  to  escape  through  the  mercury  cup,  since  the  pressure  is  inward. 
A  small  bubble  of  gas  always  remains  under  the  stopper  ;  this  is  brought  into  M 
by  lowering  the  tube  N  as  much  as  possible,  and  then  turning  the  stop-cock  so 


*  BericMe,  14,  14,  92. 


t  Z.  a.  C.  25,  309. 


LUNGE'S  IMPKOVED^NITROME^ER.  585 

that  the  flask  and  measuring  tube  are  again  placed  in  connection,  and  when  the 
bubble  has  passed  over,  quickly  reversing  the  tap  again. 

When  the  whole  of  the  gas  is  collected  in  the  nitrometer,  it  is  connected  with 
a  second  instrument,  0  P,  quite  full  of  mercury.  The  gas  is  then  transferred  by 
placing  the  tap  in  such  a  position  that  it  is  closed  in  all  directions,  and  the 
tube  M  is  heated  by  passing  steam  through  the  tube  R.  When  it  is  quite  hot 
the  tube  N  is  lowered,  causing  the  water  in  M  to  boil,  thus  expelling  every 
trace  of  dissolved  gas.  The  taps  are  then  placed  in  connection  and  the  gas 
passes  over.  It  can  then  be  cooled,  measured,  and  submitted  to  analysis.  Two 
experiments  gave  505  gm.  water  taken,  gas  evolved  5  "06  c.c.,  =10'02  per  1000  gm. 
502  gm.  water  taken,  gas  evolved  4 '94  c.c.,  =9 '84  per  1000  gm. 

Lunge's  Improved  Nitrometer  for  the  Gas- Volumetric  Analysis 
of  Permanganate4  Chloride  of  Lime,  Manganese  Peroxide,  etc. — 

Lunge*  in  describing  this  instrument  says  : — 

"  In  a  paper  published  in  the  Chemische  Industrie,  1885,  161,  I  described  the 
manifold  uses  to  which  the  nitrometer  can  be  put  as  an  apparatus  for  gas 
analysis  proper,  as  an  absorptiometer,  and  especially  for  gas-volumetric  analyses. 
To  fit  it  for  the  last-mentioned  object,  I  added  to  it  a  flask,  provided  with  an 
inner  tube  fused  on  to  its  bottom,  and  suspended  from  the  side  tube  of  the 
nitrometer,  as  shown  in  fig.  116,  which  at  the  same  time  exhibits  the  Greiner 
and  Friedrich's  patent  tap.  This  shows  how  any  ordinary  nitrometer,  such  as 
are  now  found  in  most  chemical  laboratories,  can  be  applied  to  the  before- 
mentioned  uses.  Where,  however,  the  methods  concerned  are  to  be  employed 
not  merely  occasionally,  but  regularly,  it  will  be  preferable  to  get  a  nitrometer 
specially  adapted  to  this  use,  of  which  figs.  117  and  118  show  various  forms. 
They  have  no  cup  at  the  top,  which  is  quite  unnecessary  for  this  purpose,  but 
merely  a  short  outlet  tube  for  air.  Fig.  117  shows  an  instrument  provided  with 
one  of  the  new  patent  taps,  which  are  certainly  very  handy,  and  cause -a  much 
smaller  number  of  spoiled  tests  than  the  ordinary  three-way  tap,  as  shown  in 
fig.  118,  which  at  the  same  time  exhibits  the  form  of  nitrometer  intended  for 
large  quantities  of  gas,  the  upper  part  being  widened  into  a  bulb,  below  which 
the  graduation  begins  with  either  60  or  100  c.c.,  ending  at  100  or  140  c.c. 
respectively.  There  are  also  various  shapes  of  flasks  shown  in  these  instruments, 
but  it  is  unnecessary  to  say  that  these,  as  well  as  the  bulb  arrangements,  can  be 
applied  to  any  other  form  of  the  instrument.  The  nitrometers  used  for  gas- 
volumetric  analyses  are  best  graduated  in  such  manner  that  the  zero  point^is 
about  a  centimetre  below  the  tap,  whilst  ordinary  nitrometers  have  their  zero 
point  at  the  tap  itself.  I  will  say  at  once  that  for  all  determinations  of  oxygen 
in  permanganate,  bleach  or  manganese,  it  is  quite  unnecessary  to  employ  mercury 
for  filling  the  instruments,  since  identical  results  are  obtained  with  ordinary 
tap  water  ;  but  it  is  decidedly  advisable  to  place  this  instrument,  like  any  ordinary 
nitrometer  or  any  other  apparatus  in  which  gases  are  to  be  measured,  in  a  room 
where  there  are  as  few  changes  of  temperature  by  cold  draughts  or  gas-burners, 
and  so  forth,  as  possible. 

"  It  may  be  as  well  to  give  here  a  general  description  of  the  mode  of  procedure 
for  manipulating  gas-volumetric  analysis  with  the  nitrometer,  common  to  all 
analyses  according  to  this  method.  Fill  the  nitrometer  with  water  or  mercury  by 
raising  the  pressure  tube  till  the  level  of  the  liquid  in  the  graduated  tube  is  at 
zero  (in  the  case  of  instruments  bearing  the  zero-mark  a  little  below  the  tap,  as 
in  figs.  117  and  118),  or  at  1*0  c.c.  (in  the  case  of  ordinary  nitrometers  beginning 
their  graduation  at  the  tap  itself.)  It  is  unnecessary  to  say  that  in  the  latter 
case  all  readings  must  be  diminished  by  1  c.c.  Close  the  glass  tap.  Put  the 
substance  to  be  tested  into  the  outer  space  of  the  flask,  together  with  any  other 
reagent  apart  from  the  H202  (in  the  case  of  bleaching-powder  nothing  but  the 
bleach  liquor,  in  that  of  permanganate  the  30  c.c.  of  sulphuric  acid,  etc.).  Now 
put  the  H2O2  into  the  inner  tube  of  the  flask,  after  having,  in  the  case  of  testing 
for  chlorine,  made  it  alkaline  in  the  previously  described  way.  Put  the  india- 

*  J,  S.  C.  1.9,  21. 


586 


GAS   ANALYSIS. 


rubber  stopper  still  hanging  from  the  tap,  on  to  the  flask,  without  warming  the 
latter,  as  above  described.  As  this  produces  a  compression  of  the  air  within  the 
flask,  remove  this  by  taking  out  the  key  of  the  tap  in  figs.  116,  117,  or  118,  turning 
it  for  a  moment  so  as  to  communicate  with  the  short  outlet  tube.  Now  turn  the 
tap  back,  mix  the  liquids  by  inclining  the  flask,  shake  up  and  allow  the  action  to 
proceed.  As  the  gas  passes  over  into  the  graduated  tube,  lower  the  pressure 
tube,  so  as  to  produce  no  undue  pressure  ;  at  last  bring  the  liquid  in  both  tubes  to 
an  exact  level  and  read  off. 


Fig.   117. 


Fig.  116. 


Fig.  118. 


"  In  the  case  of  bleach  analysis  all  the  oxygen  of  the  chloride  of  lime  is  given 
off,  together  with  exactly  as  much  oxygen  of  the  H202.  The  total  is  just  equal 
to  the  volume  of  chlorine  gas  which  would  be  given  off  by  the  chloride  of  lime, 
and  thus  immediately  represents  the  French  or  Gay-Lussac  chlori metric 
degrees,  of  course  after  reducing  the  volume  to  0°  and  760  mm.  pressure.  (The 
reading  of  the  barometer  must  be  corrected  by  deducting  the  tension  of  aqueous 


LUNGE'S  GAS- VOLUMETER. 


587 


vapour  for  the  temperature  observed  as  well  as  the  expansion  of  mercury,  accord- 
ing to  the  tables  found  everywhere)." 

Lunge's  Gas- volumeter  is  an  apparatus  for  dispensing  with 
reduction  calculations  in  measuring  gas  volumes  (described  by 
Lunge  in  Zeitschrift  /.  angew,  Chem.,  1890,  139-144,  and  here 
quoted  from  J.  S.  G.  I.  ix.  547). 

In  technical  gas  analysis  a  con- 
siderable amount  of  time  is  taken 
up  by  calculations  for  reducing 
gas  volumes  to  standard  temper- 
ature and  pressure.  In  pure  gas 
analysis  the  inconvenience  is  not 
so  great ;  for  technical  purposes 
the  initial  and  end  temperature 
and  pressure  may  be  taken  as 
the  same,  owing  to  the  short  dur- 
ation of  the  experiment,  and  for 
more  accurate  purpose  "  com- 
pensators "  have  been  devised. 
Where,  however,  the  gas  to  be 
measured  is  evolved  from  a 
weighed  quantity  of  a  liquid  or 
solid  (so  that  volume  and  weight 
have  finally  to  be  connected)  the 
matter  is  different,  and  readings 
of  thermometer  and  barometer 
have  to  be  made,  and  then  the 
necessary  calculations  have  to  be 
gone  through.  Tables  of  reduc- 
tion have  certainly  been  compiled 
for  reduction  of  gaseous  volumes 
at  various  temperatures  and  pres- 
sures to  N.T.P.,  but  readings  of 
thermometer  and  barometer  still 
have  to  be  made,  and  only  part 
of  the  time  is  saved.  Further  to 
reduce  the  time  occupied  and  to 
render  the  technical  chemist  in 
this  department  to  a  great  extent 
independent  of  temperature  and 
atmospheric  pressure  the  present 
apparatus  has  been  constructed. 


Fig.  119. 


By  means  of  a  T-tube  D  (fig.  119),  and  thick-walled  rubber  tubing,  are 
connected  the  three  tubes  A,  B,  C.  A  is  for  measuring  the  gas  ;  it  may  be  any 
form  of  nitrometer,  a  B  u  n  t  e '  s  burette  or  other  convenient  burette.  B  is  the 
"  reduction  tube,"  which  has  at  its  upper  end  a  spherical  or  cylindrical 
bulb.  The  volume  to  the  first  mark  is  100  c.c.,  the  remaining  narrow  portion 
of  the  tube  being  calibrated  up  to  130-140  c.c.  in  divisions  representing 
TV  c.c.  This  "  reduction  tube  "  is  set  once  for  all  at  the  beginning  of  work 
by  observing  thermometer  and  barometer,  calculating  the  volume  which  100  c.c. 


GAS   ANALYSIS. 


of  perfectly  dry  air,  measured  at  0°  C.  and  760  mm.  would  occupy  under 
the  existing  conditions.  This  quantity  of  air  is  then  introduced,  and  the  tube 
closed  by  means  of  the  stop-cock  shown,  or  by  fusing  up  the  inlet  (having 
in  place  of  the  inlet  tube  shown  in  the  figure  a  tube  of  capillary  bore).  If  it 
be  necessary  to  measure  the  gas  moist  a  drop  of  water  is  introduced  into  this 
tube,  and  of  course  in  the  calculation  necessary  the  barometric  pressure  must 
be  reduced  by  the  vapour  tension  of  water  ;  if  the  gases  are  to  be  measured 
perfectly  dry  (as,  for  instance,  when  using  the  nitrometer  with  sulphuric  acid), 
a  drop  of  sulphuric  acid  takes  the  place  of  the  water. 
C  is  the  pressure  tube. 


120. 


If  .'necessary  for  the  purpose  of  regulating  the  temperature  A  and  B  may  be 
surrounded  with  water-jackets.  A,  B,  and  C  are  supported  by  spring  clamps. 
It  is  easily  seen  that  when  by  raising  C  the  level  of  the  mercury  in  B  has  been 
forced  up  to  the  mark  100,  exactly  the  amount  of  pressure  is  exerted  by  C  as  will 
compress  the  gas  in  B  to  its  volume  under  standard  conditions. 

In  taking  a  reading  A  and  B  must  be  levelled  and  the  mercury  level  in  B  must 
have  been  brought  up  to  100.  The  volume  shown  on  A  is  then  the  volume  reduced 


LUNGE'S  GAS-VOLUMETER.  589 

to  standard  temperature  and  pressure.  In  cases  where  the  gas  is  generated  in 
A  itself,  or  where  the  gas  is  transferred  to  A,  this  is  all  that  need  be  done.  If, 
however,  the  gas  is  generated  in  a  side  apparatus,  as  shown  in  fig.  119,  A  and  C 
must  first  be  levelled  and  the  stop-cock  of  A  then  closed  so  that  the  gas  in  A  is 
collected  at  atmospheric  pressure.  After  this  reduction  may  be  effected  as 
already  explained. 

In  nitrogen  determinations  by  Dumas'  method,  A  contains  caustic  potash  as 
well  as  mercury  ;  this  is  compensated  by  having  on  the  reduction  tube,  B,  a  mark 
at  a  distance  below  the  100  mark  equal  to  one-tenth  of  the  height  of  the  caustic 
potash  column  (sp.  gr.  of  the  caustic  potash  equals  one-tenth  sp.  gr.  of  mercury) ; 
when  taking  a  reading  the  mercury  in  B  must  be  at  100,  and  that  in  A  must  be 
on  a  level  with  this  new  lower  mark  of  B.  Similar  allowance  may  be  made  in 
riitrometric  determinations,  but  the  case  is  here  more  difficult,  owing  to  the 
variations  in  the  quality  and  specific  gravity  of  the  sulphuric  acid  used.  It  is 
better  in  such  cases  to  liberate  the  gas  in  a  separate  vessel  and  transfer  subsequently 
to  the  burette  for  reduction  and  measurement.  Fig.  120  shows  a  convenient  form 
of  apparatus.  Of  course  the  working  part  E,  F  need  not  be  graduated.  Before 
beginning  the  operation  the  mercury  is  made  to  fill  E  with  the  side  tube  a,  which 
side  tube  is  then  capped  with  a  caoutchouc  stopper  to  prevent  escape  of  the  mercury 
during  subsequent  shaking.  A,  with  its  side  tube,  e,  is  also  completely  filled 
with  mercury.  The  substance  under  examination,  and  subsequently  the  acid, 
are  added  through  C  as  usual.  To  transfer  the  gas  from  E  to  A,  the  cap  6  is 
removed  and  e.  is  fitted  to  a  by  means  of  the  rubber  connection  d.  F  is  then 
raised  and  C  lowered,  the  taps  are  carefully  opened,  and  transference  effected 
until  the  acid  in  E  just  fills  e. 

A  further  saving  of  time  may  be  effected  in  works,  where  the 
instrument  is  to  be  used  always  for  one  and  the  same  object,  by 
marking  on  the  gas  burette  or  nitrometer  the  weight  in  milligrams 
corresponding  to  certain  volumes  ;  this  may  be  done  either  instead 
of  or  alongside  the  c.c.  divisions  ;  or,  by  using  a  fixed  quantity  of 
substance,  percentages  may  be  marked  off  directly.  For  nitrogen 
determinations  by  Dumas'  method  1  c.c.  of  nitrogen  under  normal 
conditions  weighs  1-2507  mgm.  In  the  case  of  azotometric 
determinations  of  ammoniacal  nitrogen  (by  sodium  hypobromite) 
the  graduations  may  be  made  to  represent  ammonia.  Correction 
must  be  made  in  graduating,  however,  for  the  incompleteness  of 
the  reaction.  Tables  giving  the  corrections  have  been  introduced, 
but  the  author  has  shown  that  these  may  be  dispensed  with,  and 
that  it  is  sufficient  to  make  a  correction  of  2-5  per  cent.  For  urea, 
however,  the  correction  is  9  per  cent. 

The  following  table  shows  substances  for  which  gasometric 
methods  are  used  : — 


590 


GAS   ANALYSIS. 


Substances. 

Basis  to  which 
Percentages  are 
Calculated. 

Method 
Employed. 

Gas 
Evolved, 

1  c.c.  of  Gas. 
=  mgm.  of  Basis, 
(Col.  II.) 

Organic  substances 

Nitrogen 

Dumas' 

N 

1-2507 

Ammonia  salts    .  . 

j» 

Hypobrmte. 

N 

1-285* 

»           »> 

Ammonia 

}> 

N 

1-561* 

Urine    

Urea 

N 

2  '952* 

Bone-charcoal,  etc. 

Carbon  dioxide 

9  J 

Decomposed 
with  HC1 

C02 

1  -960 

»>         » 

Calcium  carbonate 

» 

C02 

4-468 

Pyrolusite    

Manganese  dioxide 

Bv  HoO, 

o 

3  -882 

Bleaching  powder 

Chlorine 

•"J    J-LZ^-'2 

0 

1-5835 

Potassium  perman- 
ganate    . 

Oxygen 

,, 

0 

0-715 

Chili  saltpetre.  .  .  . 

Sodium  nitrate 

Nitrometer 

NO 

3-805 

Nitrous  bodies     .  . 

N203 

» 

NO 

1-701 

»         »> 

HNO3 

NO 

2-820 

»>         » 

Nitric  acid  36°  B. 

?> 

NO 

5-330 

>»                    5> 

Sodium  nitrate 

, 

NO 

3-805 

Nitroglycerol,    dy- 
namite, etc  

Trinitroglycerol 

„ 

NO 

3-387 

» 

Nitrogen 

, 

NO 

0-6267 

Nitrocellulose,  py- 
roxylin 

» 

jf 

NO 

0-6267 

*  The  corrections  above  referred  to  have  here  already  been  made. 

Japp*  describes  a  modification  of  Lunge's  gas- volumeter, 
by  means  of  which  with  accurately  graduated  ordinary  50  c.c.  gas 
burettes  any  required  single  gas  may  without  observation  of 
temperature  or  pressure,  and  without  calculation,  be  measured 
under  such  conditions  that  each  c.c.  represents  a  milligram  of  the 
gas.  The  name  "  gravi volumeter  "  is  appropriately  given  to  this 
instrument,  and  it  undoubtedly  possesses  this  advantage  over 
Lunge's  instrument  that  it  obviates  the  necessity  of  having 
a  number  of  different  gas-volumeters  for  different  substances,  and 
moreover  its  manufacture  involves  no  large  amount  of  skill,  as  the 
ordinary  graduation  in  c.c.  in  T^  or  -^  is  all  that  is  required. 

The  apparatus  is  represented  in  fig.  121.  It  consists  of  two  gas  burettes,  of 
50  c.c.  capacity  each,  both  furnished  with  obliquely  bored  taps.  One  of  these 
burettes,  A,  which  has  a  three-way  tap,  is  the  gas  measuring  tube  ;  the  other,  B, 
which  need  only  have  a  single  tap,  performs  the  function  of  the  regulator  in 
Lunge's  gas-volumeter,  and  may  be  termed  the  "regulator  tube."  As  in 
Lunge's  instrument,  both  tubes  are  moistened  internally  with  a  drop  of  water, 
in  order  that  the  gases  they  contain  may  be  saturated  with  aqueous  vapour,  and 
both  are  connected,  by  means  of  stout,  flexible  tubing  and  a  T-piece,  with  the 
same  movable  reservoir  of  mercury,  C.  And  since,  in  certain  determinations, 
the  level  of  the  mercury  reservoir  is  considerably  below  the  lower  end  of  the  two 
burettes,  and  an  inward  leakage  of  air  might  thus  occur  at  the  junctions  of  the 
burettes  with  the  india-rubber  tubing,  these  junctions  are  surrounded  with  pieces 
of  wider  india-rubber  tubing,  D,  D,  tied  round  the  bottom  and  open  at  the  top, 
and  filled  with  water,  so  as  to  form  a  water  joint. 

The  25  c.c.  division  of  the  regulator  tube  is  taken  as  the  starting  point  in 


*  J.  C.  S.  59,  894 


JAPP-LUNGE    GAS-VOLUMETER. 


591 


calculating  what  may  be  termed  the  "  gravi volumetric  values  "  of  the  different 
gases  to  be  measured.  Thus  in  the  case  of  nitrogen  it  is  necessary  to  calculate 
to  what  volume  25  c.c.  of  standard  dry  nitrogen  must  be  brought  in  order  that 
1  c.c.  may  correspond  with  1  mgm.  of  the  gas  ;  that  is  to  say,  25  c.c.  of  standard 
dry  nitrogen  weigh  0*0012507  x25=0'0313  gm.  ;  and,  therefore,  these  31 '3  mgm. 
must  be  brought  to  the  volume  of  3T3  c.c.  The  division  31 '3  ori  the  regulator 
tube  is  marked  N2.  Corresponding  points  are  in  like  manner  determined  for  the 
various  other  gases  which  it  is  desired  to  measure,  and  these  points  are  marked 
O2,  C02,  etc.,  as  the  case  may  be,  on  the  regulator  tube.  Finally,  the  thermometer 
and  barometer  are  read  (a  process  only  necessary  once  for  all  in  setting  the 
regulator),  the  volume  which  25  c.c.  of  standard  dry  air  would  occupy  if  measured 
moist  at  the  observed  temperature  and  pressure  is  calculated,  and  this  calculated 
volume  of  air  is  admitted  at  atmospheric  temperature  and  pressure  into  the 
regulator  tube  and  the  tap  closed.  The  instrument  is  now  ready  for  use. 


121. 


Suppose  it  is  desired  to  ascertain  the  weight  of  a  quantity  of  nitrogen  contained 
in  the  measuring  tube.  The  mercury  reservoir  is  raised  or  lowered  until  the 
mercury  in  the  regulator  tube  stand  at  the  nitrogen  mark,  31 '3,  at  the  same  time 
adjusting  the  regulator  tube  itself  by  raising  or  lowering  it  bodily,  so  that  the 
mercury  level  in  the  measuring  tube  and  the  regulator  tube  may  be  the  same. 
In  these  circumstances  each  cubic  centimetre  of  gas  in  the  measuring  tube  represents 
1  mym.  of  nitrogen.  For  since  in  the  regulator  tube  25  c.c.  of  standard  dry  air 
have  been  made  to  occupy  the  volume  of  31  -3  c.c.,  and  since  the  gases  in  the  two 
tubes  are  under  the  same  conditions  as  regards  temperature,  pressure,  and 
saturation  with  aqueous  vapour,  therefore,  in  the  measuring  tube,  every  25  c.c. 


592  GAS    ANALYSIS. 

of  standard  dry  nitrogen  have  also  been  made  to  occupy  the  volume  of  31 '3  c.c. 
But  25  c.c.  of  standard  dry  nitrogen  weigh,  as  we  have  seen,  31  '3  mgm.  ;  so  that 
the  problem  is  solved,  and  the  cubic  centimetres  and  tenths  of  cubic  centimetres' 
give  directly  the  weight  of  the  gas  in  milligrams  and  tenths  of  milligrams. 

The  various  other  single  (i.e.,  unmixed)  gases  may  be  weighed  in  like  manner 
by  bringing  the  mercury  in  the  regulator  tube  to  the  "  gravivolumetric  mark  " 
of  the  gas  in  question,  and  adjusting  the  levels  as  before.  An  exception  would 
be  made  in  the  case  of  hydrogen,  which  would  be  brought  to  such  a  volume  that 
the  cubic  centimetre  would  contain  a  tenth  of  a  milligram. 

Mixtures  of  gases  may  also  be  weighed,  provided  that  the  density  of  the  mixture 
is  known. 

Lastly,  if  the  mercury  in  the  regulator  tube  be  brought  to  the  mark  25  and 
the  levels  adjusted,  a  gas  or  mixture  of  gases  in  the  measuring  tube  will  have  the 
volume  which  it  would  occupy  in  the  standard  dry  state.  In  this  form  the 
instrument  is  merely  a  gas-volumeter,  as  described  by  Lunge,  and  may  be  used 
for  ordinary  gas  analysis. 

The  experiments  made  by  Japp  with  the  view  of  ascertaining 
the  degree  of  accuracy  of  which  the  apparatus  is  capable  were  very 
satisfactory,  details  being  given  in  the  paper  mentioned.  The 
substances  experimented  on  were  Methane,  with  a  gravivolumetric 
value  of  17-9  ;  Nitrogen,  31-3  ;  Air,  32-35  ;  and  Carbon  dioxide, 
49-4 

The  measuring  tube  and  regulator  tube  were  held  by  a  double  clamp,  the  arms 
of  which  could  be  moved  horizontally,  so  as  to  admit  of  bringing  the  tubes  close 
together  when  necessary.  The  two  tubes  were  so  arranged  that,  after  adjusting 
the  levels  and  ascertaining  that  the  mercury  in  the  regulator  tube  was  at  the 
gravivolumetric  mark,  it  was  possible  to  read  both  levels  without  moving  the 
position  of  the  eye.  The  object  of  this  was  that  any  possible  error  of  parallax 
might  operate  equally  and  in  the  same  direction  in  both  tubes,  in  which  case  the 
two  errors  would  tend  to  neutralize  one  another  in  the  final  result.*  The  mercury 
reservoir  was  held  by  a  clamp  attached  to  a  separate  stand,  so  that  in  the  case  of 
extreme  differences  of  pressure  the  entire  stand  could  be  placed  on  a  different 
level  from  the  rest  of  the  apparatus. 

Assuming  the  graduation  of  a  gravivolumeter  to  be  correct,  or  the  defects  of 
graduation  to  be  eliminated  by  calibration,  the  sources  of  error  in  such  an 
instrument  are,  broadly  speaking,  four  in  number,  and  are  to  be  found  in 
imperfections  (1)  in  filling  the  regulator.  (2)  in  adjusting  the  levels,  (3)  in  reading 
the  regulator,  and  (4)  in  reading  the  measuring  tube.  The  first  of  these  operations, 
that  of  filling  the  regulator,  is  performed  once  for  all  with  very  great  care,  and  may, 
for  all  practical  purposes,  be  disregarded  as  a  source  of  error.  Again,  in  adjusting 
the  levels,  the  two  tubes  can  be  brought,  by  means  of  the  double  clamp,  within 
such  a  short  distance  of  one  another  that  the  adjustment  is  also  practically 
accurate.  The  real  sources  of  error  lie  in  the  last  two  operations.  The  burettes 
are  divided  into  tenths  of  cubic  centimetres,  and  can  be  read  with  the  eye  alone 
accurately  to  ^V  c-c.  Calculating  this  error  on  25  c.c.  as  the  average  volume 
of  gas  contained  in  the  regulator  tube  and  measuring  the  tube  respectively,  we 
have  I/  (20  x  25)  =-^U  as  the  error  for  each  tube.  But  as  the  error  in  the 
regulator  repeats  itself  in  exact  proportion  in  the  altered  volume  of  gas  in  the 
measuring  tube,  we  must  add  the  error  of  the  regulator  to  the  independent  error 
of  the  measuring  tube,  in  order  to  ascertain  the  maximum  error,  which  would  thus 
be  -5%-$  ;  and  this,  calculated  as  assumed,  upon  25  c.c.  of  gas,  would  be  equal  to  an 
error  of  reading  O'l  c.c.  in  the  final  result.  An  inspection  of  the  foregoing  experi- 

*  Suppose  the  eye  in  reading  to  be  too  high,  the  mercury  in  the  regulator  would 
stand  below  the  gravivolumetric  mark,  and  the  gas  in  the  measuring  tube  would  con- 
sequently be  expanded  beyond  its  proper  volume.  But  owing  to  the  eye  being  too 
high,  this  too  great  volume  in  the  measuring  tube  would  be  read  off  as  smaller  than  it 
actually  is.  In  the  case  of  equal  volumes  of  gas  in  regulator  and  measuring  tube, 
there  would  thus  be  a  total  correction  of  the  error  committed  (since  the  two  tubes  are 
of  equal  bore),  and  in  every  case  a  diminution. 


FINIS.  593 

mental  results,  however,  discloses  the  fact  that  the  maximum  error  is  only  half 
this  amount,  or  0'05  c.c.  ;  and  this  the  author  attributes  to  the  fact  that,  owing 
to  the  method  of  reading  employed,  the  errors  of  reading  in  the  regulator  and 
measuring  tube  are  not,  as  assumed  in  the  foregoing  calculation,  independent, 
but  tend  to  neutralize  one  another. 

This  error  of  0'05  c.c.  is,  however,  the  error  of  reading  of  any  gas  burette  which 
is  read  with  the  eye  alone  ;  and  the  gravivolumeter  may,  therefore,  claim  to 
possess  the  same  degree  of  accuracy  as  instruments  of  this  class  generally. 


2  Q 


594 


TABLES   FOR   WATER   ANALYSIS. 


TABLE    1. 

Elasticity  of  Aqueous  Vapour  for  each  ^th  degree  centigrade  from  0° 
to  30°  C.  (Regnault). 


Temp. 

c. 

Tension  in 
Millimetres 
of  Mercury. 

Temp. 
C. 

Tension  in 
Millimetres 
of  Mercury. 

Temp. 

C. 

Tension  in 
Millimetres 
of  Mercury. 

Temp 

C. 

Tension  in 
Millimetres, 
of  Mercury. 

Temp. 
C. 

Tension  in 
Millimetres 
of  Mercury. 

•o° 

4-6 

6-0° 

7'0 

12-0° 

10-5 

18'0° 

15-4 

24'0° 

22-2 

•1 

4'6 

•1 

7-0 

•1 

10-5 

•1 

15-5 

•1 

22-3 

•2 

4'7 

•2 

7'1 

'2 

10*6              '2 

1ST) 

•2 

22-5 

•3 

4'7 

•3 

7-1 

•3 

10'7              '3 

15'7 

•3 

22-6 

•4 

4-7 

•4 

7'2 

•4 

10-7 

•4 

15-7 

•4 

22-7 

•5 

4'8 

•5 

7'2 

•5 

10'8 

•5 

15'8 

"5 

22-9 

•6 

4'8 

•6 

7-3 

•6 

10-9 

•6 

15-9 

•6 

23-0 

•7 

4'8 

•7 

7'3 

•7 

10-9 

•7 

16-0 

•7 

23'1 

•8 

4-9 

•8 

7'4 

•8 

ll'O 

•8 

16-1 

•8 

23-3 

•9 

4'9 

•9 

7'4 

•9 

11*1 

•9 

16-2 

•9 

23-4 

I'O 

4-9 

7'0 

7'5 

13-0 

11'2 

19-0 

16'3 

25'0 

23-5 

•1 

5'0 

•1 

7-5 

•1 

11-2 

•1 

16-4 

•1 

23-7 

•2 

5-0 

•2 

7'6 

•2 

11-3 

•2 

16-6 

•2 

23-8 

•3 

5-0 

•3 

7'6 

•3 

11-4 

•3 

16-7 

•3 

24-0 

•4 

5'1 

•4 

7'7 

•4 

11'5 

•4 

16-8 

•4 

24-1 

•5 

5-1 

•5 

7'8 

•5 

11'5 

•5 

16'9 

•5 

24-3 

•6 

5'2 

•6 

7'8 

•6 

11-6 

•6 

17-0 

•6 

24'4 

•7 

5'2 

•7 

7'9 

•7 

in 

•7 

17'1 

•7 

24-6 

•8 

5'2 

•8 

7-9 

•8 

11-8 

•8 

17'2 

•8 

24'7 

•9 

5-3 

•9 

8-0 

•9 

11-8 

•9 

17-3 

•9 

24-8 

2-0 

5'3 

8-0 

8-0 

14'0 

11-9 

20-0 

17'4 

26-0 

25-0 

•1 

5-3 

•1 

8-1 

•1 

12-0 

•1 

17-5 

•1 

25-1 

•2 

5'4 

•2 

8-1 

•2 

12-1 

•2 

17'6 

•2 

25-3 

•3 

5*4 

•3 

8-2 

•3 

12-1 

•3 

17'7 

•3 

25'4 

•4 

5'5 

•4 

8'2 

•4 

12-2 

•4 

17-8 

•4 

25-6 

•5 

5'5 

•5 

8'3 

•5 

12-3 

•5 

17-9 

•5 

25-7 

•6 

5'5 

•6 

8-3 

•6 

12-4 

•6 

18-0 

•6 

'   25-9 

•7 

5'6 

•7 

8-4 

•7 

12-5 

*7 

18-2 

•7 

26-0 

•8 

5-6 

•8 

85 

•8 

12-5 

•8 

18-3 

•8 

26-2 

•9 

5'6 

•9 

8.5 

•9 

12-6 

•9 

18-4 

•9 

2f>-4 

3-0 

5*7 

9-0 

8T> 

15'0 

12-7 

21-0 

18'5 

27-0 

26-5 

•1 

5'7 

•1 

8-6 

•1 

12'8 

•1 

18-6 

•1 

26'7 

•2 

5-8 

•2 

8'7 

•2 

12-9 

•2 

18'7 

•2 

26-8 

•3 

5'8 

•3 

8-7 

•3 

12-9 

•3 

18-8 

•3 

27-0 

•4 

5'8 

•4 

8-8 

•4 

13-0 

•4 

19'0 

•4 

27-1 

•5 

5'9 

•5 

8-9 

•5 

13-1 

•5 

19'1 

'5 

27-3 

•6 

5-9 

•6 

8-9 

•6 

13-2 

•6 

19-2 

•6 

27'5 

•7 

6-0 

•7 

9-0 

•7 

13-3 

'7 

19-3 

•7 

27-6 

•8 

6-0 

•8 

9-0 

•8 

13-4 

•8 

19-4 

•8 

27-8 

•9 

6-1 

•9 

9-1 

•9 

13-5 

•9 

19-5 

•9 

27'9 

4-0 

6-1 

10-0 

9-2 

16-0 

13'5 

22-0 

19-7 

28-0 

28-1 

•1 

6-1 

•1 

9-2 

•1 

13-6 

•1 

19-8 

•1 

28-3 

•2 

6'2 

•2 

9-3 

•2 

13'7 

•2 

19-9 

•2 

28-4 

•3 

6'2 

•3 

9'3 

•3 

13'8 

•3 

20'0 

•3 

28-6 

•4 

6'3 

•4 

9'4 

•4 

13'9 

•4 

20-1 

•4 

28-8 

•5 

6-3 

•5 

9-5 

•5 

14-0 

•5 

20-3 

•5 

28-9 

•6 

6-4 

•6 

9-5 

•6 

14-1 

•6 

20'4 

•6 

29-1 

•7 

6'4 

•7 

9-6 

•7 

14-2 

•7 

20-5 

•7 

29-3 

•8 

6-4 

•8 

9-7 

•8 

14'2 

•8 

20-6 

•8 

29-4 

•9 

6'5 

•9 

9-7 

•9 

14-3 

•9 

20-8 

•9 

29-6 

5-0 

6-5 

11-0 

9-8 

17-0 

14-4 

23-0 

20-9 

29-0 

29-8 

•1 

6'6 

•1 

9'9 

•1 

14'5 

•1 

21-0 

1 

30'0 

•2 

6-6 

•2 

9-9 

•2 

14-6 

•2 

21'1 

•2 

30'1 

•3 

6'7 

•3 

10-0 

•3 

14-7 

•3 

21-3 

•3 

30-3 

•4 

6-7 

•4 

10-1 

•4 

14'8 

•4 

21'4 

•4 

30-5 

•5 

6'8 

'5 

10-1 

•5 

14-9 

"5 

21'5 

•5 

30-7 

•6 

6'8 

•6 

10-2 

•G 

15'0 

•6 

21-7 

•6 

30-8 

•7 

6-9 

•7 

10-3 

•7 

15-1 

•7 

21-8 

•7 

31-0 

•8 

6-9 

•8 

10-3 

•8 

15-2 

•8 

21-9 

•8 

31-2 

•9 

7-0 

•9 

10-4 

•9 

15-3 

•9 

22-1 

•9 

31-4 

TABLES    FOR   WATER   ANALYSIS. 


595 


Log. 


TABLE   2. 
Reduction  of  Cubic  Centimetres  of  Nitrogen  to  Grams. 


.  o-00367<)760 


for  each  tenth  °f  a 


from  °°  *°  30°  C< 


t.C 

o-o 

o-i 

0-2 

0-3 

0-4 

0-5 

0-6 

0-7 

0-8 

0-9 

0° 

6-21824 

808 

793 

777 

761 

745 

729 

713 

697 

681 

1 

665 

649 

633 

617 

601 

586 

570 

554 

538 

522 

2 

507 

491 

475 

459 

443 

427 

412 

396 

380 

364 

3 

349 

333 

318 

302 

286 

270 

255 

239 

223 

208 

4 

192 

177 

161 

145 

130 

114 

098 

083 

067 

051 

5 

035 

020 

004 

*989 

*973 

*957 

*942 

*926 

*911 

*895 

6 

6*20879 

864 

848 

833 

817 

801 

786 

770 

755 

739 

7 

723 

708 

692 

676 

661 

645 

629 

614 

598 

583 

8 

567 

552 

536 

521 

505 

490 

474 

459 

443 

428 

9 

413 

397 

382 

366 

351 

335 

320 

304 

289 

274 

10 

259 

244 

228 

213 

198 

182 

167 

151 

136 

121 

11 

106 

090 

075 

060 

045 

029 

014 

*999 

*984 

*969 

12 

6T19953 

938 

923 

907 

892 

877 

862 

846 

831 

816 

13 

800 

785 

770 

755 

740 

724 

709 

694 

679 

664 

14 

648 

633 

618 

603 

588 

573 

558 

543 

528 

513 

15 

497 

482 

467 

452 

437 

422 

407 

392 

377 

362 

16 

346 

331 

316 

301 

286 

271 

256 

241 

226 

211 

17 

196 

181 

166 

151 

136 

121 

106 

091 

076 

061 

18 

046 

031 

016 

001 

*986 

*971 

*956 

*941 

*926 

*911 

19 

"6-18897 

882 

867 

852 

837 

822 

807 

792 

777 

762 

20 

748 

733 

718 

703 

688 

673 

659 

644 

629 

614 

21 

600 

585 

570 

555 

540 

526 

511 

496 

481 

466 

22 

452 

437 

422 

408 

393 

378 

363 

349 

334 

319 

23 

305 

290 

275 

261 

246 

231 

216 

202 

187 

172 

24 

158 

143 

128 

114 

099 

084 

070 

055 

041 

026 

25 

012 

*997 

*982 

*968 

*953 

*938 

*924 

*909 

*895 

*880 

26 

6~-17866 

851 

837 

822 

808 

793 

779 

764 

750 

735 

27 

721 

706 

692 

677 

663 

648 

634 

619 

605 

590 

28 

576 

561 

547 

532 

518 

503 

489 

475 

460 

446 

29 

432 

417 

403 

388 

374 

360 

345 

331 

316 

302 

\ 

2  Q  2 


596 


TABLES   FOR   WATER   ANALYSIS. 


TABLE   3. 

Loss  of  Nitrogen  by  Evaporation  of  NH!. 
With  Sulphurous  Acid. 

Parts  per  100,000. 


Nas 

Loss 

Nas 

Loss 

Nas 

Loss 

Nas 

Loss 

Nas 

Loss 

Nas 

Loss 

NH». 

of  N. 

NH3. 

of  N. 

NH3. 

of  N. 

NH3. 

of  N. 

NH3, 

of  N. 

NH3. 

of  X. 

5-0 

1-741 

3-9 

1-425 

2-8 

•898 

•7 

•370 

•6 

•145 

•04 

•009 

4-9 

1-717 

3-8 

1-378 

2-7 

•850 

•6 

•338 

•5 

•109 

•03 

•007 

4-8 

1-693 

3-7 

1-330 

2-6 

•802 

•5 

•324 

•4 

•075 

•02 

•005 

4-7 

1-669 

3-6 

1-282 

2-5 

•754 

•4 

•309 

•3 

•057 

•01 

•003 

4'6 

1-645 

3-5 

1-234 

2-4 

•706 

•3 

•295 

•2 

•038 

•008 

•002 

4-5 

1-621 

3-4 

1-186 

2-3 

•658 

•2 

•280 

•1 

•020 

•007 

•001 

4'4 

1-598 

3-3 

1-138 

2-2 

•610 

•1 

•266 

•09 

•018 

4'3 

1-574 

3-2 

1-090 

2-1 

•562 

•o 

•252 

•08 

•017 

4-2 

1-550 

3-1 

1-042 

2-0 

'514 

•9 

•237 

'07 

•015 

4-1 

1  521 

3'0 

•994 

1-9 

•466 

•8 

•217 

•06 

•013 

4-0 

1-473 

2-9 

•946 

1'8 

•418 

•7 

•181 

•05 

•on 

TABLE    4. 

Loss  of  Nitrogen  by  Evaporation  of  NH  . 
With  Hydric  Metaphosphate. 

Parts  per  100,000. 


4 

a 

fc 

ll 

s 

fe 

§1 

s 

fc 

d 

o>J> 

&>' 

w 

£ 

_  O 

fc 

m 

o 

II 

fc 

S 

n  eg 
5o 

£ 

"o 

11 

X 

o 

og, 

§ 

• 

o  — 

§ 

OQ 

op, 

§ 

• 

OP, 

§ 

% 

t>cS 

fc 

3 

>& 

fe 

O 

h9 

>$ 

<B 

55 

o 

^ 

>£ 

o 

% 

o 

A 

100  c.c. 

8-2 

•482 

100  c.c. 

5'9 

•385 

100  c.c. 

3-6 

•281 

100  c.c. 

1-3 

•142 

8-1 

•477 

5-8 

•381 

3-5 

•277 

1-2 

•136 

8-0 

•473 

5'7 

•377 

3'4 

•272 

a 

1*1 

•129 

7-9 

•469 

5'6 

•373 

3-3 

•267 

^ 

I'O 

•123 

7-8 

•465 

5'5 

•368 

3-2 

•261 

m 

•9 

•117 

7-7 

•461 

5'4 

•364 

3-1 

•255 

•8 

•111 

7-6 

•456 

5'3 

•360 

3-0 

•249 

250*  c.c. 

•7 

•088 

7-5 

•452 

52 

•356 

2'9 

•242 

•6 

•073 

7'4 

•448 

5'1 

•352 

2'8 

•236 

•5 

•061 

7'3 

•444 

5'0 

•347 

2-7 

•230 

500  c.c. 

•4 

•049 

7-2 

•440 

4-9 

343 

2'6 

•223 

•3 

•036 

7'1 

•435 

4-8 

•338 

2-5 

•217 

1000  c.c. 

•2 

•024 

7'0 

•431 

4'7 

•334 

2-4 

•211 

•1 

•012 

6'9 

•427 

4-6 

•329 

2-3 

•205 

•09 

•on 

6'8 

•423 

4-5 

•324 

2'2 

•198 

•08 

•010 

6-7 

•419 

4'4 

•319 

2-1 

•192 

•07 

•008 

6'6 

•414 

4-3 

•315 

2-0 

•186 

•06 

•007 

6'5 

•410 

4-2 

•310 

1-9 

•180 

•05 

•006 

6'4 

•406 

4-1 

•305 

1-8 

•173 

•04 

•005 

6-3 

•402 

4-0 

•301 

1-7 

•167 

•03 

•004 

6-2 

•398 

3'9 

•296 

1-6 

•161 

.  . 

•02 

•002 

6-1 

•394 

3-8 

•291 

1-5 

•154 

. 

'01 

•001 

6*0 

•389 

3-7 

•286 

1-4 

•148 

TABLES    FOR   WATER   ANALYSIS. 

TABLE   5. 

Loss  of  Nitrogen  by  Evaporation  of  NH . 
With  Sulphurous  Acid. 

Parts  per  100,000. 


597 


NIP. 

Loss 
of  N. 

NH3. 

Loss 
of  N. 

NH3. 

Loss 
of  N. 

NH3. 

Loss 
of  N. 

NH3. 

Loss 
of  N. 

NH3. 

Loss 
of  N. 

6-0 

1-727 

4-8 

1-451 

3-6 

•977 

2'4 

•503 

1-2 

•250 

•09 

•014 

5'9 

1-707 

4'7 

•411 

3'5 

•937 

2'3 

•463 

I'l 

•238 

•08 

•013 

5'8 

1-688 

4-6 

•372 

3-4 

•898 

2'2 

•424 

i-o 

•226 

•07 

•012 

5-7 

1-668 

4*5 

•332 

3'3 

•858 

2'1 

•384 

•9 

•196 

•06 

•010 

5-6 

1-648 

4-4 

•293 

3-2 

•819 

2-0 

•345 

•8 

•166 

•05 

•009 

5'5 

1-628 

t-3 

•253 

3-1 

•779 

1-9 

•333 

•7 

•136 

•04 

•007 

5-4 

1-609 

4'2 

•214 

3-0 

•740 

1-8 

•321 

•6 

•106 

•03 

•006 

5-3 

1-589 

4*1 

•174 

2'9 

•700 

1-7 

•309 

•5 

•077 

•02 

•004 

5'2 

1-569 

4-0 

•135 

2'8 

•661 

1-6 

•297 

•4 

•062 

•01 

•003 

5-1 

1-549 

3-9 

•095 

2'7 

•621 

1'5 

•285 

•3 

•047 

•009 

•001 

5-0 

1-530 

3-8 

•056 

2'6 

•582 

1-4 

•274 

•2 

•032 

4'9 

1-490 

37 

•016 

2-5 

•542 

1-3 

•262 

•1 

•017 

TABLE   6. 

Loss  of  Nitrogen  by  Evaporation  of  NH3. 
With  Hydrie  Metaphosphate. 

Parts  per  100,000. 


§1 

^ 

§1 

g 

si 

fc 

9i 

ri 

If 

W 

fe 

I 

"o  p, 

B 

"o 

l| 

eo" 

H 

1 

II 
II 

n' 
I 

o 

S 
o 

k 

^ 

0 

• 

7 

£ 

^ 

100  c.c. 

10-0 

•483 

100  c.c. 

7'2 

•386 

100  c.c. 

4'4 

•283 

100  c.c. 

1-6 

•143 

9'9 

•480 

7-1 

•382 

4-3 

•279 

1-5 

•137 

9-8 

•476 

7-0 

•379 

4'2 

•275 

1-4 

•132 

9'7 

•473 

]  ] 

6'9 

•375 

4-1 

•271 

1-3 

•127 

9'6 

•469 

6'8 

•372 

4-0 

•267 

1-2 

•122 

9'5 

•466 

\  \ 

6'7 

•368 

3'9 

•262 

I'l 

•117 

9'4 

•462 

6'6 

•365 

3'8 

•257 

i-o 

•112 

9'3 

•459 

\  [ 

6'5 

•361 

3-7 

•252 

250  c.c. 

•9 

•096 

9'2 

•455 

6'4 

•358 

3  '6 

•247 

y 

•8 

•080 

9-1 

•452 

\  \ 

6'3 

•354 

3'5 

•242 

•7 

•070 

9-0 

•448 

6'2 

•351 

3-4 

•236 

•6 

•060 

8'9 

•445 

•348 

3'3 

•231 

500  c.c. 

•5 

•050 

8-8 

•441 

6-0 

•345 

3'2 

•226 

•4 

•040 

8-7 

•438 

°  ' 

5'9 

•341 

3-1 

•221 

•3 

•030 

8'6 

•434 

5'8 

•337 

3-0 

•216 

1000  'c.c. 

•2 

•020 

8'5 

•431 

5'7 

•333 

2-9 

•211 

t 

•1 

•010 

8'4 

•428 

5'6 

•330 

2'8 

•205 

•09 

•009 

8-3 

•424 

\  \ 

5'5 

•326 

2-7 

•200 

\ 

•08 

•008 

8'2 

•421 

5'4 

•322 

2'6 

•195 

•07 

•007 

8-1 

•417 

\  [ 

53 

•318 

2'5 

•190 

•06 

•006 

8'0 

•414 

5'2 

•314 

2'4 

•184 

•05 

•005 

7'9 

•410 

\  ' 

5-1 

•310 

2'3 

•179 

( 

•04 

•004 

7'8 

•407 

5-0 

•306 

2'2 

•174 

•03 

•003 

7-7 

•403 

4'9 

•302 

2-1 

•169 

•02 

•002 

7-6 

•400 

°  j 

4'8 

•298 

2'0 

•164 

[ 

•01 

•001 

7  '5 

•396 

4'7 

•294 

1-9 

•158 

7'4 

•393 

[  \ 

4'6 

•291 

1-8 

•153 

7'3 

•389 

4'5 

•287 

1-7 

•148 

598 


TABLES   FOR   WATER   ANALYSIS. 


TABLE   7. 
Table  of  Hardness,  Parts  in  100,000. 


ume  of 
3oap 
lution. 

o 

co  O 

oo" 

o     fl 

a  **° 

0 
^ 

oo" 

o     * 

III 

si 

oo 
53 

ume  of 
Soap 
lution. 

0 
^ 

®    i£ 

o^o 

•30 

o     o 

o  ^ 

> 

* 

>    m 

* 

>    m 

* 

;  t>    ffi 

* 

c.c. 

c.c. 

c.c. 

c.c. 

4-0 

4-57 

8-0 

10-30 

12-0 

16-43 

1 

•71 

1 

•45 

1 

•39 

2 

•86 

2 

•60 

2 

•75 

3 

5-00 

3 

•75 

3 

•90 

4 

•14 

4 

•90 

4 

17-06 

5 

•29 

5 

11-05 

5 

•22 

6 

•43 

6 

•20 

6 

•38 

0-7 

•00 

7 

•57 

7 

•35 

.  7 

•54 

0-8 

•16 

8 

•71 

8 

•50 

8 

•70 

0-9 

•32 

9 

•86 

9 

•65 

9 

•86 

1-0 

•48 

5-0 

6-00 

9-0 

•80 

13'0 

18-02 

1 

•63 

1 

•14 

1 

•95 

1 

•17 

2 

•79 

2 

•29 

2 

12-11 

2 

•33 

3 

•95 

3 

•43 

3 

•26 

3 

•49 

4 

Ml 

4 

•57 

4 

•41 

4 

•65 

5 

•27 

5 

•71 

5 

•56 

5 

•81 

6 

•43 

6 

•86 

6 

•71 

6 

•97 

7 

•56 

7 

7-00 

7 

•86 

7 

19-13 

8 

•69 

8 

•14 

8 

13-01 

8 

•29 

9 

•82 

9 

•29 

9 

•16 

9 

•44 

2-0 

•95 

6-0 

•43 

10-0 

•31 

14-0 

•60 

1 

2-08 

1 

•57 

1 

•46 

1 

•76 

2 

•21 

2 

•71 

2 

•61 

2 

•92 

3 

•34 

3 

•86 

3 

•76 

3 

20-08 

4 

•47 

4 

8-00 

4 

•91 

4 

•24 

5 

•60 

5 

•14 

5 

14-06 

5 

•40 

6 

•73 

6 

•29 

6 

•21 

6 

•56 

7 

•86 

7 

•43 

7 

•37 

7 

•71 

8 

•99 

8 

•57 

8 

•52 

8 

•87 

9 

3-12 

9 

•71 

9 

•68 

9 

21-03 

3-0 

•25 

7-0 

•86 

11-0 

•84 

15-0 

•19 

1 

•38 

1 

9-00 

1 

15-00 

1 

•35 

2 

•51 

2 

•14 

2 

•16 

2 

•51 

3 

•64 

3 

•29 

3 

•32 

3 

•68 

4 

•77 

4 

•43 

4 

•48 

4 

•85 

5 

•90 

5 

•57 

5 

•63 

5 

22-02 

6 

4-03 

6 

•71 

6 

•79 

6 

•18 

7 

•16 

7 

•86 

7 

•95 

7 

•35 

8 

•29 

8 

10-00 

8 

16-11 

8 

•52 

3-9 

•43 

7-9 

•15 

11-9 

•27 

9 

•69 

16-0 

•86 

ROSCOE   AND   LUNT*S   TABLE. 


599 


TABLE   8. 


Oxygen  Dissolved  by  Distilled  Water.    5-30°  C. 
(Roscoe  and  Lunt).  * 


Temp.      ^JT 
per  litre  Aq. 

Diff.  for 
0-5°  C. 

Temp. 

c.c.  Oxygen 
N.T.P. 
per  litre  Aq. 

Biff,  for 
0'5°  C. 

5-0°               8-68 

18-0° 

6-54 

0-07 

5-5                8-58 

o-io 

18-5 

6-47 

0-07 

6-0                8-49 

0-09 

19-0 

6-40 

0-06 

6-5                8-40 

0-09 

19-5 

6-34 

0-06 

7-0                8-31 

0-09 

20-0 

6-28 

0-06 

7-5                8-22 

0-09 

2O5 

6-22 

0-06 

8-0                8-13 

0-09 

21-0 

6-16 

0-06 

8-5                8-04 

0-09 

21-5 

6-10 

0-06 

9-0                7-95 

0-09 

22-0 

6-04 

0-05 

9-5                7-86 

0-09 

22-5 

5-99 

0-05 

10-0 

7-77 

0-09 

23-0 

5-94 

0-05 

10-5                7-68 

0-08 

23-5 

5-89 

0-05 

11-0                7-60 

0-08 

24-0 

5-84 

0-04 

11-5                7-52 

0-08 

24-5 

5-80 

0-04 

12-0                7-44 

0-08 

25-0 

5-76 

0-04 

12-5                7-36 

0-08 

25-5 

5-72 

0-04 

13-0 

7-28 

0-08 

26-0 

5-68 

0-04 

13-5 

7-20 

0-08 

26-5 

5-64 

0-04 

14-0 

7-12 

0-08 

27-0 

5-60 

0-03 

14-5 

7-04 

0-08 

27-5 

5-57 

0-03 

15-0 

6-96 

0-08 

28-0 

5-54 

0-03 

15-5 

6-89 

0-07 

28-5 

5-51 

0-03 

16-0 

6-82 

0-07 

29-0 

5-48 

0-03 

16-5 

6-75 

0-07 

29-5 

5-45 

0-02 

17-0 

6-68 

0-07 

30-0 

5-43 

17-5 

6-61 

0-07 

In  this  table  the  results  are  calculated  for  aeration  at  an  observed  barometric 
pressure  of  760  mm.  When  the  observed  pressure  is  below  760  mm.  TV  tne 
value  must  be  subtracted  for  every  10  mm.  diff.  The  same  value  must  be  added 
when  the  pressure  is  above  760  mm. 

*  ,7.  C.  S.  1889,  532. 


600 


ABSORPTION  COEFFICIENTS  OF  GASES. 

TABLE  9. 


Amounts  of  Dissolved  Oxygen  in  distilled  Water  from  0°-30°  C. 
(Bar.  760  mm.).* 


Temperature 

Oxygen 
(Parts  per  100,000). 

Temperature 

°C. 

Oxygen 
(Parts  per  100,000). 

0 

1-42 

16 

0-98 

1 

1-39 

17 

0-96 

2 

1-36 

18 

0-94 

3 

1-32 

19 

0-92 

4 

1-28 

20 

0-90 

5 

1-24 

21 

0-88 

6 

1-22 

22 

0-87 

7 

1-19 

23 

0-85 

8 

1*17 

24 

0-84 

9 

1-14 

25 

0-82 

10 

I'll 

26 

0-81 

11 

1-09 

27 

0-80 

12 

1-07 

28 

0-80 

13 

1-04 

29 

0-79 

14 

1-02 

30 

0-78 

15 

1-00 

— 

— 

*  Calculated  from  R  o  s  c  o  e  and  L  u  n  t '  s  table  from  5°— 30°  C.  and  from  determin- 
ations by  W  in  k  1  e  r '  s  process  for  the  values  given  for  0°  to  4°  C. 


TABLE  9a. 

Absorption  coefficients  of  the  Commoner  Gases  in  Water  at  15°  C. 

Carbon  dioxide       .      .      .  0-024 

Hydrogen    .      .....  0-019 

Oxygen        .....  0-030 

Nitrogen      .  '.   .      .      .      .  0-015 


Propylene      .    i  .      ,  .  0-237 

Ethylene        .    !  .      ."  .  0-162 

Methane  .      .    <  .  .  .  .  0-039 

Carbon  dioxide  .  1-002 


More  recent  determinations  by  Winkle  r  are  as  follows  : — 


Coefficient  of 
Solubility  at 

10° 

12° 

14° 

16° 

18° 

20° 

Oxygen    ..  '  .      * 

0-038 

0-036 

0-035 

0-033 

0-032 

0-031 

Nitrogen  . 

0-018 

0-018 

0-017 

0-016 

0-016 

0-015 

Hydrogen 

0-020 

0-019 

0-019 

0-019 

0-018 

0-018 

Carbon  monoxide 

0-028 

0-023 

TABLES   FOR   GAS   ANALYSIS. 


601 


TABLE  10. 
TABLE  for  Correction  of  Volumes  of  Gases  for  Temperature, 


according  to  the  Formula  Vx= 


760  X  (1  +  5 1) 

1  +  St  from  0°  to  30°.     5  =  0'003665. 


t 

l  +  8t 

Log.  (1  +  8  l) 

t  1  1  +  St 

Log.  (1  +  8  1) 

t 

l  +  8t 

Log.  (l  +  8  1; 

6-0 

i-ooooooo 

O'OOO  0000 

o 

S'01-0183250 

0-007  8864 

16-0 

1-0366500 

0-015  6321 

•i 

1-0003665 

1591 

•111-0186915 

0-008  0427 

•1 

1-0370165 

7857 

•2 

1-0007330 

3182 

•2 

1-0190580 

1989 

•2 

1-0373830 

9391 

•3 

1-0010995 

4772 

•3 

1-0194245 

4551 

•3 

1-0377495 

0*016  0925 

•4 

1-0014660 

6362 

•4 

1-0197910 

5112 

•4 

1-0381160 

2459 

0-5 

1-0018325 

7951 

5-5 

1*0201575 

6672 

10-5 

1-0384825 

3992 

•6 

1-0021991 

9540 

•6 

1-0205240 

8232 

•6 

1-0388490 

5524 

•7 

1-0025655 

O'OOl  1128 

•7 

1-0208905 

9791 

•7 

1-0392155 

7056 

•8 

1-0029320 

2715 

•8 

1-0212570 

0-009  1350 

•8 

1-0395820 

8588 

•9 

1-0032985 

4302 

5-9 

1-0216235 

2909 

10-9 

1-0399485 

0-017  0118 

i-o 

1-0036650 

0-001  5888 

6'0 

1-0219900 

0-009  4466 

ll'O 

1-0403150 

0-017  1648 

•1 

1-0040315 

7473 

•1 

1-0223565 

6024 

•1 

1-0406815 

3178 

•2 

1-0043980 

9058 

•2 

1-0227230 

7580 

•2 

1-0410480 

4708 

•3 

1-0047645 

0-002  0643 

•3 

1-02308S5 

9136 

•3 

1-0414145 

6236 

•4 

1-0051310 

2227 

•4 

1-0234560 

0-010  0692 

•4 

1-0417810 

7764 

1-5 

1-0054975 

3810 

6-5 

1-0238225 

2247 

11-5 

1-0421475 

9292 

•6 

1-0058640 

5393 

•6 

1-0241890 

3801 

•6 

1-0425140 

0-018  0819 

•7 

1-0062305 

6974 

•7 

1-0245555 

5355 

•7 

1-0428805 

2346 

•8 

1-0065970 

8556 

•8 

1-0249220 

6908 

•8 

1-0432470 

3871 

1-9 

1-0069635 

0-003  0137 

6-9 

1-0252885 

8461 

11-9 

1-0436135 

5397 

2'0 

1-0073300 

0'003  1718 

7-0 

L-0256550 

O'Oll  0013 

12-0 

1-0439800 

0-018  6922 

•1 

1-0076965 

3298 

•1 

1-0260215 

1565 

•1 

1-0443465 

8446 

•2 

1-0080630 

4877 

•2 

1-0263880 

3116 

•2 

1-0447130 

9970 

•3 

1-0084295 

6455 

•3 

1-0267545 

4666 

•3 

1-0450795 

0-019  1493 

•4 

1-0087960 

8033 

•4 

1-0271210 

6216 

•4 

1-0454460 

3016 

2'5 

1-0091625 

9611 

7-5 

1-0274875 

7765 

125 

1-0458125 

4538 

•6 

1-0095290 

0-004  1188 

•6 

1-0278540 

9314 

•6 

1-0461790 

6060 

•7 

1-0098955 

2764 

•7 

1-0282205 

0-012  0863 

•7 

1-0465455 

7581 

•8 

1-0102620 

4340 

•8 

1-0285870 

2410 

•8 

1-0469120 

9102 

2-9 

1-0106285 

5916 

7-9 

1-0289535 

3957 

12-9 

1-0472785 

0-020  0622 

3'0 

1-0109950 

0-004  7490 

8-0 

1-0293200 

0-012  5504 

13-0 

1*0476450 

0-020  2141 

•1 

1-0113615 

9064 

•1 

1-0296865 

7050 

•1 

1-0480115 

3660 

•2 

1-0117280 

0-005  0638 

•2 

1-0300530 

8596 

•2 

1-0483780 

5179 

•3 

1-0120945 

2211 

•3 

1-0304195 

0-013  0141 

•3 

1-0487445 

6697 

•4 

1-0124610 

3783 

•4 

1-0307860 

1685 

•4 

1-0491110 

8214 

3'5 

1-0128275 

5355 

8-5 

1-0311525 

3229 

13-5 

1-0494775 

9731 

•6 

1-0131940 

6926 

•6 

1-0315190 

4772 

•6 

1-0498440 

0-021  1248 

•7 

1-0135605 

8497 

•7 

1-0318855 

6315 

•7 

L'0502105 

2764 

•8 

1-0139270 

0*006  0067 

•8 

1-0322520 

7857 

•8 

1  '0505770 

4279 

3-9 

1-0142935 

1636 

8-9 

1-0326185 

9399 

13-9 

1-0509435 

5794 

4-0 

1-0146600 

0-006  3205 

9-0 

1-0329850 

0'014  0940 

14-0 

1-0513100 

0-021  7308 

•1 

1-0150265 

4774 

•1 

1-0333515 

2481 

•1 

1-0516765 

8822 

•2 

1-0153930 

6342 

•21-0337180 

4021 

•2 

1-0520430 

0-022  0335 

•3 

1-0157595 

7909 

•311-0340845 

5560 

•3 

1-0524095 

1848 

•4 

1-0161260 

9476 

•4 

1-0344510 

7099 

•4 

1-0527760 

3360 

4'5 

1-0164925 

0-007  1042 

9'5 

1-0348175 

8638 

14-5 

1-0531425 

4871 

•6 

1-0168590 

2607 

•6 

1-0351840 

0-015  0175 

•6 

1-0535090 

6382 

•7 

1-0172255 

4172 

•7 

1-0355505 

1713 

.'7 

1-0538755 

7893 

•8 

1-0175920 

5737 

•8 

1-0359170 

3250 

•8 

1-0542420 

9403 

4'9 

1-0179585 

7301 

9-9 

1-0362835 

4786 

14-9 

1-0546085 

0-023  0193 

602 


TABLES    FOR   GAS    ANALYSIS. 


TABLE   10  (continued). 
TABLE  for  Correction  of  Volumes  of  Gases—  continued. 


t 

1+St 

Log.  (1  +  S  t) 

t 

1  +5t 

Log.  (1  +  5  t) 

t 

1  +  St 

Log.  (1+5  1) 

15'0 

1-0549750 

0-023  2422 

20-0 

1-0730000 

0-030  7211 

25-0 

1-0916250 

0-038  0734 

•1 

1-0553415 

3930 

•1 

1-0736665 

8694 

•1 

1-0919915 

2192 

•2 

1-0557080 

5438 

•  O 

2 

1-0740330 

0-031  0176 

"^ 

1-0923580 

3650 

•3 

1-0560745 

6946 

-c: 

v 

1-0743995 

1658 

•3 

1-0927245 

5107 

•4 

1-0564410 

8452 

•4 

1-0747660 

3139 

•4 

1-0930910 

6563 

15'5 

1-0568075 

9959 

20-5 

1-0751325 

4620 

25-5 

1-0934575 

8020 

•6 

1-0571740 

0-024  1465 

•6 

1-0754990 

6100 

•6 

1-0938240 

9474 

•7 

1-0575405 

2970 

•7 

1-0758655 

7580 

•7 

1-0941905 

0-039  0929 

•8 

1-0579070 

4475 

•8 

1-0762320 

9059 

•8 

1-0945570 

2384 

15-9 

1-0582735 

5979 

20-9 

1-0765985 

0-032  0538 

•c 

1-0949235 

3838 

16-0 

1-0586400 

0*024  7483 

21'0 

1-0769650 

0-032  2016 

26-0 

1-0952900 

0-039  5291 

•1 

1-0590065 

8986 

•1 

1-0773315 

3493 

•1 

1-0956565 

6745 

•2 

1-0593730 

0-025  0489 

•2 

1-0776980 

4971 

•G 

1-0960230 

8197 

•3 

1-0597395 

1991 

*3 

1-0780645 

6447 

"2 

1-0963895 

9649 

•4 

1-0601060 

3493 

•4 

1-0784310 

7924 

•4 

1-0967560 

0-040  1101 

16*5 

1-0604725 

4994 

21-5 

1-0787975 

9399 

26-5 

1-0971225 

2551 

•6 

1-0608390 

6495 

•6 

1-0791640 

0-033  0874 

•6 

1-0974890 

4002 

•7 

1-0612055 

7995 

"7 

1-0795305 

2349 

'7 

1-0978555 

5452 

•8 

1-0615720 

9495 

•8 

1-0798970 

3823 

•8 

1-0982220 

6901 

16-9 

1-0619385 

0-026  0994 

21-9 

1-0802635 

5298 

"S 

1*0985885 

8351 

17'0 

1-0623050 

0-026  2492 

22-0 

1-0806300 

0-033  6771 

27-0 

1-0989550 

0-040  9800 

•1 

1-0626715 

3990 

•1 

1-0809965 

8243 

•1 

1-0993215 

0-041  1247 

•2 

1-0630380 

5488 

•2 

1-0813630 

9715 

'2 

1-0996880 

2695 

•3 

1-0634045 

6985 

•3 

1-0817295 

0-034  1186 

•3 

1-1000545 

4143 

•4 

1-0637710 

8482 

•4 

1-0820960 

2658 

•4 

1-1004210 

5589 

17'5 

1-064J  375 

9978 

22-5 

1-0824625 

4129 

27-5 

1-1007875 

7036 

•6 

1-0645040 

0-027  1473 

•6 

1-0828290 

5598 

•6 

1-1011540 

8481 

•7 

1-0648705 

2968 

•7 

1-0831955 

7069 

•7 

1-1015205 

9926 

•8 

1-0652370 

4462 

•8 

1-0835620 

8538 

•8 

1-1018870 

0-042  1371 

17-9 

1-0656035 

5956 

22-9 

1-0839285 

0-035  0006 

•9 

1-1022535 

2815 

18'0 

1-0659700 

0-027  7450 

23-0 

1-0842950 

0-035  1475 

28-0 

1-1026200 

0-042  4259 

•1 

1-0663365 

8943 

•1 

1-0846615 

2942 

•1 

1-1029865 

5703 

•2 

1-0667030 

0-028  0435 

•2 

1-0850280 

4409 

•2 

1-1033530 

7145 

•3 

1-0670695 

1927 

•3 

1-0853945 

5876 

•3 

1-1037195 

8587 

•4 

1-0674360 

3418 

•4 

1-0857610 

7342 

•4 

1-1040860 

0-043  0029 

18.5 

1-0678025 

4909 

23-5 

1-0861275 

8808 

28-5 

1-1044525 

1471 

•6 

1.0681690 

6400 

•6 

1*0864940 

0-036  0273 

•6 

1-1048190 

2911 

•71-0685355 

7889 

•7 

1-0868605 

1738 

•7 

1-1051855 

4352 

•81-0689020 

9379 

•8 

1  0872270 

3202 

•8 

1-1055520 

5792 

18-91-0692685 

0-029  0868 

23-9 

1-0875935 

4666 

•9 

1-1059185 

7231 

19-01-0696350 

0-029  2356 

24-0 

1-0879600 

0-036  6129 

29-0 

1-1062850 

0-043  8671 

•1 

1-0700015 

3844 

•1 

1-0883265 

7592 

•1 

1-1066515 

0-044  0109 

•2 

1-0703680 

5331 

•2 

1-0886930 

9054 

•2 

1-1070180 

1546 

•31-07073451     6818 

•3 

1-0890595 

0-037  0517 

•3 

1-1073845 

2985 

•41-0711010 

8304 

•4 

1-0894260 

1978 

•4 

1-1077510 

4422 

19-51-0714675 

9790 

24'5 

1-0897925 

3438 

29'5 

1-1081175 

5858 

•61-0718340 

0-030  1275 

•6 

1-0901590 

4899 

•6 

1-1084840 

7295 

•7  1-0722005 

2760 

•7 

1-0905255 

6359 

•7 

1-1088505 

8730 

•81-0725670 

4244 

•8 

1-0908920 

7817 

•8 

1-1092170 

0-045  0165 

19-91-0729335 

5728 

•9 

1-0912585 

9277 

•9 

1-1095835 

1600 

I 

30'0|l-1099500 

0-045  3035 

TABLES    FOB   GAS    ANALYSIS. 


603 


TABLE   11. 

TABLE  for  Correction  of  Volumes  of  Gases  for 
Temperature,  giving   the   Divisor   for   the   Formula 


V  x 


'  760  X  (1  +  SO- 


t 

760  x 
(l  +  8t). 

Log.  [760  x 
11+84)]. 

t 

760  x 

(i  +  at). 

Log.  [760  x 
(1  +  8ft)]. 

t 

760  x 

(14-  at,. 

Log.  [760  x 
(l+8t)1. 

O'O 

•1 

•2 
•3 
•4 

760-0000 
760-2785 
760-5571 
760-8356 
761-1142 

2-880  8136 
9727 
2-881  1319 
2908 
4498 

4-0 
•1 
•2 
•3 

•4 

771-1416 
771-4201 

771-6987 
771-9772 
772-2558 

2*887  1341 
2910 
4478 
6044 
7611 

8-0 
•1 
•2 
•3 

•4 

782-2832 
782-5617 
782-8403 
783-1188 
783-3974 

2-893  3640 
5186 
6732 
8276 
9821 

0'5 
•6 

•7 
•8 

"9 

761-3927 
761-6712 
761*9498 
762-2283 
762-5069 

6087 
7676 
9264 
2-882  0851 
2437 

4-5 
•6 

•7 
•8 
•9 

772-5343 

772-8128 
773-0914 
773-3699 
773-6485 

9178 
2-888  0743 
2309 
3872 
5437 

8-5 
•6 

•7 
•8 
•9 

783-6759 
783-9544 
784-2330 
784-5115 
784-7901 

2-894  1365 
2908 
4452 
5994 
7536 

1-0 
•1 

•2 
•3 
•4 

762-7854 
763-0639 
763-3425 
763-6210 
763*8996 

2-882  4024 
5610 
7194 
8779 
2-883  0362 

5-0 
•1 
•2 
•3 
•4 

773-9270 
774-2055 
774-4841 
774-7626 
775-0412 

2-888  7000 
8563 
2-8890125 
1686 
3248 

9'0 
•1 
•2 
•3 

•4 

785-0686 
785-3471 
785-6257 
785-9042 
786-1828 

2-894  9076 
2-895  0617 
2157 
3696 
5235 

1'5 

•6 

•7 
•8 
'9 

764-1781 
764-4566 
764-7352 
765-0137 
765-2923 

1947 
3528 
5111 
6692 
8273 

5-5 

•6 
•7 
•8 
•9 

775-3197 
775-5982 
775-8768 
776-1553 
776-4339 

4808 
6368 
7927 
9487 
2-890  1044 

9-5 
•6 

•7 
•8 
•9 

786-4613 
786-7398 
787-0184 
787-2969 
787-5755 

6774 
8311 
9849 
2-896  1385 
2923 

2-0 
•1 

•2 

'3 
•4 

765-5708 
765-8493 
766-1279 
766-4064 
766-6850 

2-883  9854 
2-884  1433 
3013 
4591 
6170 

6-0 
•1 
•2 
•3 
•4 

776-7124 
776-9909 
777-2695 
777-5480 
777-8266 

2-890  2602 
4159 
5716 

7272 
8828 

10-0 

•1 
•2 
•3 
•4 

787-8540 
788-1325 
788  4111 
788-6896 
788-9682 

2-896  4457 
5993 
7528 
9061 
2-8970595 

2-5 

•6 
•7 
•8 
•9 

766-9635 
767-2420 
767-5206 
767-7991 

768-0777 

7747 
9323 
2-885  0900 
2476 
4052 

6-5 
•6 
•7 
•8 
*9 

778-1051 
778-3836 
778-6622 
778-9407 
779-2193 

2-891  0383 
1937 
3491 
5044 
6597 

105 
•6 

•7 
•8 

*9 

789-2467 
789-5252 
789-8038 
790-0823 
790-3609 

2128 
3660 
5192 
6724 
8255 

3-0 
•1 

*r 

•3 
'4 

768-3562 
768-6347 
768-9133 
769-1918 
769-4704 

2-885  5626 
7200 
8772 
2-886  0347 
1919 

7'0 
•1 
•2 
•3 
•4 

779-4978 
779-7763 
780-0549 
780-3334 
780-6120 

2-891  8149 
9701 
2'892  1251 
2802 
4352 

11-0 
•1 
•2 
•3 
•4 

790-6394 
790-9179 
791-1965 
791-4750 
791-7536 

2-897  9785 
2-898  1315 
2844 
4373 
5901 

3'5 

•6 

•8 
•9 

769-7489 
770-0274 
770-3060 
770-5845 
770-8631 

3491 
5061 
6633 
8203 
9773 

7-5 
•6 

•7 
•8 
•9 

780-8905 
781-1690 
781-4476 
781-7261 
782-0047 

5901 
7450 
8998 
2-893  0547 
2094 

11-5 
•6 

•7 
•8 
•9 

792-0321 
792-3106 
792  5892 
792-8677 
793-1463 

7428 
8954 
2-899  0482 
2008 
3534 

604 


TABLES   FOB   GAS   ANALYSIS. 


TABLE    11    (continued). 
TABLE  for  Correction  of  Volumes  of  Gases — continued. 


t 

760  x 
(l+5t). 

Log.  [760  x 
(l+5t)]. 

t 

760  X 
(l+8t). 

Log.  [760  x 
(1  +  «*)]. 

t 

760  x 

a  +  st;. 

| 
Log.  [760  x 

(1  +  St)J- 

12-0 

•i 

•2 
•3 
•4 

793-4248 
793-7033 
793-9819 
794-2604 
794-5390 

2  899  5057 
6583 
-  8106 
9629 
2-900  1153 

16-5 
•6 

•7 
•8 
•9 

805-9591 
806-2376 
806-5162 
8.67947 
807-0733 

2-906  3131 
4630 
6131 
7631 
9130 

21-0 
•1 
•2 
•3 
•4 

818-4934 
818-7719 
819-0505 
819-3290 
819-6076 

2-913  0152 
1629 
3107 
4584 
6059 

12-5 

•6 
•7 
•8 
"S 

794  8175 
795-0960 
795-3746 
795-6531 
795-9317 

2674 
4196 
5717 
7238 
8758 

17-0 
•1 
•2 
•3 
•4 

807-3518 
807*6303 
807-9089 
808-1874 
808-4660 

2-907  0627 
2126 
3624 
5121 
6617 

21-5 
•6 

•7 
•8 
21-9 

819-8861 
820-1646 
820-4432 
820*7217 
821-0003 

7535 
9010 
2-914  0485 
1959 
3434 

13-0 
•I 
•2 
•3 

•4 

796-2102 
796-4887 
796-7673 
797-0458 
797-3244 

2-901  0277 
1796 
3316 
4833 
6351 

17-5 

•6 
•7 
•8 
•9 

808-7445 
809*0230 
809-3016 
809-5801 
809-8587 

8114 
9609 
2-908  1103 
2599 
4092 

22-0 
•1 
•2 
•3 
•4 

821-2788 
821-5573 
821-8359 
822-1144 
822-3930 

2-914  4906 
6379 
7852 
9322 
2-915  0794 

13-5 
•6 

•7 
•8 
•9 

797-6029 
797*8814 
798-1600 
798-4385 
798-7171 

7867 
9383 
2-902  0900 
2415 
3931 

18-0 
•1 

•2 
•3 

•4 

810'1372 
810-4157 
810-6943 
810-9728 
811-2514 

2-908  5586 
7079 
8572 
2-909  0063 
1554 

22-5 
•6 
•7 
•8 
•9 

822-6715 
822-9500 
823-2286 
823-5071 
823-7857 

2265 
3734 
5204 
6674 
8143 

14-0 
•1 
•2 
•3 

•4 

798-9956 
799-2741 
799-5527 
799-8312 
800-1098 

2-902  5444 
6958 
8471 
9983 
2-903  1496 

18-5 
•6 

•7 
•8 
•9 

811-5299 
811-8084 
812-0870 
812-3655 
812-6441 

3046 
4535 
6026 
7515 
9004 

23-0 
•1 
•2 
•3 
•4 

824-0642 
824-3427 
824-6213 
824-8998 
825-1784 

2-915  9610 
2-916  1078 
2546 
4012 
5478 

14'5 
•6 

•7 
•8 
•9 

800-3883 
800-6668 
800-9454 
801-2239 
801-5025 

3008 
4518 
6029 
7539 
9049 

19-0 
•1 
•2 
•3 

•4 

812-9226 
813-2011 
813-4797 
813-7582 
814-0368 

2-910  0492 
1980 
3468 
4953 
6440 

23-5 
•6 

3 

•9 

825-4569 
825-7354 
826-0140 
826-2925 
826-5711 

6944 
8409 
9874 
2-917  1339 

2802 

15-0 
•1 
•2 
•3 
•4 

801-7810 
802-0595 
802-3381 
802-6166 
802  8952 

2-904  0557 
2067 
3574 
5081 
6589 

19-5 

•6 
•7 
.'8 
•9 

814-3153 
814-5938 
814-8724 
8151500 
815-4295 

7927 
9411 
2-911  0896 
2380 
3865 

24-0 
•1 
•2 
•3 
•4 

826-8496 
827-1281 
827-4067 
827-6852 
827-9638 

2-917  4265 
5728 
7191 
8652 
2-918  0114 

15'5 
•6 

•7 
•8 
•9 

803-1737 
803-4522 
803-7308 
804-0093 
804-2879 

8095 
9601 
2-905  1106 
2612 
4116 

20-0 
•1 
•2 
•3 

•4 

815-7080 
815-9865 
816-2651 
816-5436 
816-8222 

2-911  5347 
6830 
8313 
9794 
2-912  1276 

24-5 
•6 

•7 
•8 
249 

828-2423 
828-5208 
828-7994 
829*0779 
829-3565 

1574 
3034 
4495 
5953 
7413 

16-0 
•1 
•2 
•8 
•4 

804-5664 
804-8449 
805-1235 
805-4020 
805-6806 

2-905  5618 
7122 
8625 
2-906  0127 
1629 

20-5 
•6 
•7 
•8 
•9 

817-1007 
817-3792 
817-6578 
817-9363 
818-2149 

2756 
4236 
5716 
7195 
8S74 

25-0 
•1 
•2 
•3 
•4 

829-6350 
829-9135 
830-1921 
830-4706 
830-7492 

2-918  8871 
2-919  0329 
1786 
3242 
4699 

TABLES   FOR   GAS    ANALYSIS. 


605 


TABLE   11   (continued). 
TABLE  for  Correction  of  Volumes  of  Gases— continued. 


t 

760  x 
(1  +  8t). 

Log.  [760  x 
(1  +  8*)]. 

t 

760  x 
(l+8t) 

Log.  [760  x 
(1  +  8t]. 

t 

760  x 
(1  +  5t). 

Log.  [760  x 
(l  +  3t)]. 

25-5 
•6 
•7 
•8 
25-9 

831-0277 
831-3062 
831-5848 
831-8633 
832-1419 

2-919  6155 
7610 
9065 
2-920  0520 
1974 

27-0835-2058 
•l|835-4843 
•2835-7629 
•3836-0414 
•4836-3200 

2-921  7935 
9384 
2-9220831 
2279 
3725 

28-5839-3839 
•6,839-6624 
•7839-9410 
•8840-2195 
289840-4981 

2-923  9607 
2-924  1047 
2488 
3928 
5368 

26'0 
•1 
•2 
•3 

•4 

832-4204 
832-6989 
832-9775 
833-2560 
833-534H 

2-920  3427 

4880 
6333 
7784 
9236 

27'5'836-5985 
•6836-8770 
'7837-1556 
•8837-4341 
27-98377127 

5172 
6616 
8062 
9507 
2-923  0951 

29-01840-7766 
•1841-0551 
•2841-3337 
'3841-6122 

•4841-8908 

2-924  6806 
8245 
9683 
2-9251120 
2558 

26-5 

•6 
•7 
•8 
26'9 

833-8131 
834-0916 
834-3702 
834-6487 
834-9273 

2-921  0688 
2137 
3588 
5038 
6487 

28'0 
•1 
•2 
•3 
•4 

837-9912 
838-2697 
838*5483 
838-8268 
839-1054 

2-923  2394 
3838 
5281 
6723 
8165 

29-5 

:? 

•8 
29-9 

842-1693 
842-4478 
842-7264 
843-0049 
843-2835 

3995 
5431 
6866 
8301 
9737 

30-0 

843-5620 

2-9261170 

606 


TABLES   FOR   GAS   ANALYSIS. 


TABLE  12. 

Pressure  of  Aqueous  Vapour  in  Millimetres  of  Mercury, 
from  -  9'9°  to  +  35°  C. 


in  in. 

m  m. 

m  m. 

m  m. 

m  m. 

m  m. 

-9'9 

2-096 

-5-4 

3-034 

-6'9 

4-299 

3-5 

5-889 

8-0 

8-017 

12-5 

10-804 

•8 

•114 

•3 

•058 

•8 

•331 

•6 

•930 

•1 

•072 

•6 

•875 

•7 

•132 

•2 

•082 

•7 

•364 

•7 

•972 

•2 

•126 

•7 

•947 

•6 

•150 

•1 

•106 

•6 

•397 

*8 

6-014 

•3 

•181 

•8 

11-019 

•5 

•168 

-5-0 

•131 

•5 

•430 

3-9 

•055 

•4 

•236 

12-9 

•090 

-9'4 

•186 

-4-9 

3-156 

-0-4 

•463 

4-0 

6-097 

8-5 

•291 

13-0 

11-162 

•3 

•204 

•8 

•181 

•3 

•497 

•1 

•140 

•6 

•347 

•1 

•235 

•2 

•223 

•7 

•206 

•2 

•531 

•2 

•183 

•7 

•404 

•2 

•309 

•1 

•242 

•6 

•231 

•1 

•565 

•3 

•226 

•8 

•461 

•3 

•383 

-9-0 

•261 

•5 

•257 

-o-o 

4-600 

•4 

•270 

8-9 

•517 

•4 

•456 

-8'9 

2-280 

-44 

•283 

+  0'( 

4-600 

4.5 

•313 

9-0 

8-574 

13-5 

•530 

•8 

299 

•3 

•309 

•1 

•633 

•6 

•357 

•1 

•632 

•6 

•605 

•7 

•318 

•2 

•335 

•2 

•667 

•7 

•401 

•2 

•690 

•7 

•681 

•6 

•337 

•1 

•361 

•3 

'700 

•8 

•445 

•3 

•748 

•8 

•757 

•5 

•356 

-4-0 

•387 

•4 

'733 

4'9 

•490 

•4 

•807 

13-9 

•832 

-8'4 

•376 

-3'9 

3'414 

0-5 

•767 

5-0 

6-534 

9-5 

•865 

14'0 

11-908 

•3 

•396 

•8 

•441 

•6 

•801 

•1 

•580 

•6 

•925 

•1 

•986 

•2 

•416 

•7 

•468 

•7 

•836 

•2 

•625 

•7 

•985 

•2 

12-064 

•1 

•436 

•6 

•495 

•8 

•871 

•3 

•671 

•8 

9-045 

•3 

•142 

-8-0 

•456 

•5 

•522 

0-9 

•905 

•4 

717 

9'9 

•105 

•4 

•220 

-7'9 

2'477 

-3'4 

•550 

1-0 

4-940 

5-5 

•763 

10-0 

9-165 

14-5 

•298 

•8 

•498 

•3 

•578 

•1 

•975 

•6 

•810 

•1 

•227 

•6 

•378 

•7 

•519 

•2 

•606 

•2 

5-011 

•7 

•857 

•2 

•288 

•7 

•458 

•6 

•540 

•1 

•634 

•3 

•047 

•8 

•904 

•3 

•350 

•8 

•538 

•5 

•561 

-3-0 

•662 

•4 

•082 

5-9 

•951 

•4 

•412 

14-9 

•619 

-7'4 

•582 

-2-9 

3-691 

1-5 

•118 

6-0 

6-998 

10'5 

•474 

15-0 

12-699 

•3 

•603 

•8 

•720 

•6 

•155 

•1 

7-047 

•6 

•537 

•1 

•781 

•2 

•624 

•7 

•749 

•7 

•191 

•2 

•095 

•7 

•601 

•2 

•864 

•1 

•645 

•6 

•778 

•8 

•228 

•3 

•144 

•8 

•665 

•3 

•947 

-7-0 

•666 

•5 

•807 

1-9 

•265 

•4 

•193 

10-9 

•728 

•4 

13-029 

-6'9 

2-688 

-2'4 

•836 

2-0 

5-302 

6'5 

•242 

ll'O 

9-792 

15-5 

•112 

•8 

•710 

•3 

•865 

•1 

•340 

•6 

•292 

•1 

•857 

•6 

•197 

•7 

•732 

•2 

•895 

•2 

•378 

•7 

•342 

•2 

•923 

•7 

•281 

•6 

•754 

•1 

•925 

•3 

•416 

•8 

•392 

•3 

•989 

•8 

•366 

•5 

•776 

-2-0 

•955 

•4 

•454 

6-9 

•442 

•4 

10-054 

15'9 

•451 

-6'4 

•798 

-1-9 

3-985 

2-5 

•491 

7-0 

7'492 

11-5 

•120 

16-0 

13-536 

•3 

•821 

•8 

4-016 

•6 

•530 

•1 

•544 

•6 

•187 

•1 

•623 

•2 

•844 

•7 

•047 

•7 

•569 

•2 

•595 

•7 

•255 

'2 

•710 

•1 

•867 

•6 

•078 

•8 

•608 

•3 

•647 

•8 

•322 

•3 

•797 

-6-0 

•890 

•5 

•109 

2.9 

•647 

•4 

•699 

11-9 

•389 

•4 

•885 

-5-9 

2-914 

-1-4 

•140 

3-0 

5-687 

7-5 

•751 

12-0 

10-457 

16'5 

•972 

•8 

•938 

•3 

•171 

•1 

•727 

•6 

•804 

•1 

•526 

•6 

14-062 

•7 

•962 

•2 

•203 

•2 

•767 

•7 

•857 

•2 

•596 

•7 

•151 

•6 

•986 

•1 

•235 

•3 

•807 

•8 

•910 

•3 

•6G5 

'8 

•241 

•5 

3-010 

i-o 

•267 

•4 

•848 

7'9 

•964 

•4 

•734 

16-9 

•331 

TABLES   FOB   GAS   ANALYSIS. 


607 


TABLE   12  (continued). 
Pressure  of  Aqueous  Vapour— continued. 


mm. 

m  m. 

m  m. 

m  m. 

m  m. 

m  m 

17'0 

14-421 

20-0 

17-391 

2°3'0 

20-888 

2*6-0 

24-988 

29-0 

29-782 

32-0 

35-359 

•1 

•513 

•1 

•500 

•i 

21-016 

•1 

25-138 

•1 

•956 

•1 

•559 

•2 

•605 

'    '2 

•608 

•2 

•144 

"2 

•288 

•2 

30-131 

"2 

•760 

•3 

•697 

•3 

•717 

•3 

•272 

•3 

•438 

'3 

•305 

•3 

•962 

•4 

'790 

•4 

•826 

•4 

•400 

•4 

•588 

•4 

•479 

'4 

36-165 

17'5 

•882 

20'5 

•935 

23-5 

•528 

26-5 

•738 

29'5 

•654 

32'5 

•370 

•6 

•977 

•6 

18.-047 

•6 

•659 

•6 

•891 

•6 

•833 

•6 

•576 

•7 

15*072 

•7 

•159 

•7 

790 

•7 

26-045 

•7 

31-011 

•7 

•783 

•8 

•167 

•8 

•271 

•8 

•921 

•8 

•198 

•8 

•190 

•8 

•991 

17-9 

•262 

20-9 

•383 

23'9 

22-053 

26-9 

•351 

29-9 

•369 

32-9 

37-200 

18-0 

15-357 

21'0 

18-495 

24-0 

22-184 

27-0 

26-505 

30'0 

31-548 

33'0 

37-410 

•1 

•454 

•1 

•610 

•1 

•319 

•1 

•663 

•1 

•729 

•1 

•621 

•2 

•552 

•2 

•724 

•2 

•453 

•2 

•820 

•2 

•911 

•2 

•832 

•3 

•650 

•3 

•839 

•3 

•588 

•3 

•978 

•3 

32-094 

•3 

38-045 

•4 

•747 

•4 

•954 

•4 

•723 

•4 

27-136 

•4 

•278 

•4 

•258 

18'5 

•845 

21-5 

19-069 

24'5 

•858 

27-5 

•294 

30-5 

•463 

33-5 

•473 

•6 

•945 

•6 

•187 

•6 

•996 

•6 

•455 

•6 

•650 

•6 

•689 

'7 

16-045 

•7 

•305 

•7 

23-135 

•7 

•617 

•7 

•837 

•7 

•906 

•8 

•145 

•8 

•423 

•8 

•273 

•8 

•778 

•8 

33-026 

•8 

39-124 

18-9 

•246 

21-9 

•541 

24'9 

•411 

27-9 

•939 

30-9 

•215 

33-9 

•344 

19-0 

16-346 

22-0 

19-659 

250 

23-550 

28'0 

28-101 

31-0 

33-405 

34-0 

39-565 

•1 

•449 

•1 

•780 

•1 

•692 

•1 

•267 

•1 

•596 

•1 

•786 

•2 

•552 

•2 

•901 

•2 

•834 

•2 

•433 

•2 

•787 

•2 

40-007 

•3 

•655 

•3 

20-022 

•3 

•976 

•3 

•599 

•3 

•980 

•3 

•230 

•4 

•758 

•4 

•143 

'4 

24-119 

•4 

•765 

•4 

34-174 

•4 

•455 

19-5 

•861 

22-5 

•265 

25-5 

•261 

28-5 

•931 

31-5 

•368 

34-5 

•680 

•6 

•967 

•6 

•389 

•6 

•406 

•6 

29-101 

•6 

•564 

•6 

•907 

•7 

17-073 

•7 

•514 

•7 

•552 

•7 

•271 

•7 

•761 

•7 

41-135 

•8 

•179 

•8 

•639 

•8 

•697 

•8 

•441 

•8 

•959 

•8 

•364 

19-9 

•285 

22-9 

•763 

25'9 

•842 

28-9 

•612 

31-9 

35-159 

34-9 

•595 

35-0 

827' 

1 

FACTORS   AND    LOGARITHMS. 

Coefficients  and  Logarithms  for  Volumetric  Analysis. 


Normal  H2SO4 

Coefficients. 
1   c.c.  =0-04904  gm.  H,SO4  
„    =0-04804     „   S04       
„    =0-04004    „   S03      .. 

Logarithms. 
..     2-69055 
..    2-68160 
..    2-60249 

Normal  HC1 

1  c.c.  =0-03647 
„    =0-03546 

„   HC1      

2-56194 

»    Cl         

..    2-54974 

Normal  HNO3 

1   c.c.  =0-06302 
„    =0-06201 
..,    =0-054 

„   HN03 
,.  N03      
»   N205     

..    2-79948 
..    2-79246 
..    2-73239 

Normal  H2C2O4 

1  c.c.  =0-06302 
„    =0-045 

.,   H2C204,  20H2 
„   H2C204 

..    2-79948 
..    2-65321 

Normal  Acid 

1  c.c.  =0-01703 
„    =0-03505 

„   NH3     .. 
„   NH4HO           ..         ,,    . 

..    2-23121 
..    2-54469 

„    =0-191 

„   Na2B40710H20 

..    1-28103 

„    =0-03705 
„    =0-02805 
„    =0-05005 

„    Ca2HO 
„   CaO      
„   CaC03  

..    2-56879 
..    2-44793 
..    2-69940 

„    =0-08569 
„    =0-15776 
„    =0-09869 

„   BaH202           ..         .. 
„   BaH2028H00  .  . 
„   BaC03             ... 

..    2-93293 
.  .    T-19800 
..    2-99427 

„    =0-02016 
„    =0-04215 

„  Mgo  ..  .-..  •.-'":'• 

.,   MgC03  

..    2-30449 
.  .    2-62490 

„    =0-05611 
„    =0-0691 
„    =0-1881 
„    =0-1081 
„    =0-0981 
„    =0-1411 

„   KHO    . 
„    K2C03 
„   KHC4H406      ..        '.. 
„    K3C6H507,H20 
„   KC2H302 
„   KNaC4H406,4H20     ..    ' 

..    2-74904 
.  .    2-83948 
..    1-27448 
..    1-03391 
..    2-99176 
..    1  -14953 

Normal  Acid 

1   c.c.  =0-04 
„    =0-053 
„    =0-14308 

„    =0-084 

„    NaHO             .,    "'.  ,. 
„   Na2C03 
„   Na2C03lOH20 
„   NaHC03 

..    2-60206 
..    2-72427 
.  .    1-15558 
.  .    2-92427 

Normal  NaHO 

1  c.c.  =0-040 
„    =0-031 

,   NaHO 
„   Na20    .. 

..    2-60206 
..    2-49136 

Normal  KHO 

1  c.c.  =0-05611 
„    =0-0471 

„   KHO    ..         ..         .. 
„   K20      ..         ..         .. 

..    2-74904 
..    2-67302 

Normal  Na2C03 

1   c.c.  =0-053 
„    =0-030 
„    =0-022 

„   Na2C03            ..          ..;•' 
„    C03       
„    C02       .. 

..    2-72427 
..    5-47712 
..    2-34242 

Normal  Alkali 

1  c.c,  =0-06 
„    =0-07 
„    =0-03647 
„    =0-0809 
„    =0-012793 

„   HC2H302 
„   H3C6H607H20 
„    HC1      
„    HBr     
„   HI        

..    2-77815 
..    2-84510 
..    2-56194 
..    2-90810 
..    1-10697 

„    =0-06302 
,.    =0-04904 
„    =0-07503 

„   HN03-.. 
„    H2S04  
„    H2C4H406        .. 

..    2-79948 
..    2-69055 
..    2-87523 

FACTORS    AND    LOGARITHMS. 


609 


N/io  Silver  Nitrate 


Coefficients. 

1  c.c.  =0-010788  gm.Ag 
„    =0--017        „   AgN03 
„    =0-003546  „    Cl 
„    =0-00535     „  NH4C1  . 


N/io  Iodine 


N/io  Bichromate 


N/io  Thiosulphate 


„  =0-007456 

„  =0-0119 

„  =0-01029 

„  =0-0062 

I  c.c.  =0-003203 

„  =0-004104 

„  =0-004948 

„  =0-024822 

„  =0-012609 

„  =0-009714 

1   c.c.  =0-01519 

„  =0-01699 

„  =0-0278 

„  =0-01158 

.,  =0-02316 

„  =0-007185 

I  c.c.  =0-024822 

„  =0-012692 

„  =0-003546 

,  =0-007992 


KC1 
KBr 
NaBr 
Na2HAs04 

SO2  .: 
H2S03  .. 
As203  .. 

Na.,S2035H20 


FeS04  ..  ':.r;>  , 
FeS04H20  ..  -  . 
FeS047H20  .  .  . 

FeC03..          ..         , 
Fe304   .. 
FeO      .. 

Sodium  thiosulphate 

Cl 
Br 


CALCIUM  (Ca  =40-09) 

1  c.c.  N/io  permanganate 


=0-002805  gm.  CaO 
=0-005005  gm.  CaC03 
=0-00861    gm.  CaS04, 


20H2 


„         normal  oxalic  acid  =0'0280      gm.  CaO  . . 

Cryst.  oxalic  acid  x 0-444     =CaO  .'.          

Double  iron  salt  x 0*07143-  =CaO  . .          

CHLORINE  (Cl=35-46) 

1  c.c.  N/i0  silver  solution    =0-003546  gm.  Cl 

=0-005846  gm.  Nad 

,,         arsenious  or  thiosulphate  solution  =0-003546  gm.  Cl 
CHROMIUM  (Cr=52) 

Metallic  iron  xO'3104  =Cr    . .          . .        i 

xO-5968=Cr03  ..          .,          .. 

xO-878    =K2Cr2Ov .. 

x  1-928    =PbCr04 


Logarithms. 

..  2-03294 

..  2-23044 

..  3-54974 

..  3-72835 

..  3-87251 

..  2-07555 

..  2-01242 

..  3-79239 

..  3-50555 

..  3-61321 

..  3-69443 

..  2-39484 

..  2-10068 

..  3-98740 

..  2-18156 

..  2-23019 

..  2-44404 

.  2-06371 

..  2-36474 

..  3-85643 

..  2-39484 

..  2-10353 

..  3-54974 

..  3-90266 

..  3-44793 

..  3-69940 

..  3-93500 

..  2-44715 

..  1-64738 

.  2-85388 


Double-iron-salt  x  0-0443  =Cr          .. 
x  0-0853  =Cr03      .. 
x  0-1253  =K2Cr207 
xO-2754=PbCr04 

1  c.c.  N/i0  solution    =0-003333  gm.  CrO3 

=0-0049    gm.  KaCr20- 
COPPER  (Cu=63-57) 

1  c.c.  N/io  solution    =0*006357  gm.  Cu    . . 

Iron  x  1 '138  =  copper 

Double  iron  salt  x 0-1622  =copper 
CYANOGEN  (CN  =26-01) 

1  c.c.  N/io  silver  solution 


N/io  iodine 


:0 -005202  gm.  CN  . 
0-005404  gm.  HCN 
0-013022  gm.  KCN 
0-003255  gm.  KCN 


3-54974 
3-76686 
3-54974 

1-49186 
1-77586 
1-94347 
0-28519 

2-64640 
2-93095 
1-09795 
1-43996 

3-52284 
3-69020 

3-80325 
0-05614 
1-21005 

3-71617 
3-73272 
2-11468 
3-51255 


2   R 


610 


FACTORS   AND    LOGARITHMS. 


Coefficients.  Logarithms 

POTASSIUM  FERROCYANIDE  (K4FeCy6,  30H2  =422-36) 

Metallic  iron        x  7  '563  =cryst.  potassium  ferrocyanide  ..          ..    0-87809 

Double  iron  salt  x  1 '080  =     ,,  „  „  ....    003342 

POTASSIUM  FERRTCYANIDE  (K6Fe2Cy12  =658-42) 

Metallic  iron  x5'895      = potassium  ferricyanide 0*77048 

Double  iron  salt      x  T684      =         „  „  0-22634 

N/io  thiosulphate    xO'03292=         „  „  2-51746 

GOLD  (Au=197-2) 

1  c.c.  normal  oxalic  acid  =0*0657  gm.  gold          . .          . .          . .          . .    2-81757 

IODINE  (1  =  126-92) 

1  c.c.  N/i0  thiosulphate  =0*012692  gm.  iodine 2-10353 

IRON  (Fe=55-85) 

1  c.c.  N/io  permanganate,  dichromate,  or  thiosulphate  =0*005585 

=0-007185 


LEAD  (Pb  =207-1) 

1  c.c.  N/io  permanganate     =0*010355  gm.  lead 
1  c.c.  normal  oxalic  acid      =0*10355  gm.  lead 
Metallic  iron  x  1*854  =lead 

Double  iron  salt        x  0*265  =lead  .. 
MANGANESE  (Mn  =54-93) 

MnO  =70-93.     Mn(X  =86*93. 

Metallic  iron  x  0*49 18    =  Mn  

x  0*6350    =MnO          ..          ..          . 
x  0-7783    =Mn02 


Fe        3*74702 
FeO     3*85643 
0*007985  Fe203  3-90227 


Double  iron  salt  x  0*0907  =MnO 

xO*1112=Mn02 

Cryst.  oxalic  acid  x 0*6896  =Mn02 

1  c.c.  N/10  solution  =0*003547  gm.  MnO 

„  „  =0*004347  gm.  Mn02 

MERCURY  (Hg  =200) 

Double  iron  salt  x  0*5 104  =  Hg        

x  0-6914  =HgCl2 

1  c.c.  N/i0  solution  =0'0200  gm.  Hg 
„  „  =0-0208  gm.  Hg2O 

=0*0271  gm.  HgCl2 

NITROGEN  AS  NITRATES  AND  NITRITES  (N205  =108*02. 
Normal  acid  x  0*0540  =N205 
xO-1011=KNO3 

Metallic  iron  x  0-376 1=HNO3         

xO-6035=KNO3         

xO-3224=N205  

SILVER  (Ag  =  107*88) 

1  c.c.  N/io  NaCl    =0-010788  gm.  Ag         

=0-016989  gm.  AgN03 

SULPHURETTED  HYDROGEN  (H2S  =34*086) 

1  c.c.  N/io  arsenious  solution  =0*00255  gm.  HoS 
TIN  (Sn=119) 

Metallic  iron  x  1-0654  =tin  

Double  iron  salt  x  0*1 522=  tin         

Factor  for  N/io  iodine  or  permanganate  solution  0*00595 
ZINC  (Zn  =65*37)- 

Metallic  iron  x  0*5852  =Zn 

x  0-7285  =ZnO  • 

Double  iron  salt  x  0*0836  =Zn         

xO'1041    =ZnO 

1  c.c.  N/io  solution  =0*003268  gm.  Zn 


N,03=  76*02) 


2-01515 
1*01515 
0-26813 
1-42325 


1-69179 
1-80277 
1-89115 

2*95761 
T -046 10 
1*83860 

3*54986 
3*63819 

1*70791 
1*83972 
2*30103 
2-31806 
2-43296 

2-73239 
1*00475 

1*57530 

1*78068 
1*50840 

2-03294 
2-23017 

3*40654 

0*02749 
1-18241 
3-77452 

1-76733 
1-86241 
2-92221 
I -01 745 
3*51428 


ADDENDA    AND    CORRIGENDA. 


Page  41.  Methyl  Red. — The  colour  changes  of  Methyl  Orange 
and  Methyl  Red  and  the  value  of  the  latter  as  an  indicator  have 
been  recently  discussed  by  H.  T.  Tizard.*  The  author  gives 
a  method  for  the  preparation  of  methyl  red  which  is  said  to  give 
higher  yields  than  that  recommended  by  Rupp  and  Loose. 
The  author  concludes  that  methyl  red  is  greatly  superior  to  methyl 
orange  as  indicator.  Not  only  is  the  end-point  very  much  sharper, 
but  the  neutral  point  so  found  is  very  much  nearer  the  theoretical 
point  than  with  methyl  orange. 

Page  142.  Determination  of  Chlorine  by  M  o  h  r  '  s  Method. — 
In  the  presence  of  much  organic  matter,  or  of  sulphuretted 
hydrogen,  the  solution  may  be  prepared  for  titration  as  follows  : — 

200  c.c.  are  warmed  to  about  100°  C.  and  neutral  potassium  permanganate 
solution  added  in  slight  excess.  After  boiling  for  about  five  minutes  if  the  colour 
is  destroyed  a  little  more  permanganate  solution  is  added.  The  excess  of 
permanganate  is  then  removed  by  adding  a  few  drops  of  alcohol  to  the  hot  liquid, 
which,  after  standing  for  15  minutes,  is  filtered,  and  the  filtrate  when  cooled 
diluted  to  the  original  volume.  The  solution,  which  must  be  neutral,  is  then 
divided  into  two  equal  portions  and  titrated. 

Page  154.    Antimony.     G  y  o  r  y  '  s  Method. 

J.  B.  Dune  an  f  recommends  the  following  procedure. 

He  standardizes  the  N/io  potassium  bromate  solution  by  dissolving  0'3  gm.  of 
pure  finely  divided  antimony  in  20  c.c.  HC1  and  a  few  drops  of  bromine  in  a 
covered  400  c.c.  beaker,  keeping  the  liquid  warm  and  occasionally  shaking  till 
the  metal  is  dissolved.  Then  boil  off  excess  of  bromine,  cool  a  little,  and  add 
about  0-75  gm.  of  sodium  sulphite.  Boil  the  mixture  down  to  about  half  its 
volume  to  drive  off  S02.  (In  the  case  of  alloys  this  latter  operation  will  also 
remove  any  arsenic  present.)  Rinse  the  cover  and  sides  of  the  beaker  with  hot 
water,  and  add  a  little  HC1.  Then  heat  the  liquid  to  boiling  and  run  in  the 
decinormal  bromate  from  a  biirette  until  nearly  all  the  antimony  has  been 
oxidized.  Now  add  three  drops  of  methyl  orange  and  continue  the  addition  of 
bromate  until  the  colour  of  the  methyl  orange  is  destroyed.  About  50  c.c.  will 
be  required,  as  1  c.c.  of  N/iO  bromate  =  about  0-006  grn.  Sb.  The  exact  value  of 
1  c.c.  is  thus  obtained.  The  process  is  strongly  recommended  for  the  analysis  of 
hard  lead,  alloys,  ores  of  antimony,  etc.  0'3  gm.  is  taken  for  the  determination, 

*  J.  C.  S.  1910,  97,  3477-3490.  t  C.  N.  1907,  95,  49. 

2    R    2 


612  ADDENDA   AND    CORRIGENDA. 

and  solution  is  brought  about  as  described  above.     The  author  recommends  a 
process  of  fusion  for  difficultly  decomposable  substances. 

Page  256.  Manganese.  Fischer's  Modification  of 
Volhard's  Method. — Cahen  and  Little's  critical  examination 
of  this  method  has  been  referred  to  in  the  text.  Whilst  this  book 
has  been  issuing  from  the  press,  however,  their  paper  has  been 
published  in  full.*  The  authors  find  that  a  definite  end-point 
cannot  be  obtained  if  the  titration  be  carried  out  at  a  boiling 
temperature.  They,  therefore,  vary  Fischer's  procedure  slightly 
in  the  following  manner : — After  the  first  titration  with  per- 
manganate and  before  the  addition  of  acetic  acid  the  solution  is 
cooled  under  the  tap  for  a  minute  or  two,  and  after  the  addition 
of  acetic  acid  the  liquid  is  shaken  thoroughly.  The  hot,  but  not 
boiling,  solution  is  then  further  titrated  with  permanganate, 
added  a  few  drops  at  a  time,  with  vigorous  shaking  for  half  a  minute 
after  each  addition,  until  the  supernatant  liquid  retains  its  pink 
colour  after  being  well  shaken  several  times.  This  is  the  end-point 
of  the  titration.  The  quantities  given  in  the  text  apply  to  500  c.c. 
of  solution.  They  find  that  the  method  agrees  very  satisfactorily 
with  the  bismuthate,  gravimetric,  and  Pattinson's  methods. 

Page  256.     Line  19,  for  "absorbed"  read  "adsorbed." 

Page  390.  Formaldehyde.  L  egler'  s  Method. — Herrmannt 
has  shown  that  the  inaccuracies  in  L  egler 's  method  of  determining 
formaldehyde  by  conversion  into  hexamethylenetetramine,  which 
are  due  to  the  slow  action  of  ammonia  solution  and  the  instability 
of  standard  ammonia  solutions,  may  be  obviated  by  developing 
ammonia  gas  within  the  liquid  itself  from  ammonium  chloride  by 
the  addition  of  standard  sodium  hydroxide  solution.  The  heat  of 
the  reaction  causes  the  nascent  ammonia  to  convert  the 
formaldehyde  instantaneously  and  completely  into  hexamethy- 
lenetetramine. A  weighed  quantity  (between  4  and  4' 5  gm.)  of 
ordinary  formalin  solution  is  mixed  in  a  stoppered  bottle  of  150  to 
200  c.c.  capacity,  with  3  gm.  of  pure  finely-powdered  ammonium 
chloride  and  then  with  25  c.c.  of  2N — sodium  hydroxide  solution 
added  from  a  burette  as  rapidly  as  possible,  and  the  bottle  is  closed 
and  allowed  to  stand.  As  soon  as  its  contents  have  cooled  to  the 
ordinary  temperature,  50  c.c.  of  water  and  4  drops  of  a  1  per  cent, 
solution  of  methyl  orange  are  introduced,  and  the  liquid  titrated 
with  N/!  sulphuric  acid,  to  obtain  the  number  of  c.c.  of  N/x  alkali 
solution  consumed  in  the  formation  of  the  hexamethylenetetramine. 
The  result  multiplied  by  0-06  gives  the  quantity  of  formaldehyde 
in  grams  in  the  formalin  solution.  The  results  thus  obtained  are 
closely  concordant,  and  are  practically  identical  with  those  given 
when  the  liquid  is  allowed  to  stand  for  24  hours  before  the  titration. 

*  Analyst,  1911,  36,  52.  f  Chem.  Zeit.  1911,  35,  25. 


ADDENDA   AND    CORBIGENDA.  613 

Page  403.     The  most  recent*   "  Saponification  Values"  for  the 
oils  named^below  are  as  follows  : — 

Lard 195-203 

Horse  fat 195-199 

Lard  Oil        193-198 

Niger  Oil 186-192 

Linseed          190-201 

Cotton  Seed 191-196 

Whale  184^194 

Seal . .  190-193 

Rape . .  170-175 

Cod  Liver 179-189 

Castor  175-183 

Sperm  120-137 

Shark  Liver  . .  157-164 


CORRIGENDA. 

Page  19,  line  10  from  bottom. — For  "  a  kilogram  weight "  read 

"  999-13  grams." 

Page  20,  lines  2  and  10. — For  "  half  a  minute  "  read  "  a  minute." 
Page  28,  last  line  but  one.— For  "  N/"  read  "  N/10." 
Page  35,  line  26.— Insert  *  after  "  follows." 
Page  64,  line  20. — For  "  titratiug"  read  "  titrating." 

For  "  phenophthalein  "  read  "  phenolphthalein." 
Page  65,  last  line. — For  "finely-divided"  read  "reduced." 
Page  74,  last  line  but  one. — For  "  at "  read  "  as." 
Page  82,  line  39.— For  1  c.c.  AgNO3  00= -54  gram  HCy. 

read  1  c.c.  N/10  AgN03='0054  gram  HCy. 
Page  88,  line  17. — For  "analyzed"  read  "analysed." 
Page  88,  3  lines  from  bottom. — ForRonch  se  readRonchese. 
Page  136,  line  11.— For  "p.  8"  read  "p.  225." 
Page  222,  line  7.—  For  "  Schroder  "  read  "  Schroder." 
Page  250,  last  line  )  For   "Fresenius'  and  Wills'" 

Page  259,  lines  33  and  37   [read  "Fresenius  and  Will's." 
Page  323,  lines  20  and  36  \ 

Page  324,  line  15  For  "  hydrolyzed  "  read 

Page  337,  last  line  "  hydrolysed." 

Page  338,  line  8 

Page  326,  last  line.— For  "  dialyzer  "  read  "  dialyser." 
Page  422,  line  5  from  bottom. — For  "  10  "  read  "  100." 
Page  428,  line  8  from  bottom.  -For  "Felhing"  read  "Fehling." 
Page  430,  note.— For  "  Z.  A.  <?."  read  "  Z.  a.  C" 
Page  462,  line  9.— For  "analyzed"  read  "analysed." 
Page  585,  line  13.— For  4   read   , 

•See  Allen's  Comm.  Org.  Analysis  1910,  Vol.  II.,  pp.  69-73. 


INDEX 


(For  List  of  Tables  see  Table  of  Contents.} 


D  =  Determination. 

Acetate  of  lime,  analysis  of     .          .         . 

Aeetaldehyde,  D.  of 

Acetic  acid  and  acetates,  titration  of        ... 
Acetone,  analysis  of       ...... 

Acetyl  value  of  oils  and  fats    .         .         .         . 

„        „        D.  of    . 
Acidimetry  ........ 

Acid  value  (oils  and  fats) 

Aldehydes,  various,  D.  of 

Alkalimetry  .         .         .          .         .         .          . 

Alkali  chlorides,  indirect  D.  of 

„       hydroxides,  D.  of,  in  presence  of  small  proportions  of  carbonates 
„  „  D.  of  small  quantities  of,  in  presence  of  carbonates 

„       carbonate  and  bicarbonate,  D.  of,  in  presence  of  each  other      . 
„       sulphides,  sulphites,  thiosulphates  and  sulphates,  D.  of    . 
„       salts,  titration  of  . 

„         „       mixed  caustic  and  carbonated,  titration  of 
Alkalies  in  presence  of  sulphites,  D.  of 
„          indirect  D.  of  . 

Alkaline  earths,  D.  of 

„  „       indirect  D.  of  .         ... 

Alum  cake,  free  acid  in  . 
Aluminium,  D.  of  .          .          . 

Ammonia,  D.  of  by  distillation 

„         indirect  D.  of  .....  .          . 

„        salts,  analysis  of      .         .         .         .         . 

„         semi-normal  ......... 

Ammoniacal  gas-liquor,  analysis  of 

,,  liquors,  analysis  of       ....... 

Ammonium-copper,  normal  solution  of     . 

„          thiocyanate,  decinormal        .         .          .         .         .         . 

Amphoteric  substances 

Aniline,  D,  of 

y  Antimony,  D.  of    . 

/          „          titration  with  bromate 

„  ,",          „     iodate 

Arsenic,  D.  of 

„          in  presence  of  tin,  D.  of 
„          D.  of  as  silver  arsenate      ....... 

„          in  iron  ores,  steel  and  pig  iron,  D.  of . 
in  organic  compounds,  D.  of 


Arsenite,  solution,  decinormal 


.  392 

.  90 

.  383 

.  400 

.  410 

.  89 

.  400 

.  392 

.  34 

.  144 

.  62 

.  63 

.  64 

.  343 

.  60 

.  61 

.  64 

.  143 

.  72 

.  143 

.  149 

.  148 

.  75 

76 

.  75 

.  54 

.  77 

80 

.  55 

.  145 

.  44 

.  385 

.  151 
164,  611 

.  134 

.  155 

.  158 

.  160 

.  162 

.  165 
139 


INDEX.  615 

Page. 
Arsenious  acid,  reaction  with  iodine          .          .          .          .          .  .      139 

„  ,,      titration  of     .........      134 

,,         chloride,  titration  of         .          .          .          .          .          .          .          .134 

„         sulphide,  D.  of,  by  iodine         .  .  .      161 

Aurine 40 

Azo-dyes,  D.  of ....     387 

Balance,  the  .          .          .          •          •          •          •          •  '    «          •         5 

Barium  hydroxide,  decinormal       ' .  55 

D.  of .165 

„       thiosulphate,  preparation  and  use  of    .         _„'•',     .  "•' '    .V      .          .     130 
Bifluorides,  titration  of  .          .          .          .          .         i .        .       "...          .          .112 

^Bismuth,  titration  of  .          .          .          .         .»          ...•-....          .166 

Black  ash,  analysis  of     .          .          .          .          .          .          ..       *         .          .70 

Bleaching  powder,  valuation  of        .          .          .          ...         .          .          .177 

Boric  acid  in  butter,  D.  of  .          .          .          ..•••„•        .          .          .       96 

,,         „    in  meat,  D.  of        .          .          .          .          .....          .       96 

„        „    in  milk,  butter,  etc.,  D.  of  .          .         ..          .         .          .       95 

„     titration  of ,          .       93 

Bromates,  D.  of ,.         ..          .      183 

Bromides,  iodides  and  chlorides,  D.  of     .          .          .          .          „•,**.     226 
Bromine  value  of  oils  and  fats         .          .          .          .          .          .         v;        .     400 

D.  of .V      •     168 

Burette,  the .         7 

„         calibration  of    .          .          .          .          .          .          .          ...       20 

Butter,  D.  ofReichert-Wollny  number  of  .      404,  407 

Cadmium,  D.  of .          ..        .171 

Calcimeter,  Schei bier's .          .          .      105 

Calcium,  D.  of ..          .          .          .172 

„        and  magnesium  carbonates  in  waters,  D.  of          .          .          .          .73 
,,  ,,  ,.  „  etc.,  D.  of,  by  Hehner's  method     .       73 

Calibration  of  graduated  instruments 19 

Cane-sugar,  glucose  and  dextrin,  D.  of     .          .          .          .          .          .          .     337 

Carbon  disulphide,  D.  of 389 

„  „  in  benzene,  D.  of 389 

„       in  steel,  colorimetric  D.  of(Eggertz)         .          .          .          .          .     242 

,,       dioxide  in  waters,  D.  of  .          .          .          .          .          .          .99 

,,  ,,     in  aerated  waters,  D.  of  .          .          .          .          .          .          .      101 

„  ,,     in  air,  D.  of    .      '"  .         «         .         .         .         .         .         .     102 

,,  „       D.  of,  by  volume   .'.-.*         .          .          .          .          .          .      105 

Carbonic  acid,  D.  of 97 

Caustic  and  carbonated  alkalies,  titration  of     .          .          .          .          .          .61 

,,       potash  or  soda,  D.  of  by  dichromate    .          .          .          .          .          .65 

Cerium,  D.  of 173 

Chlorates,  D.  of     .          .          *" 183 

„          titration  of     .          .  - 133 

„          indirect  D.  of      ...          .          .          .          .          .          .          .      143 

Chloric  and  nitric  acids,  iodimetric  D.  of .          .          .          .          .          .          .      180 

Chlorides,  bromides  and  iodides,  D.  of      .          .  .          .          .          .     226 

„          chlorates  and  perchlorates,  analysis  of  mixtures  of    .          .          .179 

„          hypochlorites  and  chlorates,  analysis  of  mixtures  of  .        • .          .178 

Chlorine,  direct  precipitation  of  .          .          .          .  t  .         .175 

„         titration  of  in  neutral  solution .     142 

„  „  in  acid  solution  (  V  o  1  h  a  r  d )    .....     145 

„        water,  titration  of     ...          .          .          .          .          .          .177 

Chromates,  D.  of *  .          .183 

„  titration  of ••'..          .          .          .     133 

Chrome  iron  ore,  analysis  of  ...          ",''.      .          .          .          .184 

Chromic  acid,  iodimetric  D.  of         .          .          .     ,.   .          .          .          .          .189 


616  INDEX. 

Chromium  in  steel,  D.  of .          .       ig6,  370 

Citric  acid,  D.  of  .          .          .          .          .          .          .          .          .  IQS 

)lour  reactions,  precision  in  ( D  u  p  r  e )  .....  146 

l-gas,  analysis  of       ..........     553 

Cobalt,  D.  of          .  189,  270 

Cochineal  solution  .          .          .          .          .          .          .          .          .          .36 

Congo  red     ............       49 

Copper,  D.  of  .          . 191 

„       D.  as  cuprous  iodide  .........     193 

,,       D.  by  potassium  cyanide  (Parkes)     .          .          .          .          .  195 

„        D.  as  sulphide  (Pelouze)  .          .          .          .          .          .          .      197 

,,       D.  by  stannous  chloride  (Weil)  .          .          .          .          .          .     193 

„       D.  as  cuprous  thiocyanate  (  Volhard)         .....      199 

„       D.  by  titration  of  copper  ferrocyanide  (Sanchez)       .          .          .     206 
„       colorimetric  D.  of  .          .          .  .          .          .     204 

,,       ores,  Steinbeck's  process  for  ......     201 

,,       and  iron,  D.  of  ..........     199 

Corallin          .          .          .          .          .          .  .          .          .          .          .40 

Cream  of  tartar,  analysis  of    .          .          .          .          .          .          .          .          .119 

Cresols,  D.  of 415 

Cyanides  used  in  gold  extraction,  analysis  of    .          .  '       .  .          .     210 

Cyanogen,  titration  of    .          .          .          .          ...          .          .          .     207 

Decem,  the  .          .          .          .          .          .  .          .          .          .27 

Dextrin,  D.  of  ...       325,  337 

Electrolysis  of  water  (Bunsen).          .          .          ..          .  .       .          .531 

Erythrosin    .  .  74 

Ester  value  of  oils  and  fats     .........     403 

Ethyl  orange          .          .          .          .          .          .          .          .          .          .          .38 

Eudiometer,  graduation  of      .          .          .          .          .          .        •*....        .          .     505 

Fats,  analysis  of    .                              ...:....  400 

F  e  h  1  i  n  g '  s  solution,  preparation  of         .          .          .          .          .          .          .  327 

Ferric  compounds,  reduction  to  ferrous    ...          .          .          .          .          .  233 

„             „             titration  of         ...          .          .          .          .          .          .  234 

„     indicator  for  Volhard '  s  process          .          .          .          .                    .  146 

Ferricyanides,  titration  of 220 

Ferrocyanides,  analysis  of  .          .  .          .          .          .          .216 

„              commercial,  analysis  of     .          .          .          .          .          .          .  217 

Filter,  Be  ale's .19 

Float,  Erdmann's 18 

Formaldehyde,  D.  of      .                              .          ...          .                    .  390 

Formalin       ...                             .                    .....  390 

Formic  acid,  D.  of 109 

Frankland  and  Ward's  gas  apparatus        .......  543 

Gases,  analysis  of 505 

„       absorption  of  by  liquid  reagents    .......     543 

,,       determined  directly      .          .          .          .          .          .          .          .518,  519 

„  indirectly    . 518,  526 

titration  of  .    '     .         .         .         .         .         .         .         .          .573 

Gas  liquor,  analysis  of   . .77 

Gas-volumeter,  Lunge's 587 

Glucose,  D.  of  (Fe  hi  ing) 327 

,,         D.  of  by  Pa vy's  method  .          .          ...          .          .     335 

„         D.  of  by  mercury  solutions         .......     332 

„         D.  of  by  cyano-cnpric  process  (Gerrard) 337 

„         D.  of  by  Sid er sky's  method 334 

„         sucrose  and  dextrin,  D.  of  .          .          .          .      *  .          .          .     337 


INDEX.  617 

Page. 

Glycerol,  D.  of 393 

Gold,  D.  of 222 

Graduated  instruments,  the  correct  reading  of 17 

Grain  system,  the           ....                    27 

Hardness  of  waters,  H eh ner's  method  for  D.  of 73 

Hehner  value  of  oils  and  fats        .          .                    .....  401 

Hem  pel  gas  burette     ....       .'...-        ..        ....  575 

H  e  m  p  e  1' s  gas  absorption  pipettes         .          •          •          •          •         *      578,581 

Hydrofluoric  acid,  D.  of  .          .  •-=..'••     '.          .          .          .          .110 

Hydrofluosilicic  acid,  D.  of      .          .      .    ,         ..          .          ..•'•••     .          ...       .  110 

Hydrogen  peroxide,  analysis  of        .          .          .-..'.:..'        .          .  305 

Hydroxyl,  analysis  of     .          .          .          ...          .          .          .          .  305 

Hydroxides  and  carbonates,  mixed,  D.  of         ..•         ..,'..          .  72 

Indicators     .          .          .          .          .          ...          .          .                    .  34 

classification  of                .          .          ...          .          .          .          .  45 

extra  sensitive       .          .          .          .          .          .          .          .          .  40 

general  characteristics  of         .          .    '      .          .          .          .  •  v  42 

G 1  a  s  e  r '  s  classification  of     .          .          .          ;         .          .   ;      .  45 

theory  of                .          .                   -,        '".          .        .,         ,          .  46 

Thomson's  results  with      .          ."•  .     .>     .- •„  "•      .          .          .  41 

use  of  two    .          .          .  '      .          .          *          .....         .  62 

Indicator,  ferric     .       -•.-.',          -.        ,          .          .          ,          .         *          .  146 

Indigo,  valuation  of                  .          .-         .          .          .          .          .          .          .  397 

Indirect  methods  of  analysis  .          .          .          .          .          .          .  .      :.          .  32 

lodates,  D.  of         .          .          .          .                    .          .          .          ;          .          .  183 

Iodides,  titration  of        .          .          .          .          .   ;      .          .         ,     '  _ ,,         .  132 

„               „        with  decinormal  silver 228 

„        bromides  and  chlorides,  D.  of     .          .          .          .          .  '       .          .  226 

Iodine,  D.  by  distillation 224 

„      combined,  oxidation  by  chlorine 228 

„  ,,  „          in  presence  of  bromides  and  chlorides    .      229,  230 

,,      free,  titration  of 133 

„       value  of  oils  and  fats            ...                     ....  400 

„       D.  of          .          .          .          .          .          .       412,  414 

Iron,  colorimetric  D.  of .          .  238 

,,     alum,  standard  solution  of                .          .          .          ....          .  368 

,,     ores,  analysis  of     .          .          .          .          .          .          .          .          .          .  239 

,,     (ferrous),  titration  with  dichromate  (Penny)  .          .          .          .          .  126 

„          of 231 

,,     (ferric),  D.  with  iodine  and  thiosulphate           .          .          .          .          .  237 

„           ,,       reduction  to  ferrous  state   .      '    .  *                            .          .          .  233 

,,           „       titration  by  sodium  thiosulphate          ,,         .          ...          .  236 

„           ,,                 ,,           stannous  chloride      .          *  .    „         .          .          .  128 

,,           ,,                 „           titanous  chloride       .          .          ...          .          .  234 

Kjeldahl's  method  for  nitrogen  .          .      '    .          .          .          .          .          .  83 

Kjeldahl-Gunning  process        .          .          ...     .          .  "_-.     .         .  87 

Kje  Idahl-Gunn ing- Jodlbauer  process.          .          .        ...        .          .  88 

Kottstorfer  value  of  oils  and  fats       .       .   . ;        .          .        • ..    .     .          .  400 

Lacmoid        .          .          .          ...          .          .          .          .          .          .          .  40 

Lactose,  D.  of        .          .         V 338 

Lead,  D.  of                      .^       .                   .         ;.        .         .         /        .         .  245 

„      in  citric  and  tartaric  acids,  D.  of    .          .          .          i          .         .    '  247 

„      in  cream  of  tartar,  D.  of    -.  .  •        .                •   .         .          .          .          .  247 

,,      in  presence  of  iron,  colorimetric  D.  of    .          .         .    :     .          .          .  248 

„      in  waters,  colorimetric  D.  of            .          .         .  »      .          .          .          .  248 

„      and  zinc,  D.  of     .          .          .          .         .          .          .     ,    .          .          .  381 


618  INDEX. 

Tage. 
Leffmann  and  Beam's  modification  of  Reichert  process  for  butter  .     400 

Litmus  paper 36 

,,       solution     ...........       34 

Litre,  the 24 

Magnesium,  D.  of  ..........     249 

Manganese,  D.  of .          .     250 

D.  by  bismuthate  process      .......     256 

D.  by  conversion  into  permanganic  acid          ....     256 

D.  by  permanganate  (Volhard) 255 

D.  by  persulphate  (K  nor  re) 258 

colorimetric  D.  of  (Dnfty) 257 

dioxide,  valuation  of     ........     261 

ores,  technical  examination  of        ......     259 

Me  Leod' s  gas  apparatus     .          .          .          .          .          .          .          .          .     545 

Measuring  flask,  the        .          .          .          .          .          .          .          .          .          .15 

„  „       calibration  of          ........        19 

Mercuric  chloride,  titration  of  ........     264 

Mercury,  D.  of 262 

Methyl  orange 37 

„      red  .      , 41 

„       salicylate,  analysis  of  .          .          .          .          .          .          .          .419 

Methylene  blue  indicator         .          .          .          .          .          .          ..          .131 

Metric  system,  the  .          .          .          .          .  .          .          .          .23 

Milk-sugar,  D.  of .          .338 

Nessler's  solution       ..........     437 

Neutral  point  indicator  .          .          .          .          .          .          .          .          .          .40 

Nickel,  D.  of 267,  271 

„      in  steel,  D.  of 269,  270 

Nitrate  of  lime,  indirect  D.  of  nitrogen  in         ......       73 

Nitrates,  D.  by  conversion  into  ammonia  (Schulze)         .          .          .          .     271 

(Devarda's  method)        .          .     285 
,,         D.  as  nitric  oxide  (Crum)         .          .          .          .          .          .          .     463 

(Lunge) 582 

D.  by  Kjeldahl-Gunning- Jodlbauer  process  .  .       88 

D.  by  oxidation  of  ferrous  salts  (Pelouze)         .          .          .          .     273 

(Schlosing)     .          .          .       277,284 

D.  of,  in  the  absence  of  ammonium  salts  (Ulsch)      .          .          .     283 
Nitr  c  acid,  iodimetric  D.  of   .          .          .          .          .          .          .          .          .180 

Nitr  tes,  iodimetric  D.  of 286 

„         gasometric  D.  of  .          .          .          .          .          .          .          .     289 

,,         L\  by  permanganate  ........     288 

„         in  presence  of  alkali  sulphites  and  thiosulphates,  D.  of          .          .     290 
Nitrogen,  indirect  D.  of .          .          .          .          .          .          .          .          .          .     144 

,,         in  the  absence  of  nitrates,  D.  of  .          .          .          .          .87 

,,         in  the  presence  of  nitrates,  D.  of  .          .          .          •          .88 

„         D.  by  Rone  he  se' s  method    .......       88 

Nitro-compounds.  D.  of.          .          .          .          .          .          .          .          .          •     387 

Nitroso-compounds,  D.  of  .          .          .          .          .          .          •          •     387 

Nitrometer,  Lunge's   .          .          .          .  .          .          .          .     582 

Nordhausen  sulphuric  acid,  analysis  of     .  .          .          .          •          .116 

Normal  solution,  definition  of          ........       28 

,,        acid  and  alkali  solutions,  preparation  of 

„        hydrochloric  acid        .........       53 

,,        nitric  acid  .          .          .  .          .          .          .          .          .53 

„        oxalic  acid          ..........        =r>2 

„        potash  and  soda          .  .          .          .          .          .          .          .54 

„        potassium  carbonate  .........       49 

sodium  carbonate  ....       49 


INDEX. 


619 


Page. 
Normal  sulphuric  acid    ..........       49 

„        solutions  for  gas  analysis    ........     574 

Oils,  analysis  of     .          .          .          .          .          .          .          .          .          .    '  400 

Organic  salts  of  the  alkalies,  titration  of .68 

Orsat-Lunge  gas  apparatus         ........     569 

Oxalic  acid,  titration  of.          .          .          .          .          .          .          .          .          .114 

Oxygen,  dissolved,  D.  of          .          .          .          .          .          .          .          .          .     290. 

iodimetric  D.  of  (Thresh) 297 

„  „  in  waters  and  effluents,  D.  of 303 

Perchlorate  in  Chili  saltpetre,  D.  of 180 

Persulphates,  D.  of ••;.".          .          .354 

Phenacetolin 38 

Phenolphthalein ....       39 

,,  and  methyl  orange,  mixture  of  .          .          .          .40 

,,  behaviour  with  boric  acid        .          .          .          .          .          .45 

Phenols,  D.  of        ....  415 

Phosphates,  D.  by  uranium  solution         .          .          .          .          .          .          .     307 

„  D.  by  silver  nitrate  (Hoi le man)  .....     315 

Phosphoric  acid,  titration  of  .          .          .          .          .          .          .          .          .      114 

Phosphorus  in  iron  and  steel,  D.  of          .          .          .          .          .          .          .     244 

Pinch-cocks 13 

Pipette,  the  .  .14 

,,        calibration  of    . •         .          .          .          .          .          .          .          .          .20 

Polenske  value  of  fats,  D.  of  .  409 

Potassium  bi-iodate  as  standardizing  reagent    ......       48 

D.  by  sodium  cobaltinitrite     .......       68 

and  sodium  chlorides,  indirect  D.  of          .          .          .          .          .      144 

dichromate,  decinormal  solution  of 127,  231 

ferrooyanide,  titration  of          .          .          .          .          .          .          .217 

„  „  220 

permanganate,  decinormal  solution  of       ....       122,  231 

„  „  „  „  calculation  of  results  of 

analyses  made  with  .          .          .          .          .          .          .          .          .125 

Preservation  of  standard  solutions  .          .          ...          .          .          .21 

Red  liquors,  analysis  of            .          .          .          .          .          .          .  69 

Reichert  value  of  oils  and  fats    ........  400 

Reichert-Meissl  and  Reichert-Wollny  value,  D.  of .    .          .          .  403 

Residual  method  of  analysis    .........  32 

Rosolic  acid            ...........  40 

Salicylic  acid,  D.  of 418 

,,  „      limits  of,  allowed  in  food  .          .          .          .          .          .          .419 

Salt  cake,  analysis  of      ..........       70 

Saponification  value  of  oils  and  fats         •',.'••          •         "•          •          •          •     400 

Septem,  the .          .          .          .27 

Sewage,  analysis  of  .          .          .          .          .          .          .          .          .437 

Silver,  D.  by  decinormal  sodium  chloride          .          .          .          .          .          .316 

m  acid  solution,  D.  of  (Volhard)         .          .          .          .          .          .     145 

in  ores  and  alloys,  D.  of       .          .          .          .          .          .          .          .317 

in  plate,  coin,  etc.,  D.  of  .          .          .          .          .          .          .     318 

solutions  used  in  photography,  titration  of    .          .          .          .          .     322 

solution,  decinormal     .........      141 

Soap,  analysis  of   .          .          .          .          .          .          .          .          .          .          .71 

Soda  ash,  analysis  of  .          .          .          .          .          .          .          .          .        69 

S  o  d  e  a  u '  s  gas  apparatus       .........     565 

Sodium,  D.  by  potassium  dihydroxytartrate     .          .          .  .          .65 

„        carbonate  as  standardizing  reagent      ......       47 


620  INDEX. 

Page. 
Sodium  oxalate  as  standardizing  reagent          ......       48 

chloride,  decinormal  solution  of  .          .          .          .          .          .      142 

peroxide,  analysis  of  ........     306 

thiosulphate  and  iodine,  reaction  between    .          .          .          .          .128 

,,  decinormal  solution  of  .          .          .          .          .130 

„  Volhard's  method  of  standardization     .          .          .413 

Spent  liquor,  ammoniacal,  analysis  of  .          .          .          .          .          .78 

Standard  solutions,  adjustment  of  ...          .          .          .          .57 

Stannous  chloride  solution,  preparation  of        .          .          .          .          .          .128 

Starch  indicator,  preparation  of       .          .         ;          .          .          .          .          .131 

„      paper,  iodized      .          .....  .          .          .      140 

„       hydrolysis  of  .          .          .         '.; '  •'       ...  .     325 

Sugars  .          .          .          .     "    .         .         • 323 

„       conversion  into  glucose        .          ...          .          .          .          .     324 

„       comparative  reducing  powers  of  .      "    .          .          .          .          .          .     333 

Sulphides,  alkali,  D.  of .     342 

Sulphur  in  coal-gas,  D.  of  .          .          .          .          .          .          .          .     340 

„        in  iron  and  steel,  D.  of  .          .          .          .          .          .          .     244 

„         in  pyrites,  etc.,  D.  of          .          .          .          .  .          .          •     339 

,,        in  sulphides,  D.  of     ....          .          .          .          .          .          •     341 

Sulphuretted  hydrogen,  D.  of  .....          .          .          .347 

Sulphuric  acid,  D.  of 349 

„  „     fuming,  analysis  of  .          .      '-...          .          .          .          .116 

Tannic  acid,  D.  of 355 

Tannin  in  tea,  D.  of  .          .          .          .          .          .          .          •          .359 

„       in  wine,  cider,  etc.  D.  of     .          .          .          .          .          .          .          •     360 

„      materials,  rapid  method  for          .......     363 

,,      precipitation  by  gelatine      .  .          .          .          .          •          •     363 

Tartaric  acid,  titration  of        ........ 

Tartrates,  analysis  of  .          .          .          .  •          •          •          .117 

Test-mixers  .          .          .          .          .          .          •          •          •          ..17 

Thiocarbonates,  D.  of .          .          -          .389 

Thiocyanates,  D.  of  . 221 

D.  of  (Volhard)      .  145 

Thomas's  gas  apparatus     .........     559 

Tin,  D.  of 364 

„    in  white-metal  alloys,  D.  of 36( 

„    ore,  analysis  of  ........ 

„    (stannous),  titration  by  ferric  chloride      ......     365 

Titanium,  D.  of     .          . 367 

Titanous  chloride,  standard  solution  of    .          .         «          .          .          •          •     234 

Uranium.  D.  of .  v  •        •         •  T      •          .368 

Urea,  D.  of 423 

Urine  analysis 421 

„      D.  of  albumen  in  ......... 

„      D.  of  ammonia  in          ......•«•  433 

„      D.  of  free  acid  in  ...  435 

„      D.  of  glucose  in    .          .          .          .    '      •          •          •          •          •          •  428 

„      D.  of  lime  and  magnesia  in    ...  ....  432 

,,      D.  of  phosphoric  acid  in         ...  •  •  42' 

„      D.  of  soda  and  potash  in       .....  •  435 

„      D.  of  sulphuric  acid  in •  42J 

„      D.  of  total  nitrogen  in  .....«•• 

„      D.  of  uric  acid  in 429 

Vanadium,  D.  of 369 

in  steel,  D.  of        .  .  370 

Vinegar,  D.  of  free  mineral  acids  in          .          .          . 


INDEX.  621 

Page. 

Waters,  analysis  of                   .          .          .  .          .          .          .  .  .437 

Water  analysis,  Special  Index  of  Processes  .          .          .          .  .  .     444 

„           ,,         preparation  of  reagents  for  ...  .  437 

Waxes,  analysis  of          .          .          .          .  .  .  .     400 

Wij  s' solution  of  iodine         .          .          .  .•    '     .          .          .  .  .     414 

Zinc  dust,  analysis  of     .          .          .          .  ....  .  .     382 

D.  as  ferrocyanide    .       .          .          .  .          „         ...  .     376 

D.  as  phosphate     .          .          .          ...          ...  .  .     380 

indirect  D.  of          .          .          .          ,  ....          .  .  .371 

criticism  of  methods  for  D.  of       :    .    .  .          .          .          .  .  .     381 

precipitation  as  sulphide,  etc.                  -.  .          .          .'        .  .  371-373 

and  lead,  D.  of       .          .          .          .  ..._..-.         :  .  .     381 

oxide  and  carbonate,  titration  of  .                   '.  .  .     383 


NOTES. 


NOTES. 


NOTES. 


NOTES. 


NOTES. 


NOTES. 


JAN    2919331 


APR  13  1937 


1937 


»* 


FEB  161935 


DEC    101935 


s* 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


